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Initial guidance to obtain representative meteorological observations at urban sites

R E P O R T No. 81
Tim R. Oke, Canada
WMO/TD No. 1250
The designations employed and the presentation of material in this publication do not imply the
expression of any opinion whatsoever on the part of the Secretariat of the World Meteorological
Organization concerning the legal status of any country, territory, city or area, or its authorities, or
concerning the limitation of the frontiers or boundaries.
This report has been produced without editorial revision by the Secretariat. It is not an official
WMO publication and its distribution in this form does not imply endorsement by the Organization
of the ideas expressed.
The Thirteenth Session of the Commission for Instruments and Methods of Observation
(CIMO) recognized the need to include in the WMO Guide to Instruments and Methods of
Observation, WMO-No.8 (CIMO Guide) a new chapter on Urban Observations. Mr Tim Oke,
University of British Columbia, Canada, transformed his long-time experience in this field into a
new chapter of the CIMO Guide, scheduled for publication in the beginning of 2006. This IOM
Report is therefore an important tool for early dissemination of CIMO guidance to Members on
the observation of meteorological elements in urban areas. I would like to express my
appreciation to Mr Oke for this excellent publication and his ongoing contribution to the work of
The IOM Report stresses the need to fully appreciate the scales of urban climates
(micro-, local- and meso-scale) as they impact phenomena and measurement methods. In
particular, the presence of the urban canopy layer defines a micro-scale dominated layer
beneath roof-level (UCL) and a layer above roof level and the roughness sub-layer (RSL), which
responds to the local scale. The above roof layer represents a blended influence that brings with
it questions of the rate of internal boundary layer growth and the location of the source areas
(‘footprints’) for meteorological sensors.
The essential first step in selecting urban station sites is to evaluate the physical nature
of the urban terrain. This will reveal areas of ‘homogeneity’ and conversely areas of transition
and inhomogeneity. A new site classification system has been devised to describe any urban
site. It is based on measures of the urban structure, land cover, building fabric and metabolism
(anthropogenic heat, water and pollution), rather than land-use zones which only relate to
function, which is not necessarily climatically significant. The suggested classes are called
Urban Climate Zones (UCZ).
The IOM Report deals with the realities for those faced with the establishment of a
meteorological station at an urban site where application of the CIMO Guide is often either
impossible or nonsensical. The overall objective is to obtain observations of those elements that
are representative of the UCZ. For measurements involving a station located in the UCL the
suggestion is to centre the sensors in a representative space. For measurements in the blended
layer special attention is paid to the height of measurement because of the need to avoid
unwanted advective influences so that the source areas are fully representative of the UCZ.
A section of the IOM Report is devoted to the special requirements for documenting
metadata in urban environments. Because the environment of urban stations change frequently
as development proceeds, metadata (and their frequent update) are as important as the
meteorological data gathered.
In preparation of the IOM Report, information on the results of the Questionnaire to
NMHSs were taken into account as well as the feedback received after presentations made at
several conferences (in Casablanca, Morocco; Nice, France; Sydney, Australia; Ottawa,
Canada; Lodz, Poland; and Albuquerque, NM, USA) and circulation to several experts in urban
meteorology. It was also made available for comment to members of the International
Association for Urban Climate.
I would like to thank Professor Tim Oke for the remarkable work done in preparing this
Initial Guidance to Obtain Representative Meteorological Observations at Urban Sites. I also
wish to thank Mr Eric Leinberger, the cartographer, the Department of Geography of the
University of British Columbia, for preparing figures that appear in this publication.
(Dr. R.P. Canterford)
Acting President
Commission for Instruments
and Methods of Observation
T.R. Oke
University of British Columbia, Vancouver, BC, Canada V6T 1Z2
1 General
There is a growing need for meteorological observations conducted in urban areas.
Urban populations continue to expand and meteorological services are increasingly
required to supply meteorological data in support of detailed forecasts for citizens,
building and urban design, energy conservation, transport and communications, air
quality and health, storm water and wind engineering, insurance and emergency
measures. At the same time meteorological services have difficulty in taking urban
observations that are not severely compromised. This is because most developed sites
make it impossible to conform to the standard guidelines for site selection and
instrument exposure given in the Guide to Meteorological Instruments and Methods of
Observation (WMO 1996) [hereinafter referred to as the Guide] due to obstruction of
airflow and radiation exchange by buildings and trees, unnatural surface cover and
waste heat and water vapour from human activities.
This chapter provides information to enable the selection of sites, installation of a
meteorological station and interpretation of the data from an urban area. In particular it
deals with the case of what is commonly called a ‘standard’ climate station. Despite the
complexity and inhomogeneity of urban environments, useful and repeatable
observations can be obtained. Every site presents a unique challenge. To ensure
meaningful observations requires careful attention to certain principles and concepts
that are virtually unique to urban areas. It also requires the person establishing and
running the station to apply those principles and concepts in an intelligent and flexible
way that is sensitive to the realities of the specific environment involved. Rigid ‘rules’
have little utility. The need for flexibility runs slightly counter to the general notion of
standardization that is promoted as WMO observing practice. In urban areas it is
sometimes necessary to accept exposure over non-standard surfaces at non-standard
heights, to split observations between two or more locations, or to be closer than usual
to buildings or waste heat exhausts.
The units of measurement, and the instruments used in urban areas are the same
as those for other environments. Therefore only those aspects that are unique to urban
areas, or are made difficult to handle because of the nature of cities, such as the choice
of site, the exposure of the instruments and the documentation of metadata are covered
in this chapter.
Timing and frequency of observations, and coding of reports should follow
appropriate standards (WMO, 1995, 1998, 2002).
For automated stations and the requirements for message coding and transmission,
quality control, maintenance (noting any special demands of the urban environment)
and calibration, the recommendations of Chapter I, Part II of the Guide (WMO 1996)
should be followed.
1.1 Definitions and concepts
The clarity of the reason for establishing an urban station is essential to its success.
Two of the most usual reasons are, the wish to represent the meteorological
environment at a place for general climatological purposes; and the wish to provide data
in support of the needs of a particular user. In both cases the spatial and temporal
scales of interest must be defined and, as outlined below, the siting of the station and
the exposure of the instruments in each case may have to be very different.
1.1.2 H
There is no more important input to the success of an urban station than an
appreciation of the concept of scale. There are three scales of interest (Oke, 1984,
Figure 1):
Figure 1 — Schematic of climatic scales and vertical layers found in urban areas. PBL –
planetary boundary layer, UBL – urban boundary layer, UCL – urban canopy
layer [modified from Oke, 1997]
(a) Microscale – every surface and object has its own microclimate on it and in its
immediate vicinity. Surface and air temperatures may vary by several degrees in
very short distances, even millimetres, and airflow can be greatly perturbed by even
small objects. Typical scales of urban microclimates relate to the dimensions of
individual buildings, trees, roads, streets, courtyards, gardens, etc. Typical scales
extend from less than one metre to hundreds of metres. The formulation of the
guidelines in Part I of the Guide specifically aims to avoid microclimatic effects. The
climate station recommendations are designed to standardize all sites, as far as
practical. Hence the use of a standard height of measurement, a single surface
cover, minimum distances to obstacles and little horizon obstruction. The aim is to
achieve climate observations that are free of extraneous microclimate signals and
hence they characterize local climates. With even more stringent standards at first
order stations they may be able to represent conditions at synoptic space and time
scales. The data may be used to assess climate trends at even larger scales.
Unless the objectives are very specialized, urban stations should also avoid
microclimate influences, but this is hard to achieve.
(b) Local scale – this is the scale that standard climate stations are designed to monitor.
It includes landscape features such as topography but excludes microscale effects.
In urban areas this translates to mean the climate of neighbourhoods with similar
types of urban development (surface cover, size and spacing of buildings, activity).
The signal is the integration of a characteristic mix of microclimatic effects arising
from the source area in the vicinity of the site. The source area is the portion of the
surface upstream that contributes the main properties of the flux or meteorological
concentration being measured (Schmid, 2002). Typical scales are one to several
(c) Mesoscale – a city influences weather and climate at the scale of the whole city,
typically tens of kilometres in extent. A single station is not able to represent this
An essential difference between the climate of urban areas and that of rural or airport
locations is that in cities the vertical exchanges of momentum, heat and moisture does
not occur at a (nearly) plane surface, but in a layer of significant thickness called the
urban canopy layer (UCL) (Figure 1). The height of the UCL is approximately equivalent
to that of the mean height of the main roughness elements (buildings and trees), zH (see
Figure 4 for parameter definitions). The microclimatic effects of individual surfaces and
obstacles persist for a short distance away from their source but are then mixed and
muted by the action of turbulent eddies. The distance before the effect is obliterated
depends on the magnitude of the effect, the wind speed and the stability (i.e. stable,
neutral or unstable). This blending occurs both in the horizontal and the vertical. As
noted, horizontal effects may persist up to a few hundred metres. In the vertical, the
effects of individual features are discernable in the roughness sublayer (RSL), that
extends from ground level to the blending height zr, where the blending action is
complete. Rule-of-thumb estimates and field measurements indicate zr can be as low as
1.5zH at densely built (closely spaced) and homogeneous sites but greater than 4zH in
low density areas (Grimmond and Oke, 1999; Rotach, 1999; Christen, 2003). An
instrument placed below zr may register microclimate anomalies but above that it ‘sees’
a blended, spatially-averaged signal that is representative of the local scale.
There is another height restriction to consider. This arises because each local scale
surface type generates an internal boundary layer, in which the flow structure and
thermodynamic properties are adapted to that surface type. The height of the layer
grows with increasing fetch (the distance upwind to the edge where the transition to a
distinctly different surface type occurs). The rate at which the internal boundary layer
grows with fetch distance depends on the roughness and the stability. In rural conditions
height:fetch ratios might vary from as small as 1:10 in unstable conditions to as large as
1:500 in stable cases and the ratio decreases as the roughness increases (Garratt,
1992; Wieringa, 1993). Urban areas tend towards neutral stability due to enhanced
thermal and mechanical turbulence associated with the heat island and their large
roughness, therefore, a height:fetch ratio of about 1:100 is considered typical. The
internal boundary layer height is taken above the displacement height zd, which is the
reference level for flow above the blending height. (For explanation of zd see Figure 4,
Section 3.5.1 and footnote 2 of Table 2)
For example, take a hypothetical densely-built district with zH of 10 m. This means
that zr is at least 15 m. If this height is chosen to be the measurement level, then the
fetch requirement over similar urban terrain is likely to be at least 0.8 km, since fetch =
100 (zr zd ), and zd is going to be about 7 m. This can be a significant site restriction
because the implication is that if the urban terrain is not similar out to at least this
distance around the station site, then observations will not be representative of the local
surface type. At less densely developed sites, where heat island and roughness effects
are less, the fetch requirements are likely to be greater.
At heights above the blending height, but within the local internal boundary layer,
measurements are within an inertial sublayer (Figure 1) where standard boundary layer
theory applies. Such theory governs the form of the mean vertical profiles of
meteorological variables (including air temperature, humidity, and wind speed) and the
behaviour of turbulent fluxes, spectra and statistics. This provides a basis for:
(a) calculation of the source area (or ‘footprint’, see below) from which the turbulent flux
or the concentration of a meteorological variable originates; hence this defines the
distance upstream for the minimum acceptable fetch; and
(b) extrapolation of a given flux or property through the inertial layer and also
downwards into the RSL (and, although it is less reliable, into the UCL). In the
inertial layer fluxes are constant with height and the mean value of meteorological
properties are invariant horizontally. Hence observations of fluxes and standard
variables possess significant utility and are able to characterize the underlying local
scale environment. Extrapolation into the RSL is less prescribed.
A sensor placed above a surface ‘sees’ only a portion of its surroundings. This is called
the ‘source area’ of the instrument which depends on its height and the characteristics
of the process transporting the surface property to the sensor. For upwelling radiation
signals (short- and longwave radiation and surface temperature viewed by an infrared
thermometer) the field-of-view of the instrument and the geometry of the underlying
surface set what is seen. By analogy sensors such as thermometers, hygrometers, gas
analyzers, anemometers ‘see’ properties such as temperature, humidity, atmospheric
gases, wind speed and direction that are carried from the surface to the sensor by
turbulent transport. A conceptual illustration of these source areas is given in Figure 2.
The source area of a downfacing radiometer with its sensing element parallel to the
ground is a circular patch with the instrument at its centre (Figure 2). The radius (r) of
the circular source area contributing to the radiometer signal at height (z1) is given by
Schmid et al. (1991):
where F is the view factor, i.e. the proportion of the measured flux at the sensor for
which that area is responsible. Depending on its field-of-view, a radiometer may see
only a limited circle, or it may extend to the horizon. In the latter case the instrument
usually has a cosine response, so that towards the horizon it becomes increasingly
difficult to define the actual source area seen. Hence the use of the view factor which
Figure 2 — Conceptual representation of source areas contributing to sensors for
radiation and turbulent fluxes or concentrations. If the sensor is a radiometer, 50
or 90% of the flux originates from the area inside the respective circle. If the
sensor is responding to a property of turbulent transport, 50 or 90% of the signal
comes from the area inside the respective ellipses. These are dynamic in the
sense that they are oriented into the wind and hence move with wind direction
and stability.
defines the area contributing a set proportion (often selected as 50, 90, 95, 99, or
99.5%) of the instrument’s signal.
