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Urban Heat island in a Coastal City Interlaced by Wetlands

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The most pronounced effect of urbanisation on the microclimate of a region is the development of the Urban Heat Island (UHI), which is the elevation of air temperature within the region with respect to adjoining rural areas. There are many factors like coverage area, vegetation, soil moisture, street geometry, building materials, etc., which cause or modify the intensity of UHI. The UHI in Kochi, a fast growing urban region in coastal South India, interlaced by a network of canals and wetlands which are part of the Vembanad Lake system, was investigated. The UHI during summer and winter periods were recorded through mobile traverses. The intensity of the heat island during summer was 2.2 K and during winter was 2.8 K. The heating and cooling rates in different locations within the region were also derived through stationary recorders installed at selected locations. The intensity of the UHI here is moderate compared to that observed in other cities in the region. It is inferred that the wetlands is controlling the intensity of the Urban Heat Island here.
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February 2011, Volume 5, No.2 (Serial No.39)
Journal of Environmental Science and Engineering, ISSN 1934-8932, USA
Urban Heat Island in a Tropical City Interlaced by
Wetlands
G. Thomas and E.J. Zachariah
Atmospheric Sciences Division, Centre for Earth Science Studies, PB 7250, Thuruvikkal PO, Thiruvananthapuram 695031. India
Received: August 4, 2010 / Accepted: September 25, 2010 / Published: February 20, 2011.
Abstract: The most pronounced effect of urbanisation on the microclimate of a region is the development of the Urban Heat Island
(UHI), which is the elevation of air temperature within the region with respect to adjoining rural areas. There are many factors like
coverage area, vegetation, soil moisture, street geometry, building materials, etc., which cause or modify the intensity of UHI. The UHI
in Kochi, a fast growing urban region in coastal South India, interlaced by a network of canals and wetlands which are part of the
Vembanad Lake system, was investigated. The UHI during summer and winter periods were recorded through mobile traverses. The
intensity of the heat island during summer was 2.2 K and during winter was 2.8 K. The heating and cooling rates in different locations
within the region were also derived through stationary recorders installed at selected locations. The intensity of the UHI here is
moderate compared to that observed in other cities in the region. It is inferred that the wetlands is controlling the intensity of the Urban
Heat Island here.
Key words: Urban heat island, air temperature, tropical city, cooling rate.
1. Introduction
A large proportion of the global population lives in
urban regions and the trend is intensifying. The
percentage of global urban population is expected to
reach 60% by the year 2030. The urban population in
India is projected to increase from its 2008 value of
30% of the total to 40% by the year 2030. The urban
climate and the impact of urban growth on the climate
thus affect vast majority of global population.
Differences in the surface materials between a city
and the surrounding rural area lead to different climates
within a city and its adjoining rural areas. The warmer
temperature in the urban setting, a feature known as the
Urban Heat Island (UHI), is one of the well
documented urban modifications to the climate [1].
High rate of urbanization have resulted in drastic
Corresponding author: G. Thomas (1983- ), male, M.Sc.,
research fields: urban climate, greenhouse gases. E-mail:
geonce@gmail.com.
E.J. Zachariah (1954- ), male, Ph.D., scientist, research fields:
urban climate, greenhouse gases, renewable energy, radiowave
propagation. E-mail: ejzach@vsnl.com.
demographic, economic, land use, and climate
modifications in the urban regions. The growth and
expansion of urban centers leads to the construction of
new roads, buildings, and other various human made
structures, replacing the natural ground cover and
landscape. This urbanisation of the natural landscape
can have profound meteorological impacts causing
UHI of the order of 2-10 K.
Study of the urban heat island has often been
accomplished through measurements, usually with
mobile thermometer [2-5]. These short term studies are
useful in determining specific relationships between
urban temperature and meteorological conditions. UHI
develops through a difference in cooling rate between
the urban area and its rural surroundings. The studies of
cooling rates in the urban and rural regions [6, 7] have
shown that they differ and that this difference itself
varies as night advances.
