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Measuring nocturnal near-surface urban heat island intensity in the small, mid-latitude city of Inverness, Scotland

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Few publications have considered the urban heat island (UHI) effect in small settlements. This paper, therefore, presents the findings of a UHI study conducted in the small, mid-latitude city of Inverness, Scotland (population: 63,220). The study aimed to provide an initial appraisal of the scale of UHI phenomena in Inverness and to understand the factors associated with its presence. Mobile vehicular traverses of a study transect were conducted on 30 dates during June, July & August 2019. Measurements of near-surface air temperature were recorded at 19 local climate zone observation points between 20:00 and 23:00 BST. Daily mean UHI intensity (UHII) during the study ranged from 0.7 to 3.5°C, with an overall mean UHII of 1.6°C. Land use characteristics impacted the UHII, with areas with higher fractions of impervious surface cover returning significantly higher air temperatures. UHII increased with the onset of sunset, and the highest UHII occurred on dates with up to 1–2 oktas of cloud cover, low relative humidity, and high cloud base height. The results align with previous studies and comparisons are drawn between Inverness and other settlements globally. Further research in Inverness is recommended to better understand UHI effects and influence national planning policy.
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Scottish Geographical Journal
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Measuring nocturnal near-surface urban heat
island intensity in the small, mid-latitude city of
Inverness, Scotland
George F. Gunn
To cite this article: George F. Gunn (2023): Measuring nocturnal near-surface urban heat
island intensity in the small, mid-latitude city of Inverness, Scotland, Scottish Geographical
Journal, DOI: 10.1080/14702541.2023.2242819
To link to this article: https://doi.org/10.1080/14702541.2023.2242819
© 2023 The Author(s). Published by Informa
UK Limited, trading as Taylor & Francis
Group
Published online: 09 Aug 2023.
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Measuring nocturnal near-surface urban heat island intensity
in the small, mid-latitude city of Inverness, Scotland
George F. Gunn
University of the Highlands and Islands, Inverness, Scotland
ABSTRACT
Few publications have considered the urban heat island (UHI) eect
in small settlements. This paper, therefore, presents the findings of a
UHI study conducted in the small, mid-latitude city of Inverness,
Scotland (population: 63,220). The study aimed to provide an initial
appraisal of the scale of UHI phenomena in Inverness and to
understand the factors associated with its presence. Mobile
vehicular traverses of a study transect were conducted on 30 dates
during June, July & August 2019. Measurements of near-surface air
temperature were recorded at 19 local climate zone observation
points between 20:00 and 23:00 BST. Daily mean UHI intensity
(UHII) during the study ranged from 0.7 to 3.5°C, with an overall
mean UHII of 1.6°C. Land use characteristics impacted the UHII,
with areas with higher fractions of impervious surface cover
returning significantly higher air temperatures. UHII increased with
the onset of sunset, and the highest UHII occurred on dates with
up to 1–2 oktas of cloud cover, low relative humidity, and high
cloud base height. The results align with previous studies and
comparisons are drawn between Inverness and other settlements
globally. Further research in Inverness is recommended to better
understand UHI eects and inuence national planning policy.
ARTICLE HISTORY
Received 7 September 2022
Accepted 25 July 2023
KEYWORDS
Urban heat island; small city;
Inverness; urbanisation;
sustainable development
Introduction
Rapid urbanisation is occurring globally to the extent that most of the world’s population
now resides in urban areas (United Nations, 2019). The urban heat island (UHI) eect
has recognised the extent of climate change resulting from urbanisation since first
studied by Howard in 1883.
The UHI eect is observed where air temperatures in urban areas exceed those of sur-
rounding rural environments. Several classifications of UHI exist, determined by their
reference location within the urban area (Oke, 1995). The near-surface UHI (hereafter
referred to simply as UHI) is the eect directly experienced by human populations
and is determined at approximately 1.5 m above ground level. This eect is known to
exacerbate the physical impact of heatwaves (Ward et al., 2016) which have increased
© 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/
licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author
(s) or with their consent.
CONTACT George F. Gunn george.gunn@uhi.ac.uk University of the Highlands and Islands, 12b Ness Walk,
Inverness, IV3 5SQ Scotland
SCOTTISH GEOGRAPHICAL JOURNAL
https://doi.org/10.1080/14702541.2023.2242819
in prevalence and scale over the past 60 years, particularly in response to anthropogenic
activities (Perkins-Kirkpatrick & Lewis, 2020). In addition, significant impacts on
phenology are noted in areas experiencing the UHI eect (Zipper et al., 2016).