The source area of a sensor that derives its signal via turbulent transport is not
symmetrically distributed around the sensor location. It is elliptical in shape and is
aligned in the upwind direction from the tower (Figure 2). If there is a wind the effect of
the surface area at the base of the mast is effectively zero, because turbulence cannot
transport the influence up to the sensor level. At some distance in the upwind direction
the source starts to affect the sensor, these rise to a peak, thereafter decaying at
greater distances (for the shape in both the x and y directions see Kljun et al., 2002;
Schmid, 2002). The distance upwind to the first surface area contributing to the signal,
to the point of peak influence, to the furthest upwind surface influencing the
measurement, and the area of the so-called ‘footprint’ vary considerably over time. They
depend on the height of measurement (larger at greater heights), surface roughness,
Radiati on source area isopleths
Turbulence source area is opleths
90% 50%
atmospheric stability (increasing from unstable to stable) and whether a turbulent flux or
a meteorological concentration is being measured (larger for the concentration) (Kljun et
al., 2002). Methods to calculate the dimensions of flux and concentration ‘footprints’ are
available (Schmid, 2002; Kljun et al., 2004).
The situation illustrated in Figure 2 is general but it applies best to instruments
placed in the inertial sublayer, well above the complications of the RSL and the complex
geometry of the three-dimensional urban surface. Within the UCL the way that effects of
radiation and turbulent source areas decay with distance has not yet been reliably
evaluated. It can be surmised that they depend on the same properties and resemble
the overall forms of those in Figure 2. However, obvious complications arise due to the
complex radiation geometry, and the blockage and channelling of flow, that are
characteristic of the UCL. Undoubtedly the immediate environment of the station is by
far the most critical and the extent of the source area on convective effects grows with
stability and the height of the sensor. The distance influencing screen-level (~1.5 m)
sensors may be a few tens of metres in neutral conditions, less when it is unstable and
perhaps more than a hundred metres when it is stable. At a height of three metres the
equivalent distances probably extend up to about three hundred metres in the stable
case. The circle of influence on a screen-level temperature or humidity sensor is
thought to have a radius of about 0.5 km typically, but this is likely to depend upon the
building density.
1.1.5 M
It follows from the preceding discussion that if the objective of an instrumented
urban site is to monitor the local scale climate near the surface, there are two viable
(a) locate the site in the UCL at a location surrounded by average or ‘typical’ conditions
for the urban terrain, and place the sensors at heights similar to those used at non-
urban sites. This assumes that the mixing induced by flow around obstacles is
sufficient to blend properties to form a UCL average at the local scale; or
(b) mount the sensors on a tall tower above the RSL and obtain blended values that can
be extrapolated down into the UCL.
In general approach (a) works best for air temperature and humidity, and approach
(b) for wind speed and direction and precipitation. For radiation the only significant
requirement is for an unobstructed horizon. Urban stations, therefore, often consist of
instruments deployed both below and above roof-level and this requires that site
assessment and description include the scales relevant to both contexts.
The magnitude of each urban scale does not agree exactly with those commonly given
in textbooks. The scales are conferred by the dimensions of the morphometric features
that make up an urban landscape. This places emphasis on the need to adequately
describe properties of urban areas that affect the atmosphere. The most important
basic features are the urban structure (dimensions of the buildings and the spaces
between them, the street widths and street spacing), the urban cover (built-up, paved,
vegetated, bare soil, water), the urban fabric (construction and natural materials) and
the urban metabolism (heat, water and pollutants due to human activity). Hence
characterization of the sites of urban climate stations needs to take account of these
descriptors, to use them in selecting potential sites, and to incorporate them in metadata
that accurately describes the setting of the station.
These four basic features of cities tend to cluster together to form characteristic
urban classes. For example, most central areas of cities have relatively tall buildings
that are densely packed together so the ground is largely covered with buildings or
paved surfaces made of durable materials such as stone, concrete, brick and asphalt
and where heat releases from furnaces, air conditioners, chimneys and vehicles are
large. Near the other end of the spectrum there are districts with low density housing of
one- or two-storey buildings of relatively light construction and considerable garden or
vegetated areas with low heat releases but perhaps large irrigation inputs.
No universally accepted scheme of urban classification for climatic purposes exists.
A good approach to the built components is that of Ellefsen (1990/91) who developed a
set of Urban Terrain Zone types. He initially differentiates according to 3 types of
building contiguity (attached (row), detached but close-set, detached and open-set).
These are further divided into a total of 17 sub-types by function, location in the city, and
building height, construction and age. Application of the scheme needs only aerial
photography, which is generally available, and it has been applied in several cities
around the world and seems to possess generality.
Ellefsen’s scheme can be used to describe urban structure for roughness, airflow,
radiation access and screening. It can be argued that it indirectly includes aspects of
urban cover, fabric and metabolism because a given structure carries with it the type of
cover, materials, and degree of human activity. Ellefsen’s scheme is less useful,
however, when built features are scarce and there are large areas of vegetation (urban
forest, low plant covers grassland, scrub, crops), bare ground (soil or rock), and water
(lakes, swamps, rivers). A simpler scheme of Urban Climate Zones (UCZ) is illustrated
in Table 1. It incorporates groups of Ellefsen’s zones, plus a measure of the structure,
zH/W, (see Table 1, Note 2) shown to be closely related to both flow, solar shading and
the heat island, and also a measure of the surface cover (%Built) that is related to the
degree of surface permeability.
The importance of UCZ, is not their absolute accuracy to describe the site but their
ability to classify areas of a settlement into districts, that are similar in their capacity to
modify the local climate, and to identify potential transitions to different urban climate
zones. Such a classification is crucial when beginning to set up an urban station so that
the spatial homogeneity criteria are met approximately for a station in the UCL or above
the RSL. In what follows it is assumed that the morphometry of the urban area, or a
portion of it, has been assessed using detailed maps, and/or aerial photographs,
satellite imagery (visible and /or thermal), planning documents or at least a visual
survey conducted from a vehicle and/or on foot. Land use maps can be helpful but it
should be appreciated that they depict the function and not necessarily the physical
form of the settlement. The task of urban description should result in a map with areas
of UCZ delineated.
Herein the UCZ as illustrated in Table 1 are used. The categories may have to be
adapted to accommodate special urban forms characteristic of some ancient cities or of
unplanned urban development found in some less-developed countries. For example,
many towns and cities in Africa and Asia do not have as large a fraction of the surface
covered by impervious materials, roads may not be paved.
Table 1: Simplified classification of distinct urban forms arranged in approximate
decreasing order of their ability to impact local climate [Oke, 2004 unpublished]
Urban Climate Zone, UCZ1
% Built
1. Intensely developed urban with
detached close-set high-rise
buildings with cladding, e.g.
downtown towers
8 > 2 > 90
2. Intensel
developed high densit
urban with 2 – 5 storey, attached
or very close-set buildings often
of brick or stone, e.g. old city core
7 1.0 – 2.5 > 85
3. Highly developed, medium
density urban with row or
detached but close-set houses,
stores & apartments e.g. urban
7 0.5 – 1.5 70 - 85
4. Highly developed, low or
medium density urban with large
low buildings & paved parking,
e.g. shopping mall, warehouses
5 0.05 –
0.2 70 - 95
5. Medium development, low
density suburban with 1 or 2
storey houses, e.g. suburban
6 0.2 – 0.6,
up to >1
with trees 35 - 65
6. Mixed use with large buildings in
open landscape, e.g. institutions
such as hospital, university,
5 0.1 – 0.5,
on trees < 40
7. Semi-rural development,
scattered houses in natural or
agricultural area, e.g. farms,
4 > 0.05,
on trees < 10
Key to image symbols: buildings; vegetation; impervious ground; pervious ground
1 A simplified set of classes that includes aspects of the schemes of Auer (1978) and Ellefsen (1990/91) plus physical measures
relating to wind, thermal and moisture controls (columns at right). Approximate correspondence between UCZ and Ellefsen’s
urban terrain zones is: 1(Dc1, Dc8), 2 (A1-A4, Dc2), 3 (A5, Dc3-5, Do2), 4 (Do1, Do4, Do5), 5 (Do3), 6 (Do6), 7 (none).
2 Effective terrain roughness according to the Davenport classification (Davenport et al., 2000); see Table 2.
3 Aspect ratio = zH/W is average height of the main roughness elements (buildings, trees) divided by their average spacing, in
the city centre this is the street canyon height/width. This measure is known to be related to flow regime types (Oke 1987)
and thermal controls (solar shading and longwave screening) (Oke, 1981). Tall trees increase this measure significantly.
4 Average proportion of ground plan covered by built features (buildings, roads, paved and other impervious areas) the rest of
the area is occupied by pervious cover (green space, water and other natural surfaces). Permeability affects the moisture
status of the ground and hence humidification and evaporative cooling potential.
2 Choosing a location and site for an urban station
2.1 Location
First, it is necessary to clearly establish the purpose of the station. If there is to be only
one station inside the urban area it must be decided if the aim is to monitor the greatest
impact of the city, or of a more representative or typical district, or if it is to characterize
a particular site (where there may be perceived to be climate problems or where future
development is planned). Areas where there is the highest probability of finding
maximum effects can be judged initially by reference to the ranked list of UCZ types in
Table 1. Similarly the likelihood that a station will be typical can be assessed using the
ideas behind Table 1 and choosing extensive areas of similar urban development for
closer investigation.
The search can be usefully refined in the case of air temperature and humidity by
conducting spatial surveys, wherein the sensor is carried on foot, or mounted on a
bicycle or a car and traversed through areas of interest. After several repetitions, cross-
sections or isoline maps may be drawn (see Figure 3), revealing where areas of thermal
or moisture anomaly or interest lie. Usually the best time to do this is a few hours after
sunset or before sunrise on nights with relatively calm airflow and cloudless skies. This
maximises the potential for the differentiation of micro- and local climate differences. It
is not advisable to conduct such surveys close to sunrise or sunset because weather
variables are changing so rapidly then that meaningful spatial comparisons are difficult.
If the station is to be part of a network to characterize spatial features of the urban
climate then a broader view is needed. This consideration should be informed by
thinking about the typical spatial form of urban climate distributions. For example, the
isolines of urban heat and moisture ‘islands’ indeed look like the contours of their
topographic namesakes (Figure 3). They have relatively sharp ‘cliffs’, often a ‘plateau’
over much of the urban area interspersed with localised ‘mounds’ and ‘basins’ of
warmth/coolness and moistness/dryness. These features are co-located with patches of
greater or lesser development such as clusters of apartments, shops, factories or parks,
open areas or water. So a decision must be made: is the aim to make a representative
sample of the UCZ diversity, or is it to faithfully reflect the spatial structure?
Figure 3 — Typical spatial pattern of isotherms in a large city at night with calm, clear
weather illustrating the heat island effect [after Oke, 1982].
In most cases the latter is too ambitious with a fixed-station network in the UCL.
This is because it will require many stations to depict the gradients near the periphery,
the plateau region, and the highs and lows of the nodes of weaker and stronger than
average urban development. If measurements are to be made from a tower, with
sensors above the RSL, the blending action produces more muted spatial patterns and
the question of distance of fetch to the nearest border between UCZs, and the urban-
rural fringe, become relevant. Whereas a distance to a change in UCZ of 0.5 to 1 km
may be acceptable inside the UCL, for a tower-mounted sensor the requirement is likely
to be more like a few kilometres fetch.
Since the aim is to monitor local climate attributable to an urban area it is necessary
to avoid extraneous microclimatic influences or other local or mesoscale climatic
phenomena that will complicate the urban record. So unless there is specific interest in
topographically-generated climate patterns, such as the effects of cold air drainage
down valleys and slopes into the urban area, or the speed-up or sheltering of winds by
hills and escarpments, or fog in river valleys or adjacent to water bodies, or
geographically-locked cloud patterns, etc., it is sensible to avoid locations subject to
such local and mesoscale effects. On the other hand if a benefit or hazard is derived
from such events, it may be relevant to design the network specifically to sample its
effects on the urban climate, such as the amelioration of an overly hot city by sea or
lake breezes.
2.2 Siting
Once a choice of UCZ type and its general location inside the urban area is made the
next step is to inspect the map, imagery and photographic evidence to narrow down
candidate locations within a UCZ. What are sought are areas of reasonably
homogeneous urban development without large patches of anomalous structure, cover
or materials. The precise definition of ‘reasonably’ however is not possible; almost every
real urban district has its own idiosyncrasies that reduce its homogeneity at some scale.
A complete list is therefore not possible but examples of what to avoid are: unusually
wet patches in an otherwise dry area, individual buildings that jut up by more than half
the average building height, a large paved parking lot in an area of irrigated gardens, a
large, concentrated heat source like a heating plant or a tunnel exhaust vent. Proximity
to transition zones between different UCZ types should be avoided, as should sites
where there are plans or the likelihood of major urban redevelopment. The level of
concern with anomalous features decreases with distance away from the site itself, as
discussed in relation to source areas.
