

The Urban Heat Island (UHI) Phenomenon
Urbanization is the main anthropogenic process responsible for radical changes in the nature of the atmospheric and surface characteristics of a region. The distinctive biophysical features of the urban areas might be expected to be the main causes that amplify the effects of climate change (Figure 1) and cause the climate of urban areas to behave more abnormally.

As a consequence of continuous altering of the natural land cover to more man-made patterns, there is a modification of the radiative fluxes, namely change thermal properties of surfaces and trapped fluxes due to multiple reflections. Consequently, the solar radiation and hydrologic balances are dislocated (Figure 2), which in turn increases urban-rural contrast in air temperatures and surface radiance (Brazel et al., 2000). These differences in air temperature between an urban area and its surrounding area are called the Urban Heat Island phenomenon (UHI) (Oke, 1987).

UHI phenomenon has been confirmed by numerous studies to have a role in climatic change in most cities and is considered to be one of climatic phenomena that require more investigation as part of global efforts to address climate change and environmental degradation (Changnon, 1992; Klysik and Fortuniak, 1999; Brazel et al., 2000; Comrie, 2000;
Weng, 2003; Serra, 2007; Gaffin et al., 2008).
During daytime at pedestrian level, air temperatures might be lower inside cities than outside it (Johansson, 2006). However, it has been proven in many studies that air temperatures in downtown during the night are generally higher than air temperatures in surrounding rural areas (Meyer, 1991; Oke, 1973; Chapman 2005; Voogt 2004; EPA
2007).
An urban area is often observed to be significantly warmer than those of nearby areas dominated by vegetation, although they have equal exposure to solar energy. The reflection of the solar radiation in urban areas is intensified by the volume of concrete and other man-made materials used for buildings and road construction. These bulks of concrete with their different topographies behave as a good trap for sunlight during the day, and release it into the evening slowly as infrared heat, producing the phenomenon of UHI. Conversely, vegetated areas mostly tend to be covered by trees, It is estimated
that a matured tree has a projection area (canopy) of about 50m2 (Huang et al, 1987). The 50m2 canopy has a high potential to reduce the penetration of solar radiation. These trees absorb the sunlight and release energy by long-wave emission more readily (Lo et al., 1997; Rose and Devadas, 2009).
Large cities are measured regularly during the nighttime to be about 5 to 10 °C warmer than the rural surrounding area (United States Department of Energy, 1996). However, in some cases this temperature discrepancy has been measured to be more than 11°C (EPA, 2009a).On calm and clear nights the differential cooling rates between urban areas and the rural areas are usually most distinct. Accordingly, UHI acts as a model for climate change research due to its role in exacerbating the existing heat island phenomenon in urban areas by absorbing increased solar radiation. Furthermore, the climate modifications due to the effect of UHI that have happened in more cities over the second half of the last century are compatible and have the same trend to some extent with the results obtained from models to project the future climate (Karl et al,1988; IPCC,2001).
The magnitude of UHI is called UHI intensity. UHI intensity tends to vary both hourly and seasonally, and is influenced by factors such as local topography, climate region, city size, density, and geometry, industrial development, land use and land cover (LULC) characteristics, the characteristics of the surrounding rural areas, wind speed and vegetation abundance (Stathopoulou et al.,2005; Santana, 2007;Fortuniak, 2009). Cloud cover and incoming solar radiation also affect UHI intensity but with less significance than LULC characteristics and the abundance of urban vegetation (Fortuniak, 2009; Arrau and Pena, 2010).
Urban Heat Island Types
Surface Urban Heat Islands:
Surface UHI is also called remotely sensed UHI because it is usually observed using infrared data that allow retrieving land surface temperatures. Surface urban heat islands are typically present at daytime and nighttime, but tend to be stronger during the night (Oke, 1982). On sunny and hot summer days, the sun can heat dry, exposed urban surfaces, such as pavements and roofs, to a temperature hotter than the air temperature. Conversely the moist surfaces in rural surroundings remain often close to air temperatures (Berdahl and Bretz, 1997). The average difference in daytime surface temperatures between the urban and rural areas is 5 to 10°C, and the differences in nighttime surface temperatures are typically 10 to 15°C, considered higher than the daytime differences (Voogt and Oke, 2003). Surface UHI varies seasonally, and is usually greater in summer time, due to changes in the incoming solar radiation and drier weather conditions associated with summer in most regions (Oke, 1987).
Atmospheric Urban Heat Islands:
In urban areas the warmer air compared to cooler air in nearby rural areas defines the atmospheric urban heat islands. This heat island was divided into two different types:
1.Canopy layer Urban Heat Islands:
These exist in the layer of air where people live, usually extending from the ground to below the tops of vegetation and roofs.
2.Boundary layer Urban Heat Islands:
This layer begins from the rooftop level and extends up to the point where urban landscapes no longer affect the atmosphere. This region typically extends no longer than (1.5 km) from the surface (Oke, 1982).
Urban Heat Island Formation and Affecting Factors
The formation of UHI is determined by several factors, such as vegetative cover availability, properties of urban materials and geometry as well as the geographical location of cities. Large water bodies or mountainous terrain located near the cities can influence the general wind patterns, which in turn could influence UHI formation (EPA, 2009a).
Vegetation Cover:
Vegetation and open land typically dominate the landscape in most rural areas. However, albedo and emissivity of rural areas are different from those of sealed surfaces. The green cover generally helps to lower the surface temperature by providing shade and also helps to reduce the air temperatures through evapotranspiration (EPA, 2009a). During evapotranspiration, vegetation releases water vapor into surrounding air that contributes to reducing air temperature. Conversely, urban areas are characterized by being covered by impervious and dry surfaces, such as conventional roofs, roads and sidewalks. As a result of city development, more vegetation is removed and replaced by more sealed surfaces (i.e. buildings and paving). Any replacement of vegetation with building leads to less moisture and high surface runoff (Figure3).