Human activity and land use decisions in urban areas, such as work, travel, industry,
and residence, are sources of anthropogenic heat. These are measured together by the
anthropogenic heat ux (Q
F
) within the surface energy balance (Equation 1). Q
F
increases the net all wave radiation (Q*) of the urban area, and thus the intensity of
the UHI eect (UHII) experienced,
Q = QH+QE+QG+QF, (1)
where Q
H
is the sensible heat ux, Q
E
is the latent heat ux, and Q
G
is the ground heat
storage (all uxes also measured in rural environments). UHII provides a local measure
of one type of anthropogenic climate change (Oke et al., 2017). It is determined by cal-
culating the dierence between the urban air temperature (T
U
) and the temperature of
the surrounding rural environment (T
R
),
UHII =DTUR=TUTR(2)
To date, much of the literature has focussed on the extent of the UHI eect in larger
settlements (population >100,000). The eect poses more noticeable threats to human
populations in these areas, particularly in terms of heatwaves and air quality (Tan et al.,
2010). For example, Taylor et al. (2015) found there to be increased relative risk of mortality
due to heat for individuals living within a UHI in London, England. Particularly where
development space is limited, UHII may increase as vertical building growth encourages
a ‘street canyoning’ eect resulting in altered airow (Longley et al., 2004) and increased
pollutant residence time (Farrell et al., 2015). In addition, a lack of pervious surfaces
(e.g. parkland, trees, marshlands) leads to increased surface water runo into artificial drai-
nage systems. This situation results in an increase in Q
H
and a decrease in Q
E
within the
urban surface energy balance (Equation 1) as less water is available for evaporation.
It is acknowledged that the UHI eect may manifest in all urban areas and can often be
modelled by population size (Oke, 1973). However, interactions between land use, urban
form, climate and prevailing weather also impact the UHII (Oke, 1982). The UHII of
New York City (population: 8.3 million) can exceed 8°C (Gedzelman et al., 2003); in
the city of Hong Kong (population: 7.5 million) it can reach 3.8°C (Siu & Hart, 2013);
and in Glasgow, Scotland (population: 1.7 million) a maximum UHII of 8.3°C was
recorded by Hartley (1977). It is therefore clear that such complex interactions lead to
a unique set of UHI characteristics in each settlement (Grimmond, 2007).
Increasing the safety, resilience and sustainability of urban areas is a current goal of
the United Nations (2015). Reducing the UHI eect is therefore a key to achieving sus-
tainable development within urban areas. Reconfiguring land-use, building design and
activities within the urban area, in a way which improves the surface energy balance
(Equation 1), can reduce the impacts of the UHI eect (Akbari et al., 2015). The necessary
actions to do so are eminently more achievable in smaller or new plan settlements where
there may be greater opportunity to prevent rather than mitigate the UHI eect. Ahead of
any such interventions, it is beneficial to gain a better understanding of why and how the
phenomena manifests itself (Eastin et al., 2018).
2 G. F. GUNN
This paper therefore presents the results of an experimental campaign conducted in
the small city of Inverness, Scotland which has experienced one of the fastest urban
expansions in Europe in recent years (Highlands and Islands Enterprise, 2015). The
study sought to undertake an initial appraisal of the scale of UHI phenomena in Inver-
ness over a period of three months during summer 2019.
Study area
The city of Inverness is located in the north-east of Scotland (57.48° N, 4.23° W, Figure
1). Inverness is situated at the north-eastern end of the ‘Great Glen’ valley, on the banks
of the River Ness close to its conuence with the Beauly Firth. The city’s population
expanded by approximately 54 per cent between 2001 and 2016 and was most recently
estimated at 63,220 (National Records of Scotland, 2018). This expansion gave rise to
considerable urban sprawl, with several former villages and townships now contained
within the city’s footprint. The city presently has a built-up area of approximately
23 km
2
(Morton et al., 2020), the topography of which is determined by the U-shaped
morphology of the Great Glen. Terrain altitude varies from sea-level up to approximately
8 m a.s.l. in the city centre, and approximately 150 m a.s.l. at the valley sides.
Buildings in the city centre do not generally exceed 5 storeys, and land use is predo-
minately retail, oce, and accommodation space. Greenspace within the city confines is
expansive, extending to approximately 11 km
2
, including two golf courses and several
parks and recreation pitches. Housing developments radiate from the city centre to
the south, west, and east, and range from post-war terraced housing in the inner
suburbs to detached and semi-detached dwellings in the outer suburbs. North of the
Figure 1. Location of the city of Inverness, Scotland and its main former townships and villages.