In practice, for each candidate site a footprint should be estimated for radiation (e.g.
equation 1) and for turbulent properties. Then key surface properties such as the mean
height and density of the obstacles and characteristics of the surface cover and
materials should be documented within these footprints. Their homogeneity should then
be judged, either 'by eye' or using statistical methods. Once target areas of acceptable
homogeneity for a screen-level or high-level (above-RSL) station are selected, it is
helpful to identify potential ‘friendly’ site owners to host it. If a government agency is
seeking a site it may already own land in the area for other purposes or have good
relations with other agencies or businesses (offices, works yard, spare land, rights of
way) including schools, universities, utility facilities (electricity, telephone, pipeline) and
transport arteries (roads, railways). These are good sites, both because access may be
permitted but also because they also often possess security from vandalism and may
allow connection to electrical power. The roofs of buildings have been used often as the
site for meteorological observations. This may often have been based on the mistaken
belief that at this elevation the instrument shelter is freed from the complications of the
UCL. In fact roof tops have their own very distinctly anomalous microclimates that lead
to erroneous results. Airflow over a building creates strong perturbations in speed,
direction and gustiness that are quite unlike the flow at the same elevation away from
the building or near the ground (Figure 5). Flat-topped buildings may actually create
flows on their roofs that are counter to the main external flow and speeds vary from
extreme jetting to a near calm. Roofs are also constructed of materials that are
thermally rather extreme. In light winds and cloudless skies they can become very hot
by day and cold by night. Hence there is often a sharp gradient of air temperature near
the roof. Further, roofs are designed to be waterproof and to shed water rapidly. This
together with their openness to solar radiation and the wind makes them anomalously
dry. In general, therefore, roofs are very poor locations for air temperature, humidity,
wind and precipitation observations unless the instruments are placed on very tall
masts. They can however be good for observing incoming radiation components.
After the site is chosen it is essential that the details of the site characteristics
(metadata) are fully documented (see Section 4).
3 Exposure of instruments
3.1 Modifications to standard practice
In many respects the generally accepted standards for the exposure of meteorological
instruments set out in Part I of the Guide apply to urban sites. However, there will be
many occasions when it is impossible or makes no sense to conform. This section
recommends some principles that will assist in such circumstances, but all eventualities
cannot be anticipated. The recommendations here remain in agreement with general
objectives in Chapter 1 of Part I of the Guide (see Representativeness, Site selection,
including surface cover representative of the locality).
Many urban stations have been placed over short grass in open locations (parks,
playing fields) and as a result they are actually monitoring modified rural-type
conditions, not representative urban ones. This leads to the curious finding that some
rural-urban pairs of stations show no urban effect on temperature (Peterson, 2003).
The guiding principle for the exposure of sensors in the UCL should be to locate
them in such a manner that they monitor conditions that are representative of the
environment of the selected UCZ. In cities and towns it is inappropriate to use sites
similar to those which are standard in open rural areas. Instead it is recommended to
site urban stations over surfaces that, within a microscale radius, are representative of
the local scale urban environment. The %Built category (Table 1) is a crude guide to the
recommended underlying surface. The most obvious requirement that cannot be met at
many urban sites is the distance from obstacles— ‘the site should be well away from
trees, buildings, walls or other obstructions’ (Chapter 1, Part I of the Guide on siting and
exposure) Rather, it is recommended that the urban station be centred in an open
space where the surrounding aspect ratio (zH/W) is approximately representative of the
locality. When installing instruments at urban sites it is especially important to use
shielded cables because of the ubiquity of power lines and other sources of electrical
noise at such locations.
3.2 Temperature
3.2.1 Air temperature
The sensors in general use to measure air temperature, including their accuracy and
response characteristics, are appropriate in urban areas. Careful attention to radiation
shielding and ventilation is especially recommended. In the UCL a sensor assembly
may be relatively close to warm surfaces such as a sunlit wall, road, or a vehicle with a
hot engine, or it may receive reflected heat from glassed surfaces. Therefore shields
should be of a type to block radiation effectively. Similarly, an assembly placed in the
lower UCL may be too well sheltered, so forced ventilation of the sensor is
recommended. If a network includes a mixture of sensor assemblies with/without
shields and ventilation this may contribute to inter-site differences, so practices should
be uniform.
The surface over which air temperature is measured and the exposure of the sensor
assembly should follow the recommendations given above in the previous section, i.e.
the surface should be typical of the UCZ and the thermometer screen or shield should
be centred in a space with approximately average zH/W. In very densely built-up UCZ
this might mean it is located only 5 to 10 m from buildings that are 20 to 30 m high. If
the site is a street canyon, zH/W only applies to the cross-section normal to the axis of
the street. The orientation of the street axis may also be relevant because of systematic
sun-shade patterns. If continuous monitoring is planned, north-south oriented streets
are favoured over east-west ones because there is less phase distortion, although
daytime course of temperature may be rather peaked. At non-urban stations the screen
height is recommended to be between 1.25 and 2 m above ground level. Whilst this is
also acceptable for urban sites it may be better to relax this requirement to allow greater
heights. This should not lead to significant error in most cases, especially in densely
built-up areas, because observations in canyons show very slight air temperature
gradients through most of the UCL, as long as location is more than 1 m from a surface
(Nakamura and Oke, 1988). Measurements at heights of 3 or 5 m are little different from
those at the standard height, have slightly greater source areas and place the sensor
beyond the easy reach of damage or the path of vehicles. It also ensures greater
dilution of vehicle exhaust heat and reduces contamination from dust. Air temperatures
measured above the UCL, using sensors mounted on a tower, are influenced by air
exchanged with the UCL plus the effects of the roofs. Roofs are much more variable
thermally than most surfaces within the UCL. Most roofs are designed to insulate and
hence to minimize heat exchange with the interior of the building. As a result roof
surface temperatures often become very hot by day whereas the partially shaded and
better conducting canyon walls and floor are cooler. At night circumstances are
reversed with the roofs being relatively cold and canyon surfaces warmer as they
release their daytime heat uptake. There may also be complications due to release of
heat from roof exhaust vents. Therefore, whereas there is little variation of temperature
with height in the UCL, there is a discontinuity near roof-level both horizontally and
vertically. Hence if a meaningful spatial average is sought then sensors should be well
above mean roof-level, > 1.5zH if possible, so that mixing of roof and canyon air is
accomplished. Given air temperature data from an elevated sensor it is difficult to
extrapolate it down towards screen-level because currently no standard methods are
available. Similarly there is no simple, general scheme for extrapolating air
temperatures horizontally inside the UCL. Statistical models work but they require a
large archive of existing observations over a dense network, that is not usually
3.2.2 Surface temperature
Surface temperature is not commonly measured at urban stations but it can be a very
useful variable to use as input in models to calculate fluxes. A representative surface
temperature requires averaging an adequate sample of the many surfaces, vertical as
well as horizontal, comprising an urban area. This is only possible using infrared remote
sensing either from a scanner mounted on an aircraft or satellite, or a downward-facing
pyrgeometer, or one or more radiation thermometers of which the combined field-of-
view covers a representative sample of the urban district. Hence accurate results
require that the target is sampled appropriately and its average emissivity is known.
3.2.3 Soil and road temperature
It is desirable to measure soil temperature in urban areas. The heat island effect
extends down beneath the city and this may be of significance to engineering design for
water pipes or road construction. In practice measurement of this variable may be
difficult at more heavily developed urban sites. Bare ground may not be available, the
soil profile is often highly disturbed and at depth there may be obstructions or
anomalously warm or cool artefacts (e.g. empty, full, leaky water pipes, sewers, heat
conduits). In urban areas the measurement of grass minimum temperature has almost
no practical utility. Temperature sensors are often embedded in road pavement,
especially in areas subject to freezing. They are usually part of a monitoring station for
highway weather. It is often helpful to have sensors beneath both the tire track and the
centre of the lane.
3.3 Atmospheric pressure
At the scale of urban areas it will probably not be necessary to monitor atmospheric
pressure if there is already a synoptic station in the region. If pressure sensors are
included the recommendations of Chapter 3, Part I of the Guide, apply. In rooms and
elsewhere in the vicinity of buildings there is the probability of pressure ‘pumping’ due to
gusts, also interior-exterior pressure differences may exist if the sensor is located in an
air conditioned room. Both difficulties can be alleviated if a static pressure head is
installed (see Part I, Section 3.8 of the Guide).
3.4 Humidity
The instruments normally used for humidity (Part I, Chapter 4 of the Guide) are
applicable to the case of urban areas. The guidelines given in Section 3.2.1 for the
siting and exposure of temperature sensors in the UCL, and above the RSL, apply
equally to humidity sensors.
Urban environments are notoriously dirty (dust, oils, pollutants). Several
hygrometers are subject to degradation or require increased maintenance in urban
environments. Hence if psychrometric methods are used the wet-bulb sleeve has to be
replaced more frequently than normal and close attention is necessary to ensure the
distilled water remains uncontaminated. The hair strands of a hair hygrometer can be
destroyed by polluted urban air, hence their use is not recommended for extended
periods. The mirror of dew-point hygrometers and the windows of ultraviolet and
infrared absorption hygrometers need to be cleaned frequently. Some instruments
degrade sufficiently that the sensors have to be completely replaced fairly regularly.
Because of shelter from wind in the UCL forced ventilation at the rate recommended in
Part I Section 4.2 of the Guide is essential, as is the provision of shielding from
extraneous sources of solar and longwave radiation.
3.5 Wind speed and direction
The measurement of wind speed and direction is highly sensitive to flow distortion by
obstacles. Obstacles create alterations to the average wind flow and turbulence. Such
effects apply at all scales of concern, including the effects of local relief due to hills,
valleys and cliffs, sharp changes in roughness or in the effective surface elevation (zd,
see below), perturbation of flow around clumps of trees and buildings, individual trees
and buildings and even disturbance induced by the physical bulk of the tower or
mounting arm to which the instruments are attached.
3.5.1 Mean wind profile
However, if a site is on reasonably level ground, has sufficient fetch downstream of
major changes of roughness and is in a single UCZ without anomalously tall buildings,
then a mean wind profile such as that in Figure 4 should exist. The mean is both spatial
and temporal. Within the UCL no one site can be expected to possess such a profile.
Individual locations experience highly variable speed and direction shifts as the
airstream interacts with individual building arrangements, streets, courtyards and trees.
In street canyons the shape of the profile is different for along-canyon, versus across-
canyon flow (Christen et al. 2002) and depends on position across and along the street
(DePaul and Shieh, 1986). Wind speed gradients in the UCL are small until quite close
to the surface. As a first approximation the profile in the UCL can be described by an
exponential form (Britter and Hanna, 2003) merging with the log profile near roof-level
(Figure 4).
In the inertial sublayer Monin-Obukhov similarity theory applies, including the
logarithmic law:
where u
is the friction velocity, k is von Karman’s constant (0.40), z0 is the surface
roughness length, zd is the zero-plane displacement height (Figure 4), L is the Obukhov
stability length (= -u
v)QH]), where g is the gravitational acceleration,
v the virtual
potential temperature and QH the turbulent sensible heat flux), and ψM is a
dimensionless function that accounts for the change in curvature of the wind profile
away from the neutral profile with greater stability or instability1. In the neutral case
(typically with strong winds and cloud) when ψM is unity, equation (2) reduces to:
1 For more on L and the form of the ψM function, see a standard micrometeorology text, e.g. Stull, 1988;
Garratt, 1992 or Arya, 2001. Note that u
and QH should be evaluated in the inertial layer above the RSL.
]z/)zz){ln[(k/u(u M0d*z Ψ+=
]z/)zzln[()k/u(u 0d*z
The wind profile parameters can be measured using a vertical array of anemometers, or
measurements of momentum flux or gustiness from fast-response anemometry in the
inertial layer, but estimates vary with wind direction and are sensitive to errors
(Wieringa, 1996; Verkaik, 2000). Methods to parameterize the wind profile parameters
z0 and zd for urban terrain are also available (for reviews see Grimmond and Oke, 1999;
Britter and Hanna, 2003). The simplest involve general descriptions of the land use and
obstacles (Davenport et al., 2000, see Tables 1 and 2; Grimmond and Oke, 1999), or a
detailed description of the roughness element heights and their spacing from either a
Geographic Information System of the building and street dimensions, or maps and
aerial oblique photographs, or airborne/satellite imagery and the application of one of
several empirical formulae (for recommendations see Grimmond and Oke, 1999).
It is important to incorporate the displacement height zd into urban wind profile
assessments. Effectively this is equivalent to setting a base for the logarithmic wind
profile that recognizes the physical bulk of the urban canopy. It is like setting a new
‘ground surface’ aloft, where the mean momentum sink for the flow is located (Figure 4).
Depending on the building and tree density this could set the base of the profile at a
Table 2 : Davenport classification of effective terrain roughness (revised 2000)1.
Class z0 (m) Landscape description
4 “ Roughly open” 0.10 Moderately open country with occasional obstacles (e.g. isolated low buildings or trees)
at relative horizontal separations of at least 20 obstacle heights.
5 “Rough” 0.25 Scattered obstacles (buildings) at relative distances of 8 to 12 obstacle heights for
low solid objects (e.g. buildings). (Analysis may need zd)2
6 “Very rough” 0.5 Area moderately covered by low buildings at relative separations of 3 to 7 obstacle
heights and no high trees. (Analysis requires zd)2
7 “Skimming” 1.0 Densely built-up area without much building height variation. (Analysis requires zd)2
8 “Chaotic” 2.0 City centres with mix of low and high-rise buildings. (Analysis by wind tunnel advised)
1 Abridged version (urban roughness only) of Davenport et al., (2000); for classes 1 to 3 and for rural
classes 4 to 8 see Part I, Chapter 5, Annex and Aguilar et al. (2003).
2 First order values of zd are given as fractions of average obstacle height, viz: 0.5 zH, 0.6 zH and 0.7 zH
for Davenport Categories 5, 6 and 7, respectively.