Urban Materials Properties
During the daytime the buildings in the city core absorb and store double the amount of heat of their rural surroundings (EPA, 2009a). Building materials such as stone and steel have higher heat capacity than rural materials, such as dry soil and sand. As a result, the downtown areas in the cities have more efficiency (thermal inertia) than rural surroundings in storing the heat of sun energy inside their infrastructure (Christen and Vogt. 2004).
Solar reflectance, or albedo, is another property that influences heat island development. The highest percentage of solar energy reflected by a surface is found in the visible wavelengths; therefore solar reflectance is correlated with the color of materials. Brighter surfaces tend to have higher reflectance values than darker surfaces.
In case of urban areas, most of their surface materials (such as paved roads and building roofs) have lower effective albedo compared to those in rural areas. Beside the multiple reflections that traps long-wave radiation in the street canyons; consequently built-up areas reflect less and absorb more of the sun’s energy (Coutts et al, 2007). The consequence of low effective albedo in urban areas leads to increased surface temperatures and contributes in the formation of surface and atmospheric urban heat islands. The other important factor of materials influencing the formation of UHI is the emissivity.
Surfaces with high emissivity values might stay cooler, due to their higher efficiency in releasing heat (EPA, 2009a).
Urban Geometry
Urban geometry refers to the dimensions and spacing of buildings within a city. Urban geometry influences energy absorption, wind flow, and the surface’s ability to emit long- wave radiation back to the sky. The effect of urban geometry is very obvious during the nighttime, since in most developed areas, structures and surfaces are obstructed by objects, such as neighboring buildings; as a result, these areas become a large thermal masses that cannot release their heat readily during the night due to these obstructions (EPA, 2009a). Surface geometry obstructs the sky (due to buildings and related objects) from the urban surface; this effect has been called the sky view factor (SVF) (Figure 4). The surface geometry and surface building materials properties together are considered to be the primary surface controls for most UHIs.

Where
a: the point where the proportions of the sky are visible to the overall sky dome.
SVFa = the sky view factor at point a.
X= the radiation emitted by a planar surface.
Y= the radiation emitted by the entire hemispheric environment (Watson and Johnson, 1987). The reduced SVF of many urban surfaces, particularly those on the ground among buildings, prevent the loss of heat by radiation, due to the cold radiate sky being replaced by relatively
warm surfaces of buildings. Furthermore, reduced surface geometry might provide a sheltering effect that limits convective heat losses. The low values of SVF might limit the energy to enter an area but mostly any reduction in the SVF value is almost always accompanied by increase in UHI intensity (Zhanget al., 2012).
Atmospheric Factors
When wind speeds increase, turbulent mixing exponentially reduces the differences in air temperature near the surface (EPA, 2005a) (Figure 5). Moreover, the atmospheric humidity might reduce the net potential radiative cooling of the surface, therefore high atmospheric humidity is likely to increase the heat island intensity (Voogt, 2002). Low humidity areas such as high elevation and desert locations could generate large air temperature drops. In other words, temperature differences of about 40 °C were measured of the thermally insulated approximate black bodies in the Atacama Desert in Chile (Eriksson and Granqvist, 1982).