Created using LiDAR DTM data crown copyright Scottish Government, SEPA and Scottish Water,
and aerial imagery – copyright Getmapping plc.
SCOTTISH GEOGRAPHICAL JOURNAL 3
city centre, land use is predominantly industrial and includes a mainline railway station
with freight railhead and a small port. Industrial estates provide warehousing and work-
shops, major retail units and manufacturing plants.
Inverness experiences a temperate oceanic (Köppen Cfb) climate, with a mean annual
temperature of 9.3°C. The mean temperature of the warmest month, July, is 12.0°C and
the mean temperature of the coldest month, January, is 4.2°C. The surrounding moun-
tainous topography is responsible for a ‘rain shadowing’ eect (McClatchey, 2014). Thus,
mean annual rainfall is low at 755 mm, and is generally well-distributed throughout the
year (Figure 2). The mean windspeed in Inverness is gentle, varying from 3.9 to 5.4 ms
1
throughout the year.
Materials and methods
For such an initial appraisal of the UHI eect in Inverness, it was decided that an exper-
imental campaign focused on data collection along a well-considered study transect
would be of most value scientifically. Such a method aords a high-resolution under-
standing of the near-surface temperature conditions, otherwise unavailable through
the alternatives of remote sensing or existing meteorological station data. This method
involves observation of near-surface air temperature using a vehicle-mounted sensor
and global positioning system (GPS) and has demonstrable success in similar previous
UHI studies (e.g. Chandler, 1962; Graham, 1993; Hartley, 1977; Yokobori & Ohta, 2009).
Transect
Observation points along a 9.25 km transect were chosen to include a selection of urban,
suburban, and rural surface fabrics, and greenspace (Figure 3). It was necessary to account
Figure 2. Climate of Inverness, Scotland. Created using 1991–2020 climate data courtesy of the Met
Office.
4 G. F. GUNN
for the potential distance taken for air temperature at 1.5 m to adjust to that of the respect-
ive surface being studied (Oke, 2004). Therefore, in line with Stewart and Oke (2012),
250 m radii circle buers were applied to the observation points within a QGIS geographi-
cal information system (GIS) and used to define the observation area for each observation
point. This resulted in 19 observation points spaced equally along the study transect. Con-
sideration was given to the transect accessibility for a motor vehicle during the selection
phase, enabling travel in both directions and an ability to turn easily at each end.
To facilitate comparison between the observation points used in this study and those
of other published works, the local climate zone (LCZ) of each observation point was
determined by using Stewart and Oke’s (2012) method of using building and surround-
ings metadata. Shapefile data from the UK Centre for Ecology & Hydrology (UKCEH)
2019 land cover map (Morton et al., 2020) was used within the GIS to calculate fractions
of pervious and impervious surfaces within the LCZ observation areas. Dominant land
cover types were thereby determined using Stewart and Oke’s (2012) definitions in con-
junction with recorded site observations.
Observations
The UHI eect is generally observed at its strongest intensity following sunset (Oke et al.,
2017), hence focussing on a nocturnal study oered the best opportunity to observe UHI
signals within a limited time budget.
Near-surface air temperature was observed using a calibrated fast-response thermistor
(accurate to ±0.1°C) mounted within shielding to the roof of a hatchback car at a height
of approximately 1.5 m from ground level. Shielding for the thermistor comprised a
capped PVC tube of length, diameter, and thickness 250/30/2 mm. Louvres were
Figure 3. Location of the transect route within the study area, detailing air temperature observation
sites with local climate zones (LCZs). Dominant surface type determined using data from Morton et al.
(2020).
SCOTTISH GEOGRAPHICAL JOURNAL 5
included along the tube’s length to enable airow whilst shielding the thermistor from
precipitation and direct short- and long-wave radiation. Owing to the late onset of
sunset during the study period (reducing from 22:05 BST in early June to 20:15 BST
in late August), the outside of the shielding was coated white to increase its albedo
and reduce temperature error from direct insolation. A hole was drilled centrally in
one end of the tube through which the thermistor was inserted and held in position.
The apparatus was mounted lengthways on the centre of the roof, parallel to the direction
of travel to enable the free ow of air around the thermistor. A series of test traverses,
using a field assistant to conduct static measurements at a variety of observation
points simultaneously, confirmed that varying the speed of vehicle motion between 0
and 40 km/h did not impact the result of the mobile temperature observations.