Figure 4 Generalized mean (spatial and temporal) wind velocity )U( profile in a
densely developed urban area including the location of sublayers of the surface
layer. The measures on the height scale are the mean height of the roughness
elements (zH), the roughness sublayer (zr, or the blending height), the roughness
length (z0) and zero-plane displacement length (zd). Dashed line – profile
extrapolated from the inertial sublayer; solid line - actual profile.
height between 0.5 and 0.8zH (Grimmond and Oke, 1999), hence failure to incorporate it
in calculations causes large errors. First estimates can be made using the fractions of zH
given in the footnote of Table 2.
3.5.2 Height of measurement and exposure
The choice of height at which wind measurements should be made in urban areas is
a challenge, but if some basic principles are applied meaningful results can be attained.
Poor placement of wind sensors in cities is the source of considerable wasted resources
and effort and leads to potentially erroneous calculations of pollutant dispersion. Of
course this is even a source of difficulty in open terrain due to obstacles and
topographic effects. This is the reason why the standard height for rural wind
observations is set at 10 m above ground, not at screen-level, and why there the
Mean horizontal velocity, u
anemometer should not be at closer horizontal distance from obstructions than ten
obstacle heights (Part I, Chapter 5.9.2 of the Guide). In typical urban districts it is not
possible to find such locations, e.g. in a UCZ with 10 m high buildings and trees it would
need a patch that is at least 100 m in radius. If such a site exists it is almost certainly
not representative of the zone. It has already been noted that the roughness sublayer,
in which the effects of individual roughness elements persist, extends to a height of
about 1.5zH in a densely built-up area and perhaps higher in less densely developed
sites. Hence in the example district the minimum acceptable anemometer height is at
least 15 m, not the standard 10 m. When building heights are much taller, an
anemometer at the standard 10 m height would be well down in the UCL, and given the
heterogeneity of urban form and therefore of wind structure, there is little merit in
placing a wind sensor beneath, or even at about, roof-level.
It is well known from wind tunnel and field observations that flow over an isolated
solid obstacle, like a tall building, is greatly perturbed both immediately over and around
it. These include modifications to the streamlines, the presence of recirculation zones
on the roof and in the so-called ‘bubble’ or cavity behind it, and wake effects that persist
in the downstream flow for tens of building height multiples that affect a large part of the
neighbourhood (Figure 5 ).
Figure 5. 2-D flow around a building with flow normal to the upwind face (a) stream
lines and flow zones; A -undisturbed, B - displacement, C - cavity, D – wake (after
Halitsky, 1963), and (b) flow, and vortex structures (simplified after Hunt et al.,
There are many examples of poorly exposed anemometer-vane systems in cities.
The data registered by such instruments are erroneous, misleading, potentially harmful
if used to obtain wind input for wind load or dispersion applications, and wasteful of
resources. The inappropriateness of placing anemometers and vanes on short masts on
the top of buildings cannot be over-emphasized. Speeds and directions vary hugely in
short distances, both horizontally and vertically. Results from instruments deployed in
this manner bear little resemblance to the general flow and are entirely dependent on
the specific features of the building itself, the mast location on the structure, and the
angle-of-attack of the flow to the building. The circulating and vortex flows seen in
Figure 5 mean that if the mast is placed ahead of, on top of, or in the cavity zone behind
a building, direction measurements could well be counter to those prevailing in the flow
outside the influence of the building’s own wind climate (i.e. in zone A of Figure 5a), and
speeds are highly variable. To get outside the perturbed zone wind instruments must be
mounted at a considerable height. For example, it has been proposed that such sensors
should be at a height greater than the maximum horizontal dimension of the major roof
(Wieringa, 1996). This implies an expensive mast system, perhaps with guys that
subtend a large area and perhaps difficulties in obtaining permission to install.
Nevertheless, this is the only acceptable approach if meaningful data are to be
Faced with such realities, sensors should be mounted so that their signal is not
overly compromised by their support structure. The following recommendations are
(a) in urban districts with low element height and density (UCZ 6 and 7) it may be
possible to use a site where the ‘open country’ standard exposure guidelines can be
met. To use the 10 m height the closest obstacles should be at least 10 times their
height distant from the anemometer and not be more than about 6 m tall on
(b) in more densely built-up districts, with relatively uniform element height and density
(buildings and trees), wind speed and direction measurements should be made with
the anemometer mounted on a mast of open construction at 10 m or 1.5 times the
mean height of the elements, whichever is the greater ;
(c) in urban districts with scattered tall buildings the recommendations are as in (b) but
with special concern to avoid the wake zone of the tall structures; and
(d) it is not recommended to measure wind speed or direction in densely-built areas
with multiple high-rise structures unless a very tall tower is used.
Anemometers on towers with open construction should be mounted on booms
(cross-arms) that are long enough to keep the sensors at least two, better three, tower
diameters distance from the side of the mast (Gill et al., 1967). Sensors should be
mounted so that the least frequent flow direction passes through the tower. If this is not
possible or if the tower construction is not very open, two or three booms with duplicate
sensors may have to be installed to avoid wake effects and upwind stagnation produced
by the tower itself.
If anemometer masts are to be mounted on tall or isolated buildings the effects of
the dimensions of that structure on the flow must be considered (see Part II, Chapter
5.3.3 of the Guide). This is likely to require analysis using wind tunnel, water flume or
computational fluid dynamics models specifically tailored to the building in question, and
including its surrounding terrain and structures.
The object is to ensure that all wind measurements are made at heights where they
are representative of the upstream surface roughness at the local scale and are as free
as possible of confounding influences from micro- or local scale surface anomalies.
Hence the emphasis on gaining accurate measurements at whatever height is
necessary to reduce error rather than measuring at a standard height. This may require
splitting the wind site from the location of the other measurement systems. It may also
result in wind observations at several different heights in the same settlement. That will
necessitate extrapolation of the measured values to a common height, if spatial
differences are sought or if the data are to form input to a mesoscale model. Such
extrapolation is easily achieved by applying the logarithmic profile (equation 2) to two
where zref is the chosen reference height, z1 is the height of the site anemometer and z0
is the roughness length of the UCZ. In urban terrain it is correct to define the reference
)z/zln(/)z/zln(u/u 0ref01ref1
height to include the zero-plane displacement height, i.e. both z1 and zref have the form
(zx – zd), where the subscript x stands for ‘1’ or ‘ref’. A suitable reference height may be
50 m above displacement height.
Other exposure corrections for flow distortion, topography, and roughness effects
can be made as recommended in Chapter 5, Part I of the Guide (see exposure
correction). It may well be that suitable wind observations cannot be arranged for a
given urban site. In that case it is still possible to calculate the wind at the reference
height using wind observations at another urban station or the airport using the
‘logarithmic transformation’ model of Wieringa (1986):
where the subscripts A and B refer to the site of interest where winds are wanted and
the site where standard wind measurements are available, respectively. The blending
height zr should here either be taken as 4zH (Section 1.1 ) or be given a standard value
of 60 m; the method is not very sensitive to this term. Again, if either site has dense, tall
roughness elements, the corresponding height scale should incorporate zd.
3.5.3 Wind sensor considerations
Instruments used to measure wind speed, direction, gustiness and other characteristics
of the flow in non-urban environments are applicable to urban areas. In cities wind
direction should always be measured, as well as speed, in order to allow azimuth-
dependent corrections of tower influence to be made. If mechanical cup anemometers
are used, the dirtiness of the atmosphere requires an increased frequency of
maintenance and close attention to bearings and corrosion . If measurements are made
in the UCL gustiness may increase the problem of cup over-speeding and too much
shelter may cause anemometers to operate near or below their threshold minimum
speed. This must be addressed through heightened maintenance and perhaps the
choice of fast-response anemometers, propeller-type anemometers or sonic
anemometers. Propeller anemometers are less prone to over-speeding, and sonic
anemometers, having no moving parts are practically maintenance free. However, they
are expensive, need sophisticated electronic logging and processing and not all models
work when it is raining.
=)z/zln()z/zln( )z/zln()z/zln(
uu A0rB0B
3.6 Precipitation
The instruments and methods for the measurement of precipitation given in Chapter 6,
Part I of the Guide are also relevant to urban areas. The measurement of precipitation
as rain or snow is always susceptible to errors associated with the exposure of the
gauge, especially the wind field in its vicinity. Given the urban context and the highly
variable wind field in the UCL and the RSL, concerns arise from four main sources:
(a) the interception of precipitation during its trajectory to the ground by nearby
collecting surfaces such as trees and buildings;
(b) hard surfaces near the gauge may cause splash-in to the gauge, and over-hanging
objects may drip into the gauge;
(c) the spatial complexity of the wind field around obstacles in the UCL causes very
localised concentration or absence of rain- or snow-bearing airflow; and
(d) the gustiness of the wind in combination with the physical presence of the gauge
itself causes anomalous turbulence around it that leads to under- or over-catch.
In open country standard exposure requires that obstacles should be no closer than
two times their height. In some ways this is less restrictive than for temperature,
humidity or wind. However, in the UCL the turbulent activity created by flow around
sharp-edged buildings is more severe than that around natural obstacles and may last
for greater distances in their wake. Again, the highly variable wind speeds and
directions encountered on the roof of a building make it a site to be avoided.
On the other hand, unlike temperature, humidity and wind, the object of precipitation
measurement is often not for the analysis of local effects, except perhaps in the case of
rainfall rate. Some urban effects on precipitation may be initiated at the local scale (e.g.
by a major industrial facility) but may not show up until well downwind of the city.
Distinct patterns within an urban area are more likely to be due to relief or coastal
topographic effects.
Selecting an extensive open site in the city, where normal exposure standards can
be met, may be acceptable but it almost certainly will mean that the gauge will not be
co-located with the air temperature, humidity and wind sensors. While the latter sensors
need to be representative of the local scale urban structure, cover, fabric and
metabolism of a specific UCZ, precipitation does not have to be.
However, the local environment of the gauge is important if the station is to be used
to investigate intra-urban patterns of precipitation type. For example, the urban heat
island has an influence on the survival of different forms of precipitation, e.g. snow or
sleet at cloud-base may melt in the warmer urban atmosphere and end up as rain at the
ground. This may mean rural and suburban sites get snow when the city centre
registers rain.
It is recommended that precipitation gauges in urban areas:
(a) be located in open sites within the city where the standard exposure criteria can be
met (e.g. playing fields, open parkland with a low density of trees, an urban airport);
(b) be located in conjunction with the wind instruments if a representative exposure for
them is found. At other than low density built-up sites this probably means mounting
the gauge above roof-level on a mast. This means the gauge will be subject to
greater than normal wind speed and hence the error of estimation will be greater
than near the surface, and the gauge output will have to be corrected. Such
correction is feasible if wind is measured on the same mast. It also means that
automatic recording is favoured and the gauge must be checked regularly to make
sure it is level and that the orifice is free of debris;
(c) not be located on the roofs of buildings unless they are exposed at sufficient height
to avoid the wind envelope of the building; and that
(d) the measurement of depth of snowfall should be made at an open site or, if made at
developed sites, a large spatial sample should be obtained to account for the
inevitable drifting around obstacles. Such sampling should include streets oriented
in different directions.
Urban hydrologists are interested in rainfall rates, especially during major storm
events. Hence tipping bucket rain gauges or weighing gauges have utility. Measurement
of rain- and snowfall in urban areas stands to benefit from the development of
techniques such as optical rain gauges and radar.
Dew, ice and fog precipitation also occurs in cities and can be of significance to the
water budget, especially for certain surfaces, and may be relevant to applications such
as plant disease, insect activity, road safety and as a supplementary source of water
resources. The methods outlined in Chapter 6, Part I of the Guide are appropriate for
urban sites.
3.7 Radiation
There is a paucity of radiation flux measurements conducted in urban areas, currently.
For example, there are almost none in the Global Energy Balance Archive (GEBA) of
the World Climate Programme or in the Atmospheric Radiation Measurement (ARM)
Programme of the US Department of Energy. Radiation measurement sites are often
located in rural or remote locations specifically to avoid the aerosol and gaseous
pollutants of cities that ‘contaminate’ their records. Even when a station has the name of
a city, the metadata usually reveal they are actually located well outside the urban
limits. If they are in the built-up area only incoming solar (global) radiation is likely to be
measured, neither incoming longwave nor any fluxes with outgoing components are
monitored. It is mostly short-term experimental projects focussing specifically on urban
effects that measure both the receipt and loss of radiation in cities. All short- and
longwave fluxes are impacted by the special properties of the atmosphere and surface
of cities, and the same is true for the net all-wave radiation balance that effectively
drives the urban energy balance (Oke, 1988).
All of the instruments, calibrations, corrections, and most of the field methods
outlined in relation to the measurement of radiation at open country sites in Chapter 7,
Part I of the Guide, apply to the case of urban areas. Only differences, or specifically
urban needs or difficulties, are mentioned here.
3.7.1 Incoming fluxes
Incoming solar radiation is such a fundamental forcing variable of urban climate that its
measurement should be given a high priority when a station is established or upgraded.