Urban geometry is one of the most important factors
causing urban-rural and intra-urban air temperature
differences. The sky view factor is understood to be
Urban Heat Island in a Tropical City Interlaced by Wetlands
2
another major factor leading to the urban and rural
temperature differences. Other contributing factors
include reduction in evaporational cooling, the heat
storage of the building and pavement etc. Urban areas
are not homogenous. The heat accumulation capacity
of the building mass and the ground cover is large. This
heat accumulated during the day time is emitted at
night irradiating the surroundings causing delay in
cooling of the atmosphere [8]. The reduction of the
natural cooling of the city by mutual screening and
reflection by buildings etcetera amplify the intensity of
heat island.
Vegetation cover, which include trees, shrubs, and
other plants which can provide shade for building by
intercepting solar radiation and cool the air by
evapotranspiration of absorbed ground water through
their leaves. Water bodies such as lakes, rivers also
modify the UHI. Water has high specific heat capacity
leading to lower final temperatures than dry soil when
exposed to solar radiation.
Metabolic heat produced by human bodies also
contributes to UHI. The average rate of metabolic heat
generation by a relaxed person at home could be
between 100 to 120 W and that by a sleeping human
body is 70 W. The large population density in the urban
region makes the total amount of heat thrown out by
the human bodies be significant in terms of the heat
budget.
The effect of substitution of natural vegetation with
construction materials such as those used in buildings
and roads which are largely impermeable and which
have different thermal properties, are very complex [9].
Buildings also alter the reception of solar radiation
casting shadows, and change surface roughness and
local wind field. Most of the materials used in the
construction provide a low albedo surface, resulting in
increased absorption of solar radiation at day time.
After the sun set, the pavements and buildings will
slowly begin to release the stored heat energy it
accumulated throughout the day. However as the
buildings and pavement start to cool off, the air around
them begin to heat up, consequently maintaining
elevated temperature into the night.
The heat generated artificially by industrial,
domestic, traffic combustion process and other human
sources is also a contributing factor for the UHI.
The impact of these factors varies between cities
because the geographic setting, urban morphology,
patterns of urban use, and available construction
materials vary. City size as measured by the urban
population is seen to relate directly on UHI. Since
heat-island formation is due to human activities, it can
be reduced, or intensified by man’s effort. Studies
show that wise land use could decrease the effect of
heat islands [10].
2. Materials and Methods
The study area is Kochi urban agglomerate which
includes Kochi, Mattancheri, Aluva, Thrippunithura,
North Paravur and Perumbavoor Municipal areas,
located on the southwest coast of India between 090 45’
N and 100 20’ N latitude and between 760 10’ E and 760
35’ E longitude. To the west of this area lies the
Arabian Sea. The city is interlaced by estuaries. Much
of Kochi lies at sea level, with a coastline of 48 km.
The water-table is shallow resulting in high soil
moisture concentration. Soil in the coastal regions is
mostly ‘loamy sand’, with very rapid permeability. Soil
in the eastern regions is ‘gravelly clay’ with rapid
permeability on the top layers and ‘laterite’ with
moderate permeability underneath.
A sketch map of the study area is shown in the Fig.
1. Air temperature recording was done at five locations
in Ernakulam (Ekm) City and adjoining areas. These
locations are Ekm City, Thrikkakara, Mulanthuruthy,
North Paravur, and Pempavoor. The installations were
in accordance with the guidelines prescribed by the
WMO for urban meteorological observations [11, 12].
Table 1 gives the geographical and physical details
relating to these five locations.
Automatic temperature recorders with 0.1 K
resolution, calibrated in the temperature range 20 K to
Urban Heat Island in a Tropical City Interlaced by Wetlands
3
Fig. 1 Sketch Map of Study area.
40 K with ± 0.17 K uncertainty, were used. The
recording interval is programmable and could be set
between 2 sec and 12 hours. The data is recorded with a
time and date stamp.