5 round-trip observational traverses of the transect route were conducted between
20:00 and 23:00 BST on 30 dates between 01/06/2019 and 31/08/2019, totalling 150
round-trip traverses. Traversing the transect in both directions enabled a closed loop
of data. To eliminate observational bias associated with potentially favourable meteoro-
logical conditions, 10 observation dates per month were pre-selected with dates chosen in
pairs of two consecutive days spaced equally over each month. Observations were col-
lected at 250 m way pointed intervals pre-loaded on a GPS device (accurate to ±
3.65 m). A vehicle speed of approximately 35 km/h was maintained throughout the
observation period, except during infrequent trac light stops.
Data processing
Owing to the study area’s situation within a valley, observation points 2–19 were situated at
higher elevations than Inverness city centre (determined as observation point 1). Elevation
temperature control was, therefore, necessary to avoid misrepresentation of air tempera-
ture due to a potential decrease in temperature with height between 8 and 150 m a.s.l.
(per Hathaway & Sharples, 2012). The dry adiabatic lapse rate (9.8°C air temperature
decrease per 1,000 m elevation gained) was therefore applied to the data using the formula
Tcorrected =Traw +((zzcity)0.0098), (3)
where T
corrected
is the air temperature measurement corrected for change in altitude, T
raw
is
the uncorrected air temperature measurement, z is the observation point altitude in metres,
and z
city
is the altitude of the city centre in metres.
The mean air temperature was determined at each observation point for each date of
study. The daily nocturnal UHII was then calculated by Equation (2) where T
U
is the
maximum city centre air temperature, and T
R
is the mean of the rural air temperatures
(determined as observation points 16–19).
Results and discussion
Characteristics of urban heat island intensity in relation to land use
Measured near-surface air temperature decreased significantly between urban and rural
areas in Inverness from 20:00 to 23:00 BST (Figure 4). A mean nocturnal near-surface
UHII of 1.6°C was determined following statistical analysis of 5,700 observations made
6 G. F. GUNN
between June and August 2019 (Table 1). UHII varied between 0.7°C and 3.5°C during
the observation period (Figure 5).
UHI phenomena was increasingly well pronounced with the onset of sunset across the
study area (Figure 4). Very limited dierence in mean near surface air temperature was
observed along the transect during Traverse 1 which commenced at 20:00 BST, before
sunset on each date. In general, dierences in temperature continued to increase until
becoming steadier by Traverse 4. A notable exception to this occurred in rural obser-
vation points 16–19 (LCZs D, A, B and B respectively), where near-surface air tempera-
ture continued to decrease by a mean of 0.4°C from the previous traverse. By Traverse 5,
the final traverse of the evening and occurring well after sunset, the greatest dierences
were recorded between urban and rural observation points. This demonstrates a level of
delayed heat release from the urban environment, in line with Oke et al. (2017).
Figure 6 provides a breakdown of the ‘local’ UHII by calculating the mean near-surface
air temperature of each LCZ type within the study and highlighting the dierence between
LCZs. It shows that the highest UHI intensities were observed near to Inverness city centre
(LCZs 3, 5 and 6) where anthropogenic activity is most concentrated and building infra-
structure is larger and denser than elsewhere on the transect. LCZ 3 returned the
highest mean air temperature of 14.6°C across the study (Figure 4) with its overall mean
UHII 0.1°C higher than that recorded at the city centre observation point (LCZ 5).
A notable drop of 0.5°C in the mean urban air temperature occurred at observation
points 4 and 5 on the transect. At these locations, the transect passed through trees bound-
ing Inverness Golf Course and surrounding parkland (LCZ 6
B
). A steady decline in temp-
erature was observed as the transect progressed further out of the city towards Inverness
Campus and the city’s ‘green corridor’. At observation points 9 and 10, temperatures were
1.4°C lower than in the city centre. Much of the woodland cover within these areas (LCZ
9
A
) has since been removed and the land redeveloped for housing.
Figure 4. Mean near-surface air temperature profiles for each of the 5 traverses measured along the
study transect route from Inverness, Scotland to the surrounding rural countryside over 30 dates
between 01/06/2019 and 31/08/2019.
Table 1. Statistical analysis of nocturnal near-surface UHI intensity (UHII) study transect results
collected in Inverness between June and August 2019.