Knowledge of this term together with standard observations of air temperature, humidity
and wind speed, plus simple measures of the site structure and cover, allows a
meteorological pre-processor scheme (i.e. methods and algorithms used to convert
standard observation fields into the variables required as input by models, but not
measured; e.g. fluxes, stability, mixing height, dispersion coefficients, etc.) such as OLM
(Berkowicz and Prahm, 1982; Olesen and Brown, 1992), HPDM (Hanna and Chang,
1992) or LUMPS (Grimmond and Oke, 2002) to be used to calculate much more
sophisticated measures such as atmospheric stability, turbulent statistics, the fluxes of
momentum, heat and water vapour. These in turn make it possible to predict the mixing
height and pollutant dispersion (COST 710, 1998; COST 715, 2001). Further, solar
radiation can be used as a surrogate for daytime cloud activity and is the basis of
applications in solar energy, daylight levels in buildings, pedestrian comfort, legislated
rights to solar exposure and many other fields. At automatic stations the addition of
solar radiation measurement is simple and relatively inexpensive.
The exposure requirements for pyranometers and other incoming flux sensors are
relatively easily met in cities. The fundamental needs are for the sensor to be level, free
of vibration, free of any obstruction above the plane of the sensing element including
both fixed features (buildings, masts, trees, and hills) and ephemeral ones (clouds
generated from exhaust vents or pollutant plumes). So a high, stable and accessible
platform like the roof of a tall building is often ideal. It may be impossible to avoid short-
term obstruction of direct-beam solar radiation impinging on an up-facing radiometer by
masts, antennae, flag poles and similar structures. If this occurs the location of the
obstruction and the typical duration of its impact on the sensing element should be fully
documented (see Section 4). Methods to correct for such interference are mentioned in
Chapter 7, Part I of the Guide. It is also important to ensure there is not excessive
reflection from very light-coloured walls that may extend above the local horizon. It is
essential to clean the upper domes at regular intervals. In heavily polluted environments
this may mean daily.
Other incoming radiation fluxes are also desirable but their inclusion depends on the
nature of the city, the potential applications and the cost of the sensors. The fluxes (and
their instruments) are: incoming direct beam solar (pyrheliometer), diffuse sky solar
(pyranometer fitted with a shade ring or a shade disk on an equatorial mount), solar
ultraviolet (broadband and narrowband sensors, and spectrometers) and longwave
radiation (pyrgeometer). All have useful applied value: beam (pollution extinction
coefficients), diffuse (interior daylighting, solar panels), ultraviolet (depletion by ozone
and damage to humans, plants and materials), longwave (monitoring nocturnal cloud
and enhancement of the flux by pollutants and the heat island).
3.7.2 Outgoing and net fluxes
The reflection of solar radiation and the emission and reflection of longwave radiation
from the underlying surface, and the net result of short-, long- and all-wave radiant
fluxes are currently seldom monitored at urban stations. This means that significant
properties of the urban climate system remain unexplored. The albedo, that decides if
solar radiation is absorbed by the fabric or is lost back to the atmosphere and Space,
will remain unknown. The opportunity to invert the Stefan-Boltzmann relation and solve
for the surface radiant temperature is lost. The critical net radiation that supports
warming/cooling of the fabric, and the exchanges of water and heat between the
surface and the urban boundary layer is missing. Of these, net all-wave radiation data is
the greatest lack. Results from a well-maintained net radiometer are invaluable to drive
a pre-processor scheme and as a surrogate measure of cloud.
The main difficulty in measuring outgoing radiation terms accurately is the exposure
of the down-facing radiometer to view a representative area of the underlying urban
surface. The radiative source area (Equation 1, Figure 2), should ideally ‘see’ a
representative sample of the main surfaces contributing to the flux. In the standard
exposure cases, defined in the relevant sections of Chapter 7, Part I of the Guide, a
sensor height of 2 m is deemed appropriate over a short grass surface. At that height
90% of the flux originates from a circle of diameter 12 m on the surface. Clearly a much
greater height is necessary over an urban area in order to sample an area that contains
a sufficient population of surface facets to be representative of that UCZ. Considering
the case of a radiometer at 20 m (at the top of a 10 m high mast mounted on a 10 m
high building) in a densely developed district, the 90% source area has a diameter of
120 m at ground level. This might seem sufficient to ‘see’ several buildings and roads,
but it must also be considered that the system is three-dimensional, not quasi-flat like
the grass. At the level of the roofs in the example the source area is now only 60 m in
diameter, and relatively few buildings may be viewed.
The question becomes whether the sensor can ‘see’ an appropriate mix of
climatically active surfaces? This means not only does it see an adequate set of plan-
view surface types, but also is it sampling appropriate fractions of roof, wall, and ground
surfaces, including the correct fractions of each that are in sun or shade? This is a non-
trivial task that depends on the surface structure and the positions of both the sensor
and the Sun in space above the array. Soux et al., 2004 developed a model to calculate
these fractions for relatively simple urban-like geometric arrays, but more work is
needed before guidelines specific to individual UCZ types are available. It seems likely
that the sensor height has to be greater than that for turbulence measurements. The
non-linear nature of radiative source area effects is clear from Equation (1) (refer Figure
2). The greater weighting of surfaces closer to the mast location means the immediate
surroundings are most significant. In the previous example of the radiometer at 20 m on
a 10 m building, 50% of the signal at the roof-level comes from a circle of only 20 m
diameter (perhaps only a single building). If the roof of that building, or other surface on
which the mast is mounted, has anomalous radiative properties (albedo, emissivity or
temperature) it disproportionately affects the flux, which is supposed to be
representative of a larger area. Hence roofs with large areas of glass or metal, or with
an unusually dark or light colour, or those designed to hold standing water, should be
Problems associated with down-facing radiometers at large heights include (a) the
difficulty of ensuring the plane of the sensing element is level, (b) ensuring that at large
zenith angles the sensing element does not ‘see’ direct beam solar radiation or
incoming longwave from the sky, (c) considering whether there is need to correct results
to account for radiative flux divergence in the air layer between the instrument height
and the surface of interest. To eliminate extraneous solar or longwave radiation near the
horizon it may be necessary to install a narrow collar that restricts the field-of-view to a
few degrees less than 2π. This will necessitate a small correction to readings to account
for the missing diffuse solar input (see Chapter 7, Part I, Annex 7E of the Guide for the
case of a shade band) or the extra longwave input from the collar.
Inverted sensors may be subject to error because their back is exposed to solar
heating. This should be avoided by use of some form of shielding and insulation.
Maintaining the cleanliness of the instrument domes and wiping away deposits of water
or ice may also be more difficult. Inability to observe the rate and effectiveness of
ventilation of instruments at height means that the choice of instruments that do not
need aspiration is preferred. The ability to lower the mast to attend to cleaning,
replacement of desiccant or polyethylene domes and levelling is an advantage.
It is recommended that:
(a) down-facing radiometers be placed at a height at least as large as a turbulence
sensor (i.e. a minimum of 2 zH is advisable) and preferably higher;
(b) the radiative properties of the immediate surroundings of the radiation mast are
representative of the urban district of interest.
3.8 Sunshine duration
The polluted atmospheres of urban areas cause a reduction of sunshine hours
compared with their surroundings or pre-urban values (Landsberg, 1981). The
instruments, methods and exposure recommendations given in Chapter 8, Part I of the
Guide are applicable to the case of an urban station.
3.9 Visibility and meteorological optical range
The effects of urban areas upon visibility and meteorological optical range (MOR) are
complex because while pollutants tend to reduce visibility and MOR through their impact
on the attenuation of light and the enhancement of certain types of fog, urban heat and
humidity island effects often act to diminish the frequency and severity of fog and low
cloud. There is considerable practical value in having urban visibility and MOR
information to fields such as aviation, road and river transport and optical
communications, and thus to include these observations at urban stations.
Visual perception of visibility is hampered in cities. While there are many objects
and lights that can serve as range targets, it may be difficult to obtain a sufficiently
uninterrupted line-of-sight at the recommended height of 1.5 m. Use of a raised platform
or the upper level of buildings is considered non-standard and not recommended.
Observations near roof-level may also be affected by scintillation from heated roofs, or
the ‘steaming’ of water from wet roofs during drying, or pollutants and water clouds
released from chimneys and other vents.
Instruments to measure MOR, such as transmissometers and scatter meters
generally work well in urban areas. They require relatively short paths and if the optics
are maintained in a clean state will give good results. Naturally the instrument must be
exposed at a location that is representative of the atmosphere in the vicinity but the
requirements are no more stringent than for others placed in the UCL. It may be that for
certain applications knowledge of the height variation of MOR is valuable, e.g. the
position of the fog top or the cloud base.
3.10 Evaporation and other fluxes
Urban development usually leads to a reduction of evaporation primarily due to sealing
the surface by built features and the removal of vegetation, although in some naturally
dry regions it is possible that an increase may occur if water is imported from elsewhere
and used to irrigate urban vegetation.
Very few evaporation measurement stations exist in urban areas. This is
understandable because it is almost impossible to interpret evaporation measurements
conducted in the UCL using atmometers, evaporation pans or lysimeters. As detailed in
Chapter 10, Part I of the Guide, such measurements must be at a site that is
representative of the area; not closer to obstacles than 5 times their height, or 10 times
if they are clustered; not placed on concrete or asphalt; not unduly shaded; and free of
hard surfaces that may cause splash-in. In addition to these concerns the surfaces of
these instruments are assumed to act as surrogates for vegetation or open water
systems. Such surfaces are probably not representative of the surroundings at an urban
site. Hence, they are in receipt of micro-advection that is likely to force evaporation at
unrealistically high rates.
Consider the case of an evaporation pan installed over a long period, that starts out
at a semi-arid site that converts to irrigated agricultural uses, then is encroached upon
by suburban development and later is in the core of a heavily developed urban area. Its
record of evaporation starts out as very high, because it is an open water surface in hot,
dry surroundings, so although actual evaporation in the area is very low, the loss from
the pan is forced by advection to be large. The introduction of irrigation makes
conditions cooler and more humid so the pan readings drop, but actual evaporation is
large. Urban development largely reverses the environmental changes, and it reduces
the wind speed near the ground, so pan losses increase but the actual evaporation
probably drops. Hence throughout this sequence pan evaporation and actual
evaporation are probably in anti-phase. During the agricultural period a pan coefficient
might have been applied to convert the pan readings to those typical of short grass or
crops. No such coefficients are available to convert pan to urban evaporation, even if
the readings are not corrupted by the complexity of the UCL environment. In summary,
the use of standard evaporation instruments in the UCL is not recommended.
The dimensions and heterogeneity of urban areas renders the use of full-scale
lysimeters impractical (e.g. the requirement to be not less than 100 to 150 m from a
change in surroundings). Micro-lysimeters can give the evaporation from individual
surfaces, but they are still specific to their surroundings. Such lysimeters need careful
attention, including renewing the soil monolith to prevent drying out, and are not suitable
for routine long-term observations.
Spatially-averaged evaporation and other turbulent fluxes (momentum, sensible
heat, carbon dioxide) information can be obtained from observations above the RSL.
Several of these fluxes are of greater practical interest in urban areas than in many
open country areas. For example, the vertical flux of horizontal momentum, and the
integral wind statistics and spectra are needed in questions of wind loading on
structures and the dispersion of air pollutants. The sensible heat flux is an essential
input to calculation of atmospheric stability (e.g. the flux Richardson Number and the
Obukhov length) and the depth of the urban mixing layer. Fast response eddy
covariance or standard deviation methods are recommended, rather than profile
gradient methods. Appropriate instruments include sonic anemometers, infrared
hygrometers and gas analyzers and scintillometers. The sensors should be exposed
like wind sensors: above the RSL but below the internal boundary layer of the UCZ of
interest. Again, such measurements rely on the flux ‘footprint’ being large enough to be
representative of the local area of interest.
If such flux measurements are beyond the financial and technical resources
available, a meteorological pre-processor scheme such as OLM, HPDM or LUMPS (see
Section 3.7) can be an acceptable method to obtain aerially representative values of
urban evaporation and heat flux. Such schemes only require spatially representative
observations of incoming solar radiation, air temperature, humidity and wind speed, and
general estimates of average surface properties such as albedo, emissivity, roughness
length and the fractions of the urban district that are vegetated or built-up or irrigated.
Clearly the wind speed observations must conform to the recommendations in Section
3.5. Ideally the air temperature and humidity should also be observed above the RSL,
but if only UCL values are available this is usually acceptable because such schemes
are not very sensitive to these variables.
3.11 Soil moisture
Knowledge of urban soil moisture can be useful, e.g. to gardeners and in the calculation
of evaporation. Its thermal significance in urban landscapes is evidenced by the
remarkably distinct patterns in remotely-sensed thermal imagery. By day any patch with
active vegetation or irrigated land is noticeably cooler than built, paved or bare land.
However, the task of sampling to obtain representative values of soil moisture is
Some of the challenges presented include the fact that large fractions of the urban
surface are completely sealed over by paved and built features; much of the exposed
soil has been highly disturbed in the past during construction activity or abandonment of
old urban uses; the ‘soil’ may actually be largely formed from the rubble of old buildings
and paving materials or have been imported as soil or fill material from distant sites; or
the soil moisture may be affected by seepage from localised sources such as broken
water pipes or sewers or be the result of irrigation. All of this leads to a very patchy
urban soil moisture field that may have totally dry plots situated immediately adjacent to
over-watered lawns. Hence whilst some idea of local scale soil moisture may be
possible in areas with very low urban development, or where the semi-natural
landscape has been preserved, it is almost impossible to characterise in most urban
districts. Here again it may be better to use rural values that give a regional background
value rather than have no estimate of soil moisture availability.