Mobile traverses were done during the winter (in the
month of January) and summer (in the month of
March) periods. From may onwards, the region gets
pre-monsoon showers and Summer Monsoon normally
sets in by 1st June. Temperature data were collected
using mobile traverse method. Automatic temperature
recorder was installed inside a Stevenson’s screen on a
vehicle, taking care to keep away from the engine heat.
The vehicle was stopped for 2 minutes at each point on
the survey route. The temperature recorder was set to
log temperature data along with time stamp
automatically at 5 second interval.
Observational points were so chosen as to ensure
adequate representation of all part of the city and the
adjacent rural areas. At each point of observation, time
of observation was noted with the help of chronometer
synchronized with the clock in the temperature
recorder.
The coordinates of the locations were obtained with
a GPS. The time stamp of the temperature data could
thus be used for identifying its location and
coordinates. Days with clear sky and calm winds were
chosen in which the urban influence is presumed to be
most pronounced. Observations were carried out
during 07:00 to 10:00 PM when UHI continues to
retain the air temperature high and the city remains
active. Though UHI could become more pronounced as
night wears out into pre-dawn hours, the urban air also
would have cooled enough to make the impact of the
UHI less significant.
The western region of the city is interlaced by
wetlands. Fig. 2 is a plot of the area showing the
relative percentage of water cover to total area. It is
also noted that the densely built up region of the city
falls in this area.
The reference temperature was taken from a
temperature recorder installed in Stevenson’s screen
near Thrikkakara, a suburban location in the area. The
instantaneous temperature difference between all
observational points and the reference site was
calculated in order to determine instantaneous
temperature difference. The intensity of heat island
depends up on the cooling rate of urban and rural
environment. Fixed air temperature recorders were
installed in five locations in the Ernakulam city and the
rural areas. The observation point were chosen for
representative data from different regions of varying
Table 1 Locations where continuous monitoring of air temperature was done.
Location Soil type Immediate neighborhood
Ernakulam
(09:57 N/76:17 E) Loamy Sand, with rapid to very rapid permeability Dense urban center, with mixed low and high-
rise
buildings.
Thrikkakara
(10:03 N/76:21 E)
Gravelly Clay with rapid permeability on the surface.
Laterite with moderate permeability, below.
Suburban area with open space. Ground cover by grass
and shrubs.
Mulanthuruthy
(09:54 N/76:23 E)
Gravelly Clay with rapid permeability on the surface.
Laterite with moderate permeability, below. Semi-
urban area with medium building density, and
paved roads. Ground cover by grass and shrubs.
North Paravur
(10:08 N/76:13 E) Loamy Sand, with rapid to very rapid permeability Semi-
urban area with medium building density, and
paved roads. Interspersed by large trees. Poor ground
cover by shrubs.
Perumpavoor
(10:07 N/76:28 E)
Gravelly Clay with rapid permeability on the surface.
Laterite with moderate permeability, below.
Suburban area with open space. Ground cover by grass
and shrubs.
Urban Heat Island in a Tropical City Interlaced by Wetlands
4
Fig. 2 Percentage of water cover over study area.
levels of urbanisation, land cover, coverage areas, etc.,
as well as the rural areas. The average warming and
cooling rates at the different locations where fixed
temperature recorders were installed was also
computed for different periods of the day.
3. Results and Discussion
The existence of a moderate urban heat island is seen
from the plots. It is interesting to note that the winter
UHI is only slightly higher at 2.8 K than the summer
UHI at 2.2 K. Rural areas are typically cooler than the
urban during both seasons.
The growth and the intensity of heat island depend
up on the cooling rate at urban and rural environment.
Monthly average cooling rates for one year at five
stations mentioned above are given in Fig. 3. The
cooling rates for different period for each day were
tabulated separately. The outlying values were
eliminated by statistical method and monthly average
for each period was calculated. It is seen that lowest
cooling rate is observed at the city centre, where as
highest rate is observed at Thrikkakara and
Perumpavoor. It is also noted that during the rainy
season when the entire area is uniformly wet,
urban-rural differences appear to have minimized. The
lower day-night temperature fluctuations due to cloud
cover during monsoon months also contribute to the
lower cooling rates observed during the period.