Near-surface UHII (°C)
Mean 1.6
Minimum 0.7
Maximum 3.5
Median 1.3
Sample variance 0.5
Skewness 1.4
Standard deviation 0.7
SCOTTISH GEOGRAPHICAL JOURNAL 7
Figure 5. Observed nocturnal near-surface urban heat island intensity in Inverness, Scotland on each
observation date between 01/06/2019 and 31/08/2019.
Figure 6. Near-surface air temperature difference between the local climate zones (LCZs) studied in
Inverness, Scotland over 30 dates between 01/06/2019 and 31/08/2019.
8 G. F. GUNN
Observations within the rural countryside were generally consistent except for obser-
vation point 17 which included an area of dense trees (LCZ A) and was 0.1–0.2°C cooler
than its surrounds.
Relationship between air temperature and pervious surface cover
In the previous section, several examples were highlighted whereby air temperature
varied in response to dierent land uses, particularly woodland and parkland. Whilst
comparing dierences in air temperature with greenspace cover can eectively explain
intra-observation point UHI variability during the summer months (Hamada & Ohta,
2010), changes in plant transpiration rate render this approach less eective year-
round. van Hove et al. (2015) found that comparing dierences in air temperature
with building surface fraction enabled a strong predictor for intra-observation point
UHI variability at any time of year. A linear regression analysis was therefore carried
out between the air temperature and the fraction of pervious surfaces within respective
LCZs, based on data from Morton et al. (2020). Figure 7 illustrates the result of the linear
regression covering the full duration of the study and indicates a strong relationship
between air temperature and land use (R
2
= 0.92). A two-tailed t-test confirms that
this relationship is significant (p = .008352) at an alpha level of .05.
Understanding the impact of meteorological conditions
Inverness city is subjected to meteorological conditions arising from its proximity to
the coast, the River Ness, and wider situation within the Great Glen valley. It is
Figure 7. Relationship between mean near-surface air temperature and fraction of impervious surface
within 250 m of the observation points studied, by local climate zone (LCZ).
SCOTTISH GEOGRAPHICAL JOURNAL 9
therefore necessary to consider the how these conditions may have impacted the
results of the study. Meteorological reports from Munlochy PWS (Met Oce ID:
882856001), located at a semi-rural site approximately 8 km NNW of the city
centre (Figure 1), were obtained post hoc and analysed alongside field notes for this
purpose (Table 2).
On dates with a high UHII (UHII > 1.6°C), mean wind speed did not exceed 1.3 ms
1
.
This is consistent with findings at coastal locations elsewhere (Kolokotsa et al., 2009;
Pinho & Orgaz, 2000). On these dates wind direction was generally variable, owing to
the low wind speed.
Mean wind speeds were often higher on low UHII dates (UHII < 1.6°C), reaching up
to 3.1 ms
1
. When analysed using linear regression, there appears to be a weak negative
relationship between mean wind speed and UHII (R
2
= 0.03) (Figure 8a). However, appli-
cation of a two-tailed t-test confirms that this relationship is not significant (p = .56826)
at an alpha level of .05.
Murphy et al. (2011) found that prevailing wind direction can have a strong inuence
on UHII where a settlement is located near to a coastal location and vulnerable to sea
breeze eects. No real trend was determined between UHII and wind direction across
the observation period. Wind direction varied principally between the two prevailing
directions and ‘variable’ owing to lower wind speeds.
Cloud base height and cloud cover were variable throughout the observation period.
The highest recorded UHIIs occurred on evenings with limited cloud cover and a high
cloud base height. A positive relationship between UHII and cloud base height exists
upon linear regression analysis (R
2
= 0.25), which is significant (p < .001) at an alpha
level of .05 on application of a two-tailed t-test (Figure 8b). Cloud cover on all high
UHII dates was non-existent to few (1-2 oktas), with 75 per cent of dates exhibiting
no cloud. This is likely due to the occurrence of enhanced nocturnal radiative cooling
in rural areas where there is a lack of cloud cover (Morris et al., 2001). These results
therefore compound understanding of fundamental physical eects associated with
cloud cover and air temperature, namely the greenhouse eect and turbulent mixing
by thermals.