3.12 Present weather
If human observers, or the usual instrumentation is available, observation of present
weather events and phenomena such as rime, surface ice, fog, dust and sand storms,
funnel clouds and thunder and lightning can be valuable, especially those with practical
implications for the efficiency or safety of urban activities, e.g. transport. If archiving
facilities are available, the images provided by web cameras can provide very helpful
evidence of clouds, short-term changes in cloud associated with fronts, fog banks that
ebb and flow, low cloud that rises and falls, and the arrival of dust and sand storm
3.13 Cloud
Cloud cover observation is rare in large urban areas but such information is very useful.
All of the methods and instruments outlined in Chapter 15, Part I of the Guide are
applicable to urban areas. The large number and intensity of light sources in cities
combined with a hazy, sometimes polluted, atmosphere makes visual observation more
difficult. Where possible the observational site should avoid areas with particularly bright
3.14 Atmospheric composition
Monitoring of atmospheric pollution in the urban environment is increasingly important,
but is a specialist discipline not dealt with in this chapter. Chapter 17, Part I of the Guide
treats the subject in the broader context of the Global Atmospheric Watch (GAW).
3.15 Profiling techniques for the urban boundary layer
Urban influences extend throughout the planetary boundary layer (Figure 1), so as well
as the need to use towers and masts to obtain observations above the RSL there is a
need to probe higher. Of special interest are effects on the wind field and the vertical
temperature structure including the depth of the mixing layer and their combined role in
affecting pollutant dispersion.
All of the special profiling techniques outlined in Chapter 5, Part II of the Guide are
relevant to the case of urban areas. Acoustic sounders (sodars) are potentially very
useful but it must be recognized that they suffer from two disadvantages in settled
areas: firstly, their signals are often interfered with by various urban sources of noise
(traffic, aircraft, construction activity, even lawnmowers), and secondly, they may not be
permitted to operate because of annoyance to residents. Wind profiler radars, radio-
acoustic sounding systems (RASS), microwave radiometers, microwave temperature
profilers, laser radars (lidars) and modified ceilometers are all suitable systems to
monitor the urban atmosphere if interference from ground clutter can be avoided.
Similarly balloons for wind tracking, boundary layer radiosondes (minisondes) and
instrumented tethered balloons can all be used with good success as long as air traffic
authorities allow. Instrumented towers and masts can provide excellent means of
placing sensors above roof-level and into the inertial sublayer, and very tall structures
may permit measurements into the mixing layer above. However, it is necessary to
emphasize the cautions given in Chapter 5, Part II of the Guide, (see Instrumented
towers and masts) regarding potential interference with atmospheric properties by the
support structure. Tall buildings may appear to provide a way to reach higher into the
urban boundary layer but unless obstacle interference effects are fully assessed and
measures instituted to avoid them the deployment of sensors may be unfruitful and
probably misleading.
3.16 Satellite observations
Remote sensing by satellite with adequate resolution in the infrared may be relevant to
extended urban areas, but an exposition is outside the scope of this chapter. Some
information is available in Chapter 8, Part II of the Guide and a review is given by Voogt
and Oke, 2003.
4 Metadata
The full and accurate documentation of station metadata (refer to Chapter 1, Part I of
the Guide) is absolutely essential for any station “to ensure the final data user has no
doubt about the conditions in which data have been recorded, gathered and transmitted,
in order to extract accurate conclusions from their analysis” (Aguilar et al., 2003). It can
be argued that this is even more critical for an urban station, because urban sites
possess both an unusually high degree of complexity and a greater propensity to
change. The complexity makes every site truly unique, whereas good open country
sites conform to a relatively standard template. Change means that site controls are
dynamic so documentation must be updated frequently. In the following it is assumed
that the minimum requirements for station metadata set by Aguilar et al. (2003) are all
met and also hopefully some or all of the best practices they recommend. Here
emphasis is placed on special urban characteristics that need to be included in the
metadata, in particular under the Categories ‘Local environment’ and ‘Historical events’.
4.1 Local environment
As explained in Section 1.1, urban stations involve the exposure of instruments both
within and above the urban canopy, hence the description of the surroundings must
include both the micro- and local scales. Following Aguilar et al. (2003), with
adaptations to characterize the urban environment, it is recommended that the following
descriptive information be recorded for the station:
(a) a map at the local to mesoscale (~1 : 50,000) as in Fig. 6a, updated as necessary to
describe large scale urban development changes (e.g. conversion of open land to
housing, construction of a shopping centre or airport, new tall buildings, cutting a
forest patch, draining a lake, creation of a detention pond). Ideally an aerial
photograph of the area should also be provided and a simple sketch map (at 1 :
500,000 or 1 : 1,000,000) to indicate the location of the station relative to the rest of
the urbanized region (Fig. 6b and c) and any major geographic features such as
large water bodies, mountains and valleys or change in ecosystem type (desert,
swamp, forest). An aerial oblique
Figure 6 — Minimum information necessary to describe the local scale environment of
an urban station, consisting of (a) template to document local setting, (b) sketch
map to situate the station in the larger urban region, and (c) an aerial
photograph can be especially illuminating because the height of buildings and trees can
also be appreciated. If available, aerial or satellite infrared imagery may be instructive
regarding the presence of important controls on microclimate. For example, relatively
cool surfaces by day usually indicate the availability of moisture or materials with
anomalous surface emissivity. Hotter than normal areas may be very dry, or have a low
albedo or very good insulation. At night relative coolness indicates good insulation and
relative warmth the opposite, or it could be a material with high thermal admittance that
is releasing stored daytime heat or there is an anomalous source of anthropogenic heat.
UCZ and Davenport roughness classes can be judged using Tables 1 or 2.
rea of local
scale ma
Figure 7 — Information required to describe the microscale surroundings of an urban
climate station. (a) Template for metadata file, (b) an example of a fisheye lens
photograph of a street canyon illustrating horizon obstruction, and (c) UKMO
hemispheric reflector placed on a rain gauge.
(b) microscale sketch map (~1 : 5,000), according to metadata guidelines, updated
each year (Figure 7a);
(c) horizon mapping using a clinometer and compass survey in a circle around the
screen (as shown in the diagram at the base of the template, Figure 7a), and a
fisheye lens photograph taken looking vertically at the zenith with the camera’s back
placed on the ground near the screen, but not such that any of the sky is blocked by
it (Figure 7b). If a fisheye lens is not available a simpler approach is to take a
photograph of a hemispheric reflector (Figure 7c). This should be updated every
year, or more frequently if there are marked changes in horizon obstruction, such as
the construction or demolition of a new building nearby, or the removal of trees;
(d) photographs taken from cardinal directions of the instrument enclosure and of other
instrument locations and towers.;
(e) a microscale sketch of the instrument enclosure, updated when instruments are
relocated or other significant changes occur;
(f) if some of the station’s measurements (wind, radiation) are made away from the
enclosure (on masts, roof-tops, more open locations) repeat steps (b) to (d) above
for each site.
4.2 Historical events
Urban districts are subject to many forces of change, including new municipal legislation
that may change the types of land use allowed in the area, or the height of buildings, or
acceptable materials and construction techniques, or environmental, irrigation, or traffic
laws and regulations. Quite drastic alterations to an area may result from central
planning initiatives for urban renewal. More organic alterations to the nature of a district
also arise because of in- or out-migrations of groups of people, or when an area comes
into, or goes out of favour or style as a place to live or work. The urban area may be a
centre of conflict and destruction. Such events should be documented so that later
users of the data understand some of the context for changes that might appear in the
urban climate.
4.3 Observance of other WMO recommendations
All other WMO recommendations regarding the documentation of metadata, including
station identifiers, geographical data, instrument exposure, type of instruments,
instrument mounting and shelters, data recording and transmission, observing
practices, metadata storage and access and data processing should be observed at
urban stations.
5 Assessment of urban effects
The study of urban weather and climate possesses a perspective that is almost unique.
People are curious about the role of humans in modifying the urban atmosphere. So
unlike other environments of interest, where it is sufficient to study the atmosphere for
its own sake or value, in urban areas there is interest to know about urban effects. This
means assessing possible changes to meteorological variables as an urban area grows
or develops over time, compared to what would have happened had the settlement not
been built. This is a question that is essentially unanswerable because the settlement
has been built, and even if it hadn’t the landscape may well have evolved into a different
state than the pre-existing one anyway (e.g. due to other human activity such as
agriculture or forestry). The assessment of urban effects is therefore fraught with
methodological difficulties and no ‘truth’ is possible, only surrogate approximations. If an
urban station is being established either alone, or as part of a network, to assess urban
effects on weather and climate it is recommended that careful consideration be given to
the analysis given by Lowry (1977) and Lowry and Lowry (2001).
6 Summary of key points for urban stations
6.1 Working principles
When establishing an urban station, the rigid guidelines for climate stations are often
inappropriate. It is necessary to apply guiding principles rather than rules, and to retain
a flexible approach. This often means different solutions for individual atmospheric
properties and may mean that not all observations at a ‘site’ are made at the same
Because the environment of urban stations changes frequently as development
proceeds, frequently updated metadata are as important as the meteorological data
gathered. Without good station descriptions it is impossible to link measurements to the
surrounding terrain.
6.2 Site selection
An essential first step in selecting urban station sites is to evaluate the physical nature
of the urban terrain, using a climate zone classification. This will reveal areas of
Several urban terrain types comprise an urban area. In order to build a picture of the
climate of a settlement, multiple stations are required. Sites should be selected that are
likely to sample air drawn across relatively homogenous urban terrain and so are
representative of a single climate zone. Care is necessary to ensure that microclimatic
effects do not interfere with the objective of measuring the local-scale climate.
6.3 Measurements
(a) Air temperature and humidity measurements made within the UCL can be locally
representative if the site is carefully selected. If these variables are observed above
roof-level, including above the RSL, there is no established link between them and
those within the UCL.
(b) Wind and turbulent flux measurements should be made above the RSL but within
the internal boundary layer of the selected urban climate zone. Such measurements
must establish that the surface ‘footprint’ contributing to the observations is
representative of the climate zone. For wind, it is possible to link the flow at this level
and that experienced within the canopy.
(c) Precipitation observations can be conducted either near ground at an unobstructed
site, or above the RSL, corrected according to parallel wind measurements.
(d) With the exception of incoming solar radiation, roof top sites are to be avoided,
unless instruments are exposed on a tall mast.
(e) Net and upwelling radiation fluxes must be made at heights sufficient to sample
adequately the range of surface types and orientations typical of the terrain zone.
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... The main causes for the formation of the UHI effect are: (i) urban morphology and geometry [8,9]; (ii) thermal properties of the constituent materials and their layouts [10,11]; (iii) the heat storage capacity of building materials (such as solar factor, shading coefficient, irradiance, radiosity, emissivity, absorptance, reflectance, and transmittance) [5,12]; (iv) the change and influence on wind speed, depending on surface roughness; (v) the increased absorption of solar radiation, due to the albedo of some surfaces; (vi) albedo [13]; (vii) Width (W)/Height (H)/Length (L) and urban canyons [14]; (viii) Sky View Factor (SVF) [14]; regions, such as Bragança, is due to the movement of tectonic plates and erosion processes (which depend on other factors, such as climate) [32,33]. The specific climate is one of the parameters used to distinguish from other systems, such as valley and lowland zones, and is associated with three effects: (i) synoptic weather systems or air flows that alter dynamic and thermodynamic processes; (ii) differences generated in regional conditions (cloudiness, precipitation regimes, dynamic and thermal winds, etc.); (iii) terrain morphologies and slopes [32,33]. ...
... The main causes for the formation of the UHI effect are: (i) urban morphology and geometry [8,9]; (ii) thermal properties of the constituent materials and their layouts [10,11]; (iii) the heat storage capacity of building materials (such as solar factor, shading coefficient, irradiance, radiosity, emissivity, absorptance, reflectance, and transmittance) [5,12]; (iv) the change and influence on wind speed, depending on surface roughness; (v) the increased absorption of solar radiation, due to the albedo of some surfaces; (vi) albedo [13]; (vii) Width (W)/Height (H)/Length (L) and urban canyons [14]; (viii) Sky View Factor (SVF) [14]; regions, such as Bragança, is due to the movement of tectonic plates and erosion processes (which depend on other factors, such as climate) [32,33]. The specific climate is one of the parameters used to distinguish from other systems, such as valley and lowland zones, and is associated with three effects: (i) synoptic weather systems or air flows that alter dynamic and thermodynamic processes; (ii) differences generated in regional conditions (cloudiness, precipitation regimes, dynamic and thermal winds, etc.); (iii) terrain morphologies and slopes [32,33]. ...
... In a previous study of the UHI, Bragança was classified into seven different LCZs, following the methodological procedures indicated by Oke [14]: (i) Compact midrise (CM): areas with the existence of modern construction of medium-high height, high density, and paved surfaces-sensors 3, 7, and 13; (ii) Compact low-rise (CLR): the ancient center of Environments 2022, 9, 98 4 of 37 the city, with medium-low height, high-density, built-in rock, and brick-sensors 4 and 6; (iii) Open midrise (OM): medium density, streets of low height dwellings in bands or isolated-sensor 10, 12, 18 and 22; (iv) 8-Large low-rise (LLR): commercial and industrial, low average density with low and high buildings with paved parking-sensors 5, 17, and 21; (v) Urban Green Spaces (GAB): urban green spaces, predominantly green cover with undergrowth and trees-sensors 2, 8, 9, and 11; (vi) Sparsely built (SB): transition space between urban and rural environments, scattered houses with agricultural and forestry surroundings-sensors 1, 14, and 15; (vii) Rural Areas (RCD): isolated rural areas in the suburbs of the city are representative of the characteristics of the local landscape-sensors 16, 19, 20, and 23 [32]. ...