Impact of the Urban Heat Island
Some positive impacts could result from UHI, such as reductions in energy required for heating, the melting of ice on roads during the winter and lengthening the growing season in the city. Regardless of these positive impacts, the consequences of UHI phenomenon are more perceived on the environment and human health (Akbari, 2005). The most frequent negative impacts in urban areas are:
Energy Consumption
UHI might increase the energy demand for cooling and more pressure to be added to the electricity grid due to most buildings and houses running cooling systems to reduce the indoor air temperature particularly during extreme heat events. Akbari (2005) found that the electric demand increases 1.5 to 2 percent for every 1°F (0.6°C) increase in summertime temperature in Chicago City. Over the last several decades downtown temperatures have been increasing notably, resulting in a 5 to 10 percent increase in community-wide demand for electricity to compensate for the heat island effect (EPA, 2009a).
Fossil Fuel and Air Quality
To accommodate the increased temperature during a dry summer within a city, more energy was consumed by large cities for cooling .Fossil fuels remain the most common source of electricity production worldwide(Chow et al, 2003). The high levels of air pollution and greenhouse gas emissions throughout the world are clearly correlated with the combustion of fossil fuels (Le Treut et al, 2007). Accordingly, pollutants from most power plants form nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO), and mercury (Hg). Most of these pollutants are harmful to human health and participate in complex air quality problems such as acid rain (Ed. Hatier, 1993). Fossil- fuel-powered plants participate in global climate change by emitting greenhouse gases, particularly carbon dioxide (CO2) (EPA, 2009a).
Human Health
The nighttime atmospheric heat islands might lead to serious health implications for urban residents in the case of heat waves (Kalkstein, 1991). Respiratory difficulties, general
discomfort, heat cramps and exhaustion, and heat-related mortality are the most common health problems related to daytime increase of air temperatures and reduction in outdoor nighttime cooling (EPA, 2009a).
Quality of Water
Water quality could be degraded by surface urban heat island through thermal pollutants. High rooftop and pavement surface temperatures could heat storm water runoff. James, (2002) has conducted tests that revealed that pavements that are 100°F (38°C) can raise initial rainwater temperature from roughly 70°F (21°C) to over 95ºF (35°C). Accordingly, the heated storm water generally becomes runoff, which drains into storm sewers and elevates water temperatures as it is released into ponds, streams, rivers, and lakes. However, increased run- off temperature could be stressful for aquatic live, for example brook trout experience thermal shock and stress when the water temperature changes in a day by more than (1-2°C) (EPA, 2003).
Mitigation and Reduction of Urban Heat Island Impacts
Despite the phenomenon of UHI being acknowledged in the literature for decades, concern and community interest regarding the UHI is more recent. The increased attention afforded by climatologists to heat-related environment and health issues has participated in UHI reduction in some cities in the world by the implementation of recommended strategies, for instance promoting trees and vegetation, green roofs and cool roofs(EPA. 2003).
Vegetation and Trees
One major strategy is extensive planting of trees and vegetation. Leaves and branches participate in cooling the urban area through shading. In late spring and summer time, particularly in mid-latitudes, about 10 to 30 percent of the sun’s energy reaches the area below trees, with the remainder either reflected back into the atmosphere or absorbed by leaves and used for photosynthesis (Huang, Akbari and Taha, 1990).
Green Roofs
This technique involves growing a vegetative layer on a conventional rooftop. Green roofs act as trees and vegetation elsewhere; they shade surfaces and remove heat from the air
through evapotranspiration. Regardless of rooftop moisture content, they also change the albedo to a certain extent. The surface of a vegetated roof top can participate in cooling the ambient air, particularly on hot days, during daytime (Vandermeulen, 2011; Liu and Baskaran, 2003).
Cool Roofs
Cool roof technology employs highly reflective and emissive materials. Cool roof products are in most cases bright and white. These products obtain a high reflectance primarily by reflecting the visible portion of the spectrum. Conventional roofing materials have low solar reflectance of 5 to 15 percent, which means they absorb 85 to 95 percent of the energy that reaches them, instead of reflecting the energy back to the atmosphere. Conversely, cool roof materials have high solar reflectance that can exceed 65 percent; therefore they absorb and transfer to buildings less than 35 percent of the energy that reaches them. Furthermore, these materials reflect radiation across the entire solar spectrum, particularly in the infrared and visible wavelengths (EPA, 2009a).
Emissivity or thermal emittance is a very important consideration when selecting materials for installing a cool roof. Any surface exposed to radiant energy becomes hotter until it reaches thermal equilibrium; in other words it gives off as much heat as it receives. In order to know how much heat the material radiates per unit area at a given temperature, there is a need to know beforehand the material’s thermal emissivity. The high-emittance surface gives off its heat more readily because the surfaces with high emittance reach thermal equilibrium at a lower temperature than surfaces with low emittance (Akbari and Taha, 2003; Rose, 2005).
Objectives:
Effects of climate change are expected to be amplified in urban areas due to their distinctive physical features, which are manifested in the UHI phenomenon (Fortuniak, 2009). Therefore, the overall aim of this research was to illustrate the influence of urban growth on the air temperature trend in Kolkata City.
Kolkata City, the north eastern part of India, has been selected as study area and this study addresses the following objectives:
1- To find out if there is an increase in the minimum air temperature (mainly minimum temperature) in Kolkata City for the period 1975-2011.
2- To compare the air temperature inside the city to the air temperature of the surrounding rural areas, on a regional level.
3- To map the spatial pattern of temperature for Kolkata City by using the thermal band of ETM+ satellite for autumn 2011 to analyze the influence of different types of buildings and green areas on temperature patterns.
Research hypothesis:
There are several factors (transport, industry, commercial and household activities) that influence the air temperature in an urban area and make it differ from the air temperature of the surrounding area (EPA, 2009a). Accordingly, the hypothesis of this study was that the air temperature (mainly minimum temperature) has increased due to the urban growth of Kolkata City.