Several studies identified a negative relationship between UHII and relative humidity
(RH), whereby as RH increases the UHII decreases (Eastin et al., 2018; Kim & Baik,
2002). Linear regression of RH and UHII on each date of the study revealed a similar
negative relationship (R
2
= 0.06), confirmed as significant (p < .001) at an alpha level of
.05 within a two-tailed t-test. (Figure 8c). Thus, in practice, it might be expected that
UHII would be lower after a period of rainfall owing to increased evaporative cooling
of the urban surface. Conversely, often the dominance of impermeable materials
within the urban area leads to reduced evaporative cooling due to their provision of
prompt surface water runo. It is therefore not surprising that Kim and Baik (2002,
p. 655) regard RH as the ‘least important predictor’ of UHII.
Frontal passages, which involve the movement of air masses associated with weather
fronts, can potentially impact UHII (Gedzelman et al., 2003; Szymanowski, 2005).
During frontal passages, the arrival of cooler air and changes in wind patterns can
lead to reductions in UHII by providing greater ventilation and cooling eects. Eastin
et al. (2018) built on earlier fundamental understanding of UHI dynamics and concluded
that dates without frontal passages are optimal for higher magnitude UHI development.
10 G. F. GUNN
Table 2. Summary of wind and cloud cover conditions at Inverness during the study period.
Observation
date
Maximum UHII (°
C)
Mean windspeed
(ms
1
)
Dominant wind
direction
Cloud
cover
Cloud base height
(m)
Mean relative humidity
(%)
Precipitation
(mm)
Frontal
passage
01/06/2019 3.5 0.6 Variable No cloud n/a 86.5 0.0 None
02/06/2019 2.8 0.8 SW Few 400 86.5 1.0 Warm
08/06/2019 1.5 0.8 Variable Few 1,600 93.6 1.5 None
09/06/2019 1.3 0.5 Variable Broken 400 93.9 0.7 None
15/06/2019 2.2 1.2 Variable No cloud n/a 71.7 0.0 None
16/06/2019 1.8 1.0 SE No cloud n/a 73.3 0.0 None
22/06/2019 2.9 0.9 WNW Few 2,400 86.0 0.8 None
23/06/2019 1.2 0.9 Variable Few 600 91.5 0.0 None
29/06/2019 1.3 1.0 ENE Overcast 0 96.2 0.5 None
30/06/2019 1.3 3.1 WSW Scattered 1,200 77.0 0.0 Occluded
05/07/2019 1.3 2.2 W Broken 1,200 79.6 0.0 None
06/07/2019 1.4 0.9 Variable Scattered 1,200 82.5 0.0 None
14/07/2019 1.9 1.2 Variable No cloud n/a 81.0 0.0 None
15/07/2019 1.4 1.9 ESE Overcast 400 91.2 0.0 None
20/07/2019 1.3 1.6 WSW Few 400 78.5 0.0 None
21/07/2019 1.3 0.9 Variable Broken 1,000 85.4 0.0 Warm
27/07/2019 1.6 2.0 Variable No cloud n/a 88.6 0.0 None
28/07/2019 2.0 1.9 ENE No cloud n/a 91.6 0.0 None
30/07/2019 1.2 1.2 Variable Overcast 100 95.2 1.0 None
31/07/2019 0.8 0.5 Variable Overcast 600 96.6 1.0 None
03/08/2019 2.9 1.3 ENE No cloud n/a 85.9 0.0 None
04/08/2019 1.4 1.3 Variable Few 300 90.0 0.0 None
10/08/2019 1.3 1.2 Variable Overcast 600 84.8 0.0 None
11/08/2019 0.9 0.2 Variable Broken 400 93.6 0.3 None
17/08/2019 0.7 2.4 SW Scattered 400 79.8 0.0 None
18/08/2019 0.7 1.9 WSW Scattered 300 86.6 0.0 None
24/08/2019 1.3 0.3 Variable Few 1,400 92.3 0.0 None
25/08/2019 1.4 0.5 Variable No cloud n/a 90.2 0.0 None
30/08/2019 1.3 0.5 Variable Few 600 91.8 0.2 None
31/08/2019 0.9 1.8 W Broken 300 93.4 0.8 None
Note: Created using data from field notes and hourly reporting obtained post hoc from Munlochy PWS (Met Office ID: 882856001).
SCOTTISH GEOGRAPHICAL JOURNAL 11
During the observation period, only 2 frontal passages were experienced (Table 2). 1 of
these passages was on a high UHII date (UHII >1.6°C), and 1 was below this threshold.
Therefore, it is challenging to make definitive conclusions as to their impact on UHII
dynamics during this study.
Conditions with no precipitation have previously been found to be optimal for UHI
development (Eastin et al., 2018; Hartley, 1977). Precipitation was consistently very
low throughout the observation period. Trace levels of precipitation were observed on
9 of the 30 dates studied, with the highest recorded level reaching 1.5 mm. Such levels
made no discernible impact on UHII observed.