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Urban Heat Islands increase surface temperatures which impact the health and well-being of urban populations. Radiative forcing is impacted by changes to the land surface associated with urbanization that are particularly significant immediately after sunset. This paper aimed to analyze the behavior of UHI in different Local Climate Zones (LCZ) in Bragança city (Portugal), using Air Temperature (Ta), satellite images (Landsat 8), and on-site data. The methodology included a seasonal approach, integrating data with different scales (spatial, radiometric, and spectral) and qualitative and quantitative analyses. Google Earth Engine (GEE) optimized the processing time and computation requirement to generate the Land Surface Temperature (LST) maps. The integration of data with different scales corroborated the complementation of information/analysis and detected the correlation between the Ta and LST. However, the identification of the UHI was compromised due to the time of the passage of Landsat 8, and it was identified as the Urban Cool Island (UCI), a complementary effect of UHI, supporting the results of previous studies and for the use of Remote Sensing (RS) for thermal effects analysis.
... Unwin [18] summarized in 1978 some fundamental techniques for microclimate measurements and explained their meaning. Ozawa et al. [19] published a book in 1965 focusing on local climate measurements, introducing their experiences in instrument use, data collection, related field methods such as observing plant phenology, etc. Oke [20] developed guidance in 2006 for meteorological observations in urban areas. They considered the complexity and heterogeneousness of the urban environment. ...
... (1) Application scale Currently, the seminal meteorological observation guidelines [17,20,25] refer to standard climate stations designed to monitor the climate conditions at local scales and avoid microclimate effects. Some standards in microclimate studies are initially developed for indoor or working thermal environments [22][23][24]. ...
... The urban microclimate is a complex and heterogeneous consequence of diverse parameters involving a wide range of natural and urban processes [20,26]. The microclimate of high-density cities is strongly affected by some key factors: high-rise buildings, narrow street canyons, inhomogeneous urban fabric, changeable anthropogenic heat/cooling/moisture, diverse vertical and horizontal exchanges of momentum, complex human activities, etc. [20]. ...
To tackle urban overheating induced by the combined effect of global warming and intensive urbanization, researchers have recommended assimilating microclimate-related strategies into urban design practices. Field measurements, playing a central role in urban climatology, have been widely applied worldwide. Reviewing the last five years' field measurement studies and existing guidelines and standards from WMO (World Meteorological Organization) and ISO (International Organization for Standardization), this study identified a gap between available guidelines and researchers' practical needs to ascertain the collection of high caliber data. Therefore, dedicated guidelines are required to explain the crucial conceptual and application issues and refine systematic field measurement methods. This demand is particularly acute for microscale and urban environments. This study proposed and explained integrated and comprehensive guidelines for systematic microclimate field measurements. The suggested workflow included four main steps: formulating field measurement plan, preparing for field measurements, sustaining measurement quality, and curating data. The complex and heterogeneous environment in urban areas was carefully evaluated to hone the data acquisition campaign and ascertain data quality. Relevant concepts and practices learned from existing guidelines and standards, experiences from actual field studies, and professional recommendations were distilled and incorporated into the guidelines. The significance of a complete report with full metadata was emphasized. Detailed hints, precautions, recommendations, examples, and a metadata checklist were provided as a helpful and actionable package of research procedures.
... Etapa II -Campanhas de campo para identificação e seleção dos locais estratégicos para a instalação dos termohigrômetros Oke (2004) assinala que a escolha de locais adequados à instalação de estações requer, de antemão, uma avaliação do sítio urbano que envolva a descrição de suas propriedades. Entre elas, o autor destaca a estrutura urbana (dimensão dos prédios e dos espaços entre eles, largura das ruas), cobertura urbana (área construída, área pavimentada, área vegetada, solo exposto, massa d'água), o tecido urbano (materiais de revestimento das superfícies sejam elas naturais ou construídos) e o metabolismo urbano (calor, água e poluentes decorrentes das atividades humanas). ...
... 27 -JUL/DEZ 2020 248 nos padrões construtivos e um ponto localizado na periferia da cidade no limite do perímetro urbano. Para dar conta da definição das ZCUs, utilizou-se uma adaptação simplificada da classificação apresentada por Oke (2004) que categoriza em sete classes as distintas formas urbanas, considerando, principalmente, os padrões construtivos, as funções urbanas, a presença de vegetação e o revestimento ou não das superfícies (Quadro 2). Ao invés de calcular a altura média das construções e árvores e o espaçamento médio entre eles, optou-se pelo enquadramento das formas urbanas levando em conta o levantamento descritivo das áreas focalizadas com o apoio de imagens de satélite, imagens 3D disponibilizadas pelo Google Maps e as observações realizadas em campo. ...
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RESUMO: As alterações nas paisagens urbanas decorrentes do intenso processo de urbanização das cidades ocasionam perturbações no balanço de energia no sentido de haver maior quantidade de calor disponível para a promoção de aquecimento do ar a depender dos níveis de urbanização e das características ambientais e urbanas dos locais. A presente pesquisa tem como objetivo verificar como as diferenças espaciais e ambientais da malha urbana da cidade de Goiânia/GO afetam o comportamento espacial e temporal das variáveis climáticas temperatura e umidade relativa do ar em um mês de calor intenso na capital goiana (outubro de 2014). Para tanto, adotou-se a proposta teórico metodológica de Monteiro (1976), Sistema Clima Urbano (SCU), mais especificamente o subsistema termodinâmico. A escolha dos pontos de monitoramento das variáveis climáticas foi baseada na caracterização do sítio urbano e na classificação dessas áreas em Zonas Climáticas Urbanas (ZCUs). Ao todo, 5 pontos foram selecionados: dois deles localizados na porção mais densamente ocupada da cidade, mas com diferenças nos padrões construtivos, dois parques urbanos e um na periferia da cidade. A avaliação temporal e espacial das variáveis ocorreu em três horários do dia, às 6h, 14h e 21h e considerou: registros in loco de temperatura e umidade do ar; informações como saldo e perda energético em determinados períodos do dia; tempo máximo diário de aquecimento e desaquecimento e as amplitudes térmica e higrométrica. Os resultados obtidos apontaram variação espacial de temperatura e umidade do ar bem definida em resposta às diferenças de níveis de urbanização, dando-se maior aquecimento na área densamente urbanizada ausente de sombreamento de prédios ou de controladores naturais do campo higrotérmico e menor aquecimento na periferia e nos parques intraurbanos, em virtude da presença dos controladores naturais do campo higrotérmico. PALAVRAS-CHAVE: clima urbano, Goiânia, comportamento higrotérmico, Zonas Climáticas Urbanas. ABSTRACT: The changes in urban landscapes resulting from the intense urbanization process of cities cause disturbances in the energy balance in the sense that there is a greater amount of heat available to promote air heating depending on the levels of urbanization and the environmental and urban characteristics of the places. The present research aims to verify how the spatial and environmental differences of the urban fabric of the city of Goiânia / GO affect the spatial and temporal behavior of the climatic variables temperature and relative humidity in a month of intense heat in the capital of Goiás (October 2014). For this purpose, Monteiro's (1976) theoretical methodological proposal, Urban Climate System (UCS) was adopted, more specifically the thermodynamic subsystem. The selection of monitoring points for climatic variables was based on the characterization of the urban site and the classification of these areas in Urban Climate Zones (UCZs). In all, 5 points were selected: two of them located in the most densely occupied part of the city, but with differences in construction patterns, two urban parks and one on the suburban area of the city. The temporal and spatial evaluation of the variables occurred at three times of the day, at 6 am, 2 pm and 9 pm and considered: on-site records of air temperature and humidity; information such as gain and energy loss at certain times of the day; maximum daily time of heating and
... It is also a great challenge due to the specificity of the urban environment, where large differences in weather conditions occur in a relatively small area. The weather information collected in the urban canopy layer has a short-distance validity, which ranges from less than one meter to hundreds of meters [1]. Getting the current weather conditions with high accuracy and forecasting future weather conditions requires an increase in the density of weather measurement points. ...
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Smart-city management systems use information about the environment, including the current values of weather factors. The specificity of the urban sites requires a high density of weather measurement points, which forces the use of low-cost sensors. A typical problem of devices using low-cost sensors is the lack of legalization of the sensors and the resulting inaccuracy and uncertainty of measurement, which one can attempt to solve by additional sensor calibration. In this paper, we propose a different approach to this problem, i.e., the two-stage selection of sensors, carried out on the basis of both the literature (pre-selection) and experiments (actual selection). We formulated the criteria of the sensor selection for the needs of the sources of weather information: the major one, which is the fast response time of a sensor in a cyber-physical subsystem and two minor ones, which are based on the intrinsic information quality dimensions related to measurement information. These criteria were tested by using a set of twelve weather sensors from different manufacturers. Results show that the two-stage sensor selection allows us to choose the least energy consuming (due to the major criterion) and the most accurate (due to the minor criteria) set of weather sensors, and is able to replace some methods of sensor selection reported in the literature. The proposed method is, however, more versatile and can be used to select any sensors with a response time comparable to electric ones, and for the application of low-cost sensors that are not related to weather stations.
... Portanto, as analises evidenciam a viabilidade da estimação do índice IBUTG por meio do uso de redes neurais, o que pode fomentar o uso da técnica para monitoramento preventivo do risco ocupacional ao estresse térmico ao calor através da sua implantação em uma plataforma de monitoramento automático.O clima urbano pode ser representado por três componentes: uma regional, uma local devida aos fatores não urbanos (efeitos topográficos, por exemplo) e finalmente aquela decorrente propriamente das condições urbanas(LOWRY, 1977). Nesse sentido, para uma a abrangência mais geral das estimações, com enfoque mais no comportamento de mesoescala do que microclimático, recomenda-se que as medições meteorológicas sejam feitas por meio de estações que estejam influenciadas pelas duas primeiras componentes.Medições em áreas específicas da cidade, apesar de serem mais precisas, uma vez que os sensores de temperatura ou umidade ficam influenciados pela parcela de área no seu entorno imediato, restringem a extensão das estimativas por parte das redes devido ao conceito de footprint das estações meteorológicas, onde a estimativa fica limitada a algumas centenas de metros no entorno da estação(OKE, 2006).Com a finalidade de verificar a viabilidade de se utilizar a técnica de redes neurais, para estimar o nível de exposição ocupacional ao calor de trabalhadores, que desempenham atividades em condição de céu aberto sem fonte artificial de calor, conduziu-se medições meteorológicas de forma síncrona com as variáveis físicas ambientais de temperatura de bulbo úmido e temperatura de globo, o que permitiu determinar o índice IBUTG, indicador utilizado NR. 9 e NR. 15 para avaliar o nível de estresse térmico ao calor. As estimações proporcionadas pelas RNAs testadas permitiram demonstrar a viabilidade técnica de estimar o IBUTG com adequada precisão a partir de dados meteorológicos, usualmente medidos nas estações sem a utilização dos termômetros especificados pela norma. ...
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Este trabalho objetiva demostrar a viabilidade técnica de estimação do Índice de Bulbo Úmido Termômetro de Globo (IBUTG) para ambiente a céu aberto sem fonte artificial de calor por meio de dados medidos em estações meteorológicas convencionais, a partir da utilização de Redes Neurais Artificiais (RNA). Para tanto, procedeu-se a instalação de termômetros de bulbo úmido natural e de globo em uma estação meteorológica convencional, com a finalidade de calcular o IBUTG sincronamente com as variáveis de temperatura, umidade e velocidade do ar, bem como radiação solar global e pressão atmosférica. O treinamento da RNA foi conduzido com a utilização de 81 dias de medições. Algumas configurações da RNA foram modificadas com o intuito de encontrar a de melhor desempenho para a rede. Para o teste de validação do treinamento, selecionou-se dia de céu aberto, nublado e com precipitação, com condições sinópticas que impõem elevado estresse ao calor. O IBUTG estimado pelo RNA acompanhou o ciclo diário do IBUTG medido, com a melhor configuração de rede (três camadas e cinco neurônios) estimando erro médio quadrático diário de 0,2724°C e erro médio absoluto de 0,1818°C (com erro percentual de apenas 0,7%). Comprova-se a viabilidade técnica de estimar o IBUTG com adequada precisão a partir de dados meteorológicos, o que permite que a técnica de RNA possa ser utilizada como estratégia de orientação do gerenciamento do risco ocupacional.
... Furthermore, evidence has shown that, when air crossed the border between neighboring underlying surfaces, it would gradually adjust to a new set of internal boundary conditions: such thermal transitions would not sudden breaks at the borders, but would instead be a spatially continuous process [7]. Accordingly, not only the representative sampling points of the different underlying surfaces should be concerned [66], but also their border positions (Fig. 8), which might otherwise have often been overlooked. ...
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Micro-scale thermal interactions between underlying surfaces and their surrounding air in urban spaces critically influence urban residents' thermal comfort. On the micro-scale, heat can induce substantial changes in the air temperature (Ta) at the pedestrian height. To characterize the spatiotemporal distribution of the Ta for spaces adjacent to the underlying surfaces, an accurate measurement method is essential. This study explored the proper spatial interpolation methods (SIMs) through strategically-arranged measurement points and then assessed the spatiotemporal distribution of Ta in a tiny experimental area. Results showed that the selection of SIMs and interpolation points significantly affected the results for the micro-scale spatial interpolation. The Ta might fluctuate precipitously within extremely short distances between adjacent underlying surfaces, depending on their thermal properties or on the locations of shadows. At the pedestrian height, the Ta readings registered a maximum difference of 1.31 °C for the same areas with and without arbor shading. However, the arbors' cooling extent and location depended on their shading's position, which constantly changed during the day. The shrubs and the lawn under direct sunlight hardly regulated the microclimate, even in the periphery. Additionally, solar radiation and wind speeds influenced the Ta spatial distribution on the micro-scale. Therefore, future micro-scale urban designs should take into consideration the dynamic positioning of arbor shading, given its significant cooling effect.