However, during dry winter months, the cooling rate
at urban centre is significantly lower than that in the
rural area.
Monthly average rainfall recorded in the district
during the one year period for which cooling curves are
presented in Fig. 3, is given in Fig. 4. Due to the
Fig. 3 Monthly average cooling rates at different locations
in the study area.
Urban Heat Island in a Tropical City Interlaced by Wetlands
5
Fig. 4 Monthly rainfall recorded in the district.
occurrence of summer monsoons the summer months
from May onwards are very wet here. June-August
receives highest rainfall. The sky would be heavily
overcast during this period and air temperature falls
significantly, and day-night temperature fluctuation
becomes low.
The average diurnal variation of air temperature at
these five locations during January and March are
given in Fig. 5 and Fig. 6. The nighttime highest
temperature is always observed in the, though this is
not applicable during daytime. This agrees with the
general behavior of cities, due to reduced sky view
factor.
Most intense cooling in winter occurs just after
sunset, 1.6 K/h in the open area and ranging between
0.7 to 1.5 K/h in other locations. As night advances,
cooling rate as well as differences between the sites
reduce. It is seen that the summer cooling rates were
similar in all location with a maximum of 0.7 K/h.
It is seen from Fig. 3 that the rate of cooling varies as
night advances. In winter the sites cooled rapidly
around sunset and the cooling slowly decrease during
the rest of the night. During the summer the cooling
was less intense, with an almost constant rate during
the entire night. The difference of intra-urban surface
air temperature have been shown to be strongly
dependant up on the difference in building density, and
geometry, as well as land cover and land use [13, 14].
On average there was 2.5 K temperature difference
between the open area and built up region.
Water cover percentage in the study area range up to
Fig. 5 Average air temperature in January.
Fig. 6 Average air temperature in March.
Fig. 7 UHI in Kochi during winter.
66%, as seen from Fig. 2. The winter and summer
UHIs have been plotted and given in Fig. 7 and Fig. 8.
This large water mass act as a huge thermal storage,
Urban Heat Island in a Tropical City Interlaced by Wetlands
6
modifying the UHI here. During the heating mode this
water body absorbs a large
Fig. 8 UHI in Kochi during summer.
amount of heat resulting in a lower maximum
temperature, because specific heat capacity of water is
higher than that of dry soil. Evaporation from water
surface will take away part of the heat from
surrounding air and thereby enhance the cooling mode.
The high latent heat of vaporization of water as well as
the high heat capacity of moist air also may enhance
the cooling mode further.
It may be noted that the average UHI here is only 2.5
K, where as in other coastal cities in the region, like
Madras (Chennai), Calcutta (Kolkata),
Visakhapatanam, etc., UHI intensity of 4 K and above
have been reported. In cities located in continental
interiors like Bhopal, Delhi, Pune etc., UHI intensity in
the range of 6-10 K have been observed [15-18]. It is
noted that the summer and winter UHI in Kochi are not
very significantly different. This could be due to the
presence of large water mass having high specific heat
capacity, moderating the heating phase and offering a
large water surface for evapocooling during the cooling
phase.
4. Conclusions
The intensity of heat island in the tropical coastal
city of Kochi in South India was studied. The UHI
intensity observed here is 2.8 K in winter and 2.2 K in
summer. The growth and intensity of heat island is
found to depend on the cooling rates at the urban and
rural environments. The lowest cooling rate in winter,
0.7 K/hr, is observed in the city centre whereas that in
the rural area during same period was 1.6 K/hr.
The UHI intensity in Kochi, is seen to be
substantially lower than that in other coastal cities in
the region. It is inferred that the canals and wetlands
which lie within the city have played a significant role
in moderating the heat island intensity here.
Acknowledgments
The authors are grateful to the Director and Research
Council of Centre for Earth Science Studies for the
support extended for this study. The authors are also
thankful to Dr. P.V.S.S.K Vinayak and Mr. C.J. Johny
for their valuable co-operation.
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