Comparison with other settlements
The majority of UHI publications address the eect in larger settlements (population
>100,000), where the impacts are often more pronounced. However, it is important to
acknowledge that other factors such as weather and climate, local physical geography,
and urban form are of relevance in inter-settlement UHI variability comparison. Not
all studies consider the UHI eect during the summer months which introduces a limit-
ation to the comparisons made by this study (Table 3). Therefore, whilst comparisons are
made here between this study and others, it should be considered that the UHII may be
dierent during other seasons. This is due to reduced solar insolation, increased heating
demand and waste heat generation in urban areas, and greater presence of temperature
inversions.
Figure 8. Relationship between daily nocturnal near-surface urban heat island intensity and (a) wind
speed, (b) cloud base height, and (c) mean relative humidity on each of the 30 dates studied between
01/06/2019 and 31/08/2019.
12 G. F. GUNN
The closest comparator study in terms of population size is that of Hania, Greece
(population: 53,000). Both cities have a significant coastal inuence, but there are
notable distinctions in terms of climate and built-up area. Hania experiences a hot-
summer Mediterranean (Köppen Csa) climate and has a greater built-up area compared
to Inverness. The study conducted in Hania during the summer months registered a
mean UHII of 1°C higher than that observed in Inverness. With due consideration
given to Hania’s climate and built form, the UHII value is a useful comparator in
terms of settlement size.
The cities of Fairbanks, USA (64°N) and Brandon, Canada (50°N) are the closest small
settlement comparators in terms of latitude. Both settlements have approximately half
the population of Inverness and are located inland without significant coastal meteoro-
logical inuence. Consequently, it could be anticipated these settlements would exhibit a
stronger UHI signal. This is true in the case of Brandon where the summertime mean
UHII of 3.2°C is significantly higher than that observed in Inverness. The long-term,
year-round UHII of Fairbanks is noted as 0.3°C. Fairbanks experiences a multitude of
Table 3. Review of near-surface UHI intensity (UHII) in selected cities, by population, latitude, Köppen
climate classification and season recorded.
Settlement Study Population Latitude
Köppen climate
classification Season UHII (°C)
Ipora, Brazil Alves and Lopes (2017) 31,274 16°S Aw Autumn 3.0
Hania, Greece Kolokotsa et al. (2009) 53,000 35°N Csa Summer 2.6
Glasgow,
Scotland
Hartley (1977) 940,400 55°N Cfb Autumn 8.3
(maximum)
Fairbanks, USA Magee et al. (1999) 31,427 64°N Dfb All 0.3
Brandon,
Canada
Suckling (1981) 37,000 50°N Dfb Spring/
Summer
3.2
Aveiro, Portugal Pinho and Orgaz (2000) 38,000 39°N Csb Autumn 7.5
(maximum)
Ljutomer,
Slovenia
Ivajnšič et al. (2014) 3,270 46°N Cfb Winter 1.0
Agrinio, Greece Vardoulakis et al.
(2013)
93,000 38°N Csa Summer 3.8
Geisenheim,
Germany
Diesnst et al. (2018) 11,500 50°N Cfb Summer 0.8
Dublin, Ireland Graham (1993) 931,582 53°N Cfb Summer 8.0
(maximum)
Bodø, Norway Miles et al. (2023) 52,000 67°N Cfb/Dfb Summer 0.7
Melbourne,
Australia
Wai et al. (2022) 4,963,000 38°S Cfb All 1.0
Glasgow,
Scotland
Hartley (1977) 940,400 55°N Cfb Autumn 8.3
(maximum)
Manchester,
England
Levermore et al. (2015) 2,685,400 53°N Cfb Winter 6.0
(maximum)
Seattle, USA Ramamurthy and
Sangobanwo (2016)
710,687 48°N Cfb Summer 3.4
Stuttgart,
Germany
Ketterer and Matzarakis
(2015)
600,000 49°N Cfb Summer 5.4
Utrecht,
Netherlands
Brandsman and
Wolters (2012)
311,000 52°N Cfb All 1.5
Portland, USA Hart and Sailor (2009) 549,781 46°N Cfb Summer 2.0
Reading,
England
Nicholson (2020) 160,000 51°N Cfb Summer 1.0
Inverness,
Scotland
Current study 63,220 57°N Cfb Summer 1.6
Note: Mean UHII is presented unless otherwise stated and rounding applied for consistency.