... Floor heights are highly effective in weather temperatures, especially in terms of changing wind directions and storing temperatures due to surface characteristics and the spatial size of the city, causing cities to warm up (Oke, 2004). When we look from this perspective, the distribution and proportions of the building floors in our study area are different. ...
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The new summer index (SSI) which was established to determine climate comfort was classified according to the index. Climatic comfort areas will increase and thus raising the quality of living in areas. The temperature values felt in the study were calculated with the SSI formula, and then the maps of the study area were classified. The study area was the Hayal Park and its surroundings in the Toros Quarter of Cukurova District, which is surrounded by multi-story and dense buildings in the north of Adana. Although the vertical construction is very high, it is richer in terms of green areas compared to other large central districts. Cukurova, which is one of the largest districts of Adana (according to population), has Seyhan dam lake in the north and Seyhan district in the south. The study area is on the border of Seyhan district and is adjacent to Yüreğir and Sarıçam districts. With the portable Smart SENSOR AS 847 measuring device, temperature and relative humidity were measured at 1.5 m above the ground on Tuesday, August 20 at 07.00, at 14.00, and 21.00. Bioclimatic comfort areas were determined by measuring temperature and relative humidity at 25 points with different textures in the study area. In this study, measured instantaneous temperature (°C) and relative humidity (%) data were transferred to GIS medium by ArcGIS 10.6 program and then modeled with the Kriging method, which is one of the interpolation methods, and temperature and relative humidity maps were created. The temperature map created in degrees Celsius (°C) by the Kriging method and was converted to Fahrenheit degrees (°F) in the Raster Calculator. The reason for this change is that SSI is calculated with the formula Fahrenheit (°F). These maps were then calculated on the Raster Calculator using the SSI formula, and sensed temperature values were obtained The SSI, which normally has 8 classes, has 3 classes in the study area. These classes are 83 ≤ SSI < 91 slightly hot, 91 ≤ SSI < 100 hot, and 100 ≤ SSI < 112 very hot. When we look at the maps, the places outside of Hayal Park are in the hot class at 07.00 in the morning, and we can see that the green spaces have a positive effect on the bioclimatic comfort areas. The small area is in a very hot class. When we look at 14.00 h, it is a totally hot class. Normally, the temperature is the highest of these times and also coincides with the multi-story buildings in the study area; asphalt and concrete areas increased the temperature. As a result, there is a very hot air in the study area. It was determined that the maps identified non-comfortable areas. Since urban form and settlement affect climate comfort values, the urban plan should be revised, and the area should be brought to the bioclimatic comfort value range. For reducing the effect of temperature in the working area and creating comfortable areas is increasing the density of green areas compared to asphalt and concrete areas. Considering that the Cukurova district will grow further, urban planning should be done very well to reach the comfort range in the following settlements.
... The urbanization level is represented by three types (urban, suburban and rural), which correspond to the super-classes defined by Stearns et al. [181]. The equivalence of these classes with other classifications by Oke [152] and Stewart and Oke [183] can be found in Table 12 of Stearns et al. [181]. The equivalence with the classification used in WMO [205, vol.3 ...
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This thesis uses regional observations and global reanalysis to develop a regional meteo-climate service and assess the climate in Central Italy. The first part of the thesis identifies a general procedure to design an efficient mesoscale meteorological network and ensure the physical consistency of observations. A complex quality control is applied both on station data and metadata and it is used as a basis for the network optimization. Missing and wrong hourly temperature data have been reconstructed by using a spatio-temporal method based on Empirical Orthogonal Functions. The introduction of the global ERA5 reanalysis in the gap-filling process allowed the reconstruction of the longest gaps with statistical and physical consistency. The entire procedure is tested in the Umbria region, central Italy, where its implementation would lead to a considerable advancement in terms of regional weather and climate services. The second part of the thesis examines the link between circulation weather types, rainfall and orography in central Italy. Using ERA5 reanalysis and raingauge data from 1951 to 2019, four weather types are identified as most responsible for the spatial variability of rainfall in central Italy. They are associated with cyclonic circulations characterized by high water vapour transport coming from west, south-west, south-east, and north-east. The precipitation associated with north-easterly and westerly fluxes shows the strongest modulation by orography. However, the orographic component of rainfall is not well captured by the ERA5 reanalysis when cyclonic north-easterly and westerly flows impinge on the north-central Apennines. Future studies should employ high-resolution modeling to investigate how global warming affects the interaction between cyclonic fluxes and orography, as suggested by recent climate trends observed in central Italy. The last part of the thesis develops a climate application to support water resources management by estimating groundwater recharge in the Umbria region, central Italy. This application is based on local water table elevation measurements and soil moisture time-series derived from the ERA5 reanalysis. Water fluxes towards the aquifer are evaluated within a Richard equation-based approach. The relationship found between simulated fluxes and local observations includes both a logarithmic and linear term. The predominance of one term over the other point out the possible different recharge mechanism of the considered aquifers.
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The phenomenon of urbanization has rapidly grown worldwide and currently more than half the world's population lives in cities experiencing the urban climate that reports to a change in climatic conditions at local scale due to the development of urban areas. The most evident effect of urban climate is the Urban Heat Island (UHI) phenomenon which consists in a warmer climate in cities compared to the surrounding rural areas. Urban climate, that can be considered as an important paradigm for citizens’ quality of life, is in turn influenced by climate effects at larger scale. For this reason, one of the major challenges posed to human beings in the 21th century is to manage the rapid worldwide urban growth with the impacts generated by climate changes over cities. In particular, among the effects of climate changes, an increase in atmospheric temperature may exacerbate, the negative consequences due to UHI. The climate projections over the Mediterranean basin predict increasing temperature during summer season in the period when (especially during the heat waves) the UHI causes major effects on human health, consumptions of electricity related to air conditioning systems and air quality in urban environment. The UHI is due to an accumulation of heat arising from both structural causes (e.g. presence of man-made materials such as concrete and asphalt, reduction of public parks) and weather (e.g. strong solar radiation, low wind speed). These features are unique to each city. In this context, the objective of this Thesis is to study the UHI in a city of the Mediterranean basin during the summer season, analyzing this phenomenon by combining instrumental and modelling techniques, in order to propose strategies for temperature reduction in the urban environment. Two measurement campaigns were carried out in Lecce during summer 2011 and summer 2012. Collected data have been used to validate two numerical models: ADMS-Temperature and Humidity Model and ENVI-met 3.1. The former is an integral operational model able to assess changes in temperature and humidity within urban areas due to changes in land use. The latter is a Computational Fluid Dynamics based model able to simulate surfaces, air, plants interactions for urban microclimate. This is recognized to be a very important stage since, despite the widespread use of numerical models for the study of urban climate, their reliability have not been thoroughly determined. These models were compared for the same study areas to investigate strengths and weaknesses. Finally, hypothesis of scenario have been simulated by ENVI-met in order to propose UHI mitigation strategies. Results demonstrated how this phenomenon can be reduced by means of different techniques to lower urban temperatures and especially through the use of vegetation and natural soils instead of artificial surfaces. The resulting benefits are due to the increase of evapotranspiration that may dissipate the heat generated from energy inputs arising from solar radiation through latent heat fluxes. The findings of this Thesis may be useful for policy makers and urban planners to structure interventions of mitigation of UHI in a hot climate city, achieving important benefits for public health, environment and energy.
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Rising temperatures are causing distress across the world, and for those most vulnerable, it is a silent killer. Information about indoor air temperature in residential dwellings is of interest for a range of reasons, such as health, thermal comfort and coping practices. However, there have been only few studies that measure indoor heat exposure, and contrast these to outdoor temperatures in rural-urban areas, of which none are in South Asia. We aim to close this knowledge gap with our indoor and outdoor heat measurement dataset, covering five low-income sites in South Asia. Two sites are in rural areas (Maharashtra, India), while three sites focus on urban areas (Dhaka, Delhi and Faisalabad). Data are based on 206 indoor temperature data loggers and complemented by data from five outdoor automated weather stations. The data-set can be used to examine temperature and humidity variation in low-socioeconomic status households in rural and urban areas and to better understand factors aggravating heat stress. This is important to plan and implement actions for combating heat stress.
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Increasing urbanization and concern about sustainability and quality of life issues have produced considerable interest in flow and dispersion in urban areas. We address this subject at four scales: regional, city, neighborhood, and street. The flow is one over and through a complex array of structures. Most of the local fluid mechanical processes are understood; how these combine and what is the most appropriate framework to study and quantify the result is less clear. Extensive and structured experimental databases have been compiled recently in several laboratories. A number of major field experiments in urban areas have been completed very recently and more are planned. These have aided understanding as well as model development and evaluation.
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Distinguishes the Urban Canopy Layer (UCL) from the ground to roof level, from the Urban Boundary Layer (UBL) which extends to the height where urban influences are no longer perceptible. Methods to establish the energy balance include micrometeorological approaches and numerical models. Data are presented under the heading of radiation budget, anthropogenic heat release, subsurface (storage) heat flux, turbulent heat transfer, advection, and urban-rural energy balance differences. -K.Clayton
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1 RATIONALE If we measure rainfall, in order for the data to be useful for future users, we also need to document where and how the measurements were made. Station documentation is information about the data or data about the data: metadata. The word metadata is made up by the addition of the Greek "meta" (beyond) and the Latin " datum" (a given fact). Metadata should reflect how, where, when and by whom information was collected. Ideally, complete metadata should register all the changes a station has undergone during its lifetime, composing what is called the station history. Supplementary information about the observations, such as type of instrument or exposure, can provide additional insights into interpreting the observed quantities. Sometimes when the instruments change, the observations will show an artificial increase or decrease. Such a jump in the measured amount is an example of an inhomogeneity and adjustments to these data are often applied to account for the effects of the inhomogeneity. If a long-term time series is homogeneous, then all variability and change is due to the behavior of the atmosphere. Every user and provider of climate data has to deal with metadata and homogeneity to some extent. Many climate researchers throughout the world have developed effective approaches for dealing with the many aspects of metadata and homogeneity. The following document is based on their collective experience and should and is intended to offer guidance to the NMHSs on these matters.
Wind sensors mounted on towers and smokestacks do not always indicate the true free-air flow. To determine the probable errors in measurements of wind speed and direction around such structures, quarter-scale models have been tested in a large wind tunnel. Data on changes in wind speed and direction were obtained by using smoke, very small wind vanes, and a scale model propeller anemometer. Most emphasis has been placed on a relatively open lattice-type tower, but a solid tower and a stack were also studied. The analysis shows that in the wake of lattice-type towers disturbance is moderate to severe, and that in the wake of solid towers and stacks there is extreme turbulence, with reversal of flow. Recommendations for locating wind sensors in the wind field relative to the supporting structure are given for each of the three structures studied. Guidelines are suggested regarding probable errors in measurements of wind speed and direction around different supporting structures, as outlined below. For an open triangular tower with equal sides D, the wake is about 1-1/2D in width for a distance downwind of at least 6D. Sensors mounted 2 D out from the corner of such a tower will usually measure speeds within ± 10° of that of the undisturbed flow for an arc of about 330°. The disturbance by very dense towers and stacks is much greater. Wind sensors mounted 3 diameters out from the face of a stack will measure wind speeds within ± 10%, and directions within ± 10° of the undisturbed flow for an arc of about 180°.
The book is aimed at the beginning graduate level for students with an undergraduate background in meteorology. The chapter organization is: mean boundary layer characteristics; statistics; application of the governing equations to turbulent flow; prognostic equations for turbulent fluxes and variances; turbulent kinetic energy, stability, and scaling; turbulence closure techniques; boundary conditions and external forcings; time series; similarity theory; measurement and simulation techniques; convective mixed layer; stable boundary layer; boundary layer clouds; geographic effects. -after Author
At first glance, the large number of equations developed in Chapters 3-5 would suggest that we have a fairly complete description of turbulent flow. Unfortunately, a closer examination reveals that there are a large number of unknowns remaining in those equations. These unknowns must be dealt with in order end up with a useful description of turbulence that can be applied to real situations. In this Chapter, the unknowns are identified, and methods to parameterize them are reviewed. Simulation techniques such as large-eddy simulation are discussed in Chapter 10.
The footprint of a turbulent flux measurement defines its spatial context. With the onset of long-term flux measurement sites over forests and other inherently inhomogeneous areas, and the development of the FLUXNET program, the need for flux footprint estimations has grown dramatically. This paper provides an overview of existing footprint modeling approaches in the critical light of hindsight and discusses their respective strengths and weaknesses. The second main objective of this paper is to establish a formal connection between micrometeorological measurements of scalar fluxes and their mass conservation equation, in a surface-vegetation-atmosphere volume. An important focus is to identify the limitations of the footprint concept and to point out situations where the application of footprint models may lead to erroneous conclusions, as much as to demonstrate its utility and power where warranted. Finally, a perspective on the current state-of-the-art of footprint modeling is offered, with a list of challenges and suggestions for future directions.