Calcutta/Kolkata stands on the Eastern Bank of River Ganga. The tail end of river Ganga flows by the side of Kolkata before it reaches Bay of Bengal about 180 Km. downstream from Kolkata. The Vital Statistics of a capital city, called by the British Raj as The Jewel of the East, and remained its capital till 1911, Kolkata is now the capital of West Bengal, a state of India.
Location : Kolkata is situated at the longitude of 88º 30'E - 22º 33' N.
Altitude : 9m (30’). From sea level it is 6.4 meters (20 ft). (Mount Everest 8848 m.)
Climate : Maximum temperature rises during the summer months of May-June up to 24 - 42º C and the minimum temperature falls during winter months of December - January up to 8 - 26ºC on an average. Climate is humid varying from 85 - 65 during the summer & exceeding pleasant in winter. From June to September average rainfall in Kolkata is 158 cm.

Area in kilometers : 1480 sq. km. (London 1580 sq. km.) 205 sq. km. is within the Corporation Area.
Population : A growing population of 45,80,544 according to the 2001 Census. (Mumbai 14.8 million.)
Density of Population : 24.760/ sq. km.
Ratio of Population : Male - 1000; Female - 956;
Literacy Rate : 81.31%
Mother Tongues : Bengali 55%, Hindi 20%, English 10%, Others 15%
Position : 7th biggest city of India in area and population.
Data and Methodology
Meteorological Data
In order to analyze the effect of urbanization on the local climate of Kolkata City and its surroundings, there was a need to make two databases.
Temporal Data
Daily records of air temperatures from eight weather stations (Figure 10) were obtained from the Ministry of Agriculture in Kolkata, which includes the maximum and minimum temperatures registered daily at the weather stations, then grouped in monthly and annual averages, except for the urban station of Kolkata (1975 to 2011) all the other station have a time series of 1985- 2011. In accordance with this information, and to identify the variability of the temperature in the time series, the following statistical tests were applied.
First of all, to reject the null hypothesis, which claims that the sample data come from a population with a normal distribution and are not influenced by some non-random causes, there is a need to calculate skewness (z1), kurtosis (z2) and standardized coefficients (Siegel, 1956). In case of the absolute value of z2 or z1 being more than 1.96, then a significant deviation of the curve is indicated at the reliability level of 0.95.
For evaluating the air temperature trend, a simple regression analysis was performed with the year as independent variable and the temperature as dependent variable, with determining the significance for type I error of 5% (α = 0.05) and the coefficient of determination R2 was
0.74. The analysis was applied for the whole period of 1975-2012 for Kolkata urban station, the only time series available in the ministry of agriculture.
In order to reinforce the validity of the simple regression analysis there is a need to apply the Kendall-t rank correlation as a non-parametric alternative. Balling and Ceverny (1987) stated that to identify any temporal linear or non-linear trend in the annual data from all the stations, it is necessary to apply the Mann-Kendall rank statistic. Moreover, between
Kolkata station and each one of the other stations, the monthly and annual air temperature differences were calculated for both minimum and maximum values. The data from the rural stations were available only from 1984 onwards. All the means, correlation coefficients and standard deviations between the year of record and the temperature differences were all calculated.
Steps involving in finding Urban Heat Island
Step 1: Conversion to Top of Atmosphere (TOA) Radiance:
Using the radiance rescaling factor, Thermal Infra-Red Digital Numbers can be converted to TOA spectral radiance.
Lλ = ML * Qcal + AL-Oi
Lλ = 0.0003342*Band10+0.10000-0.29
Where:
Lλ = TOA spectral radiance (Watts/ (m2 * sr * μm)) ML = Radiance multiplicative Band number
AL = Radiance Add Band (No.)
Qcal = Quantized and calibrated standard product pixel values (DN) Oi = correction value for band 10 is 0.29
Step 2: Conversion to Top of Atmosphere (TOA) Brightness Temperature (BT):
Spectral radiance data can be converted to top of atmosphere brightness temperature using the thermal constant Values in Metadata file.
Kelvin (K) to Celsius (0C) Degrees BT = K2 / In (k1 / Lλ + 1) - 273.15 BT= (1321.0789/Ln(774.8853/ToA+1))-273.15
Where:
BT = Top of atmosphere brightness temperature (°C) Lλ = TOA spectral radiance (Watts/ (m2 * sr * μm)) K1 = K1 Constant Band (No.)
K2 = K2 Constant Band (No.)
Step 3: Normalized Difference Vegetation Index (NDVI):
The Normalized Differential Vegetation Index (NDVI) is a standardized vegetation index which Calculated using Near Infra-red (Band 5) and Red (Band 4) bands.
NDVI = (NIR – RED) / (NIR + RED)
NDVI = (Band 5-Band 4)/ (Band 5+Band 4)
Where; RED= DN values from the RED band NIR= DN values from Near-Infrared band
Step 4: Land Surface Emissivity (LSE):
Land surface emissivity (LSE) is the average emissivity of an element of the surface of the Earth calculated from NDVI values.
PV = ((NDVI – NDVI min) / (NDVI max – NDVI min))2
Where:
PV = Proportion of Vegetation NDVI = DN values from NDVI Image
NDVI min = Minimum DN values from NDVI Image NDVI max = Maximum DN values from NDVI Image
E = 0.004 * PV + 0.986
Where:
E = Land Surface Emissivity PV = Proportion of Vegetation
0.986 corresponds to a correction value of the equation
Step 5: Land Surface Temperature (LST):
The Land Surface Temperature (LST) is the radiative temperature Which calculated using Top of atmosphere brightness temperature, Wavelength of emitted radiance, Land Surface Emissivity.
LST = BT/ (1 + (λ * BT / c2) * ln(E))
Here, c2= 14388 mm K
The Values of λ for Landsat 8: For Band 10 is 10.8 and for Band 11 is 12.0
Where
BT = Top of atmosphere brightness temperature (°C) λ = Wavelength of emitted radiance E = Land Surface Emissivity
c2= h*c/s=1.4388*10-2 mK =14388 mK h=Planck’s Constant =6.626*10-34 J s s= Boltzmann constant =1.38*10-23 JK c=velocity of light =2.998*108 m/s

Urban Heat Island (UHI) and UHI (stack) Profile
Urban heat island (UHI) refers to the phenomenon where urban areas experience higher temperatures due to human activities and the built environment. Causes include land surface modification, reduced vegetation, heat from buildings and infrastructure, altered air circulation, and waste heat. UHI has adverse effects on human health, energy consumption, and air quality.
How We Can Calculate Urban Heat Island using GIS?
Where,
UHI= Urban Heat Islands
LST= Land Surface Temperature
LSTm= The mean temperature of the land surface temperature in the study area
SD= Standard deviation of temperature.


According to the result, the places Sinthi, Bagbazar, Maniktala, Beleghata, Taltala in the northern part of Kolkata, Garden Reach in northern part of Kolkata and A part of Dhapa-Manipur in the eastern part of Kolkata have higher UHI value.
Whereas major parts of Dhapa-Manpur, Ajay Nagar and Dhakuria along the eastern part have Lower UHI value.
The majority of the southern parts of Kolkata such as Sarsuna, Thakurpukur, Bekala, Garia,Tollyfanj lie in the Moderate UHI value.
Reference
Rahman, M. N., Rony, M. R. H., Jannat, F. A., Chandra Pal, S., Islam, M. S., Alam, E., & Islam, A.
R. M. T. (2022). Impact of urbanization on urban heat island intensity in major districts of Bangladesh using remote sensing and geo-spatial tools. Climate, 10(1),3