SCOTTISH GEOGRAPHICAL JOURNAL 13
confounding meteorological conditions including strong temperature inversions and
chinook winds. Notwithstanding this, the low UHII of Fairbanks may be considered
as a minimum value from which to validate the results from Inverness.
Settlements with a temperate oceanic (Köppen Cfb) climate, though varying in popu-
lation size, share similar summertime nocturnal UHII intensities to Inverness (Table 2).
These include Geisenheim and Stuttgart (Germany), Melbourne (Australia), Seattle and
Portland (USA), and Reading (UK). Collective analysis of these studies returns a mean
UHII of 2.3°C. It is important to note that while settlements in this climate classification
share a similar UHII, variations in local physical geography may inuence slight dier-
ences in UHII magnitude. For example, these comparator settlements have relatively
atter surrounding landscapes or diering extents of hill/valley topography which can
aect air movement and temperature distribution. Coastal situations also dier, particu-
larly in the case of Reading which lacks a direct coastal inuence. Inverness is also a con-
siderably smaller settlement in terms of urban centre size and scale. Building height and
density is low, potentially leading to less heat storage within the urban area. However,
based on this analysis, it could be concluded that the Inverness experiences a slightly
below average UHII when compared to other settlements with the same Köppen
climate classification.
Implications
Whilst a mean UHII of 1.6°C is low in comparison with values experienced in other
settlements, it is a cause for concern. The result is an indication of the impact that urban-
isation has had in the city of Inverness and is a direct consequence of national settlement
planning policy.
It is expected that the UHII experienced in other seasons may dier from these
findings recorded during the summer months. This study provides valuable initial
insights into the UHI eect in Inverness, which can serve as a basis for more extensive
investigations. Suggestions for this are given for this in the following section.
Limitations and future work
A limited period for observations was available owing to the research taking place as
part of an undergraduate dissertation project. It was therefore necessary to maximise
the available research time by focussing the research to nocturnal UHI eects during
the summer months of June, July & August 2019. Future studies may wish to
expand this using a similar method at dierent times of day, and during dierent
seasons, to yield results which would enable interesting comparisons of UHI behav-
iour in Inverness. Such further research is necessary to better inform development
of a mechanism accounting for the climate impact of development proposals,
currently absent from Scotland’s present national planning framework (Scottish
Government, 2023).
The lack of nearby reputable fixed weather station data in Inverness may be overcome
by setting up a network of fixed weather stations to simultaneously measure the UHI
eect across a range of LCZs, over a larger area. Alternatively, a ‘citizen science’
project could be explored, either using sensors axed to private vehicles or public
14 G. F. GUNN
transport, or home weather stations. Both methods would require that a high level of
attention be given to sensor calibration, and, in the case of the latter method, cleaning
and verification of data.
Concluding remarks
This study has identified the presence of near-surface UHI phenomena in the small, mid-
latitude city of Inverness during the summer months. A mean UHII of 1.6°C (with a
maximum of 3.5°C) was observed.
Land use and prevailing meteorological conditions have a significant impact on the
development of UHI eects in Inverness. More heavily built-up areas with a lack of per-
vious surfaces returned the highest UHI intensities. Conversely, areas of large urban
greenspace were found to partially oset the UHI with a cooling eect. Whilst UHI
phenomena was recorded on all dates of the study, the highest UHI intensities were
recorded on dates with up to 1–2 oktas of cloud cover, low relative humidity, and
high cloud base height. The city of Inverness compares closely with other, larger, settle-
ments at a variety of latitudes sharing the same Köppen climate classification.
It is recommended that further research be undertaken to determine the scale and
behaviour of UHI phenomena in Inverness over a wider time period and across
diering local climate zones. The results identify a clear need for action to prevent devel-
opment of UHI eects through the national planning framework and such policy changes
should be informed by research.
Acknowledgements
This research originates from a final year BSc (Hons) Geography dissertation project at the Uni-
versity of the Highlands and Islands, supervised by Dr Anne-Marie Nuttall. Thanks are oered to
Dr Eddy Graham for his thoughts and detailed explanation of his related (1993) Dublin study. The
author would also like to thank Gary Johnston for collation and provision of data from Munlochy
PWS. Acknowledgements are extended to Dr Stuart Black and Tim Stott at the Highland Council
Planning Service for their support and interest in the project.
Disclosure statement
No potential conict of interest was reported by the author(s).
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