its about the urban heat island effect and thee stratergies to overcome it

1. Chapter-1 Introduction
1
Introduction
From the prehistoric days the humans are together with each other, seeking
cooperation in the battle of survival. Humans life has been changing from the
ancient tribal village to the mega cities with luxurious life of today, they have
thrived by living shoulder-to shoulder. By living together as community’s they
have experienced lots of benefits, as a result they has flourished across the world.
But with those benefits there are some unseen consequences. One of the
consequences is the urbanization effect of the cities on the weather and climate
around them. Today more than half of the world’s population is living in the cities
are subjected each and every day to various types urban effects such as,
meteorological effects, air pollution, urban heat island effect, precipitation, etc.
These growth in the urbanization leads landscape to transform from small,
isolated population centers to expansive, interconnected physical features of the
landscape. The physical infrastructure of cities has replaced native soil and
vegetation with concrete, asphalt and buildings which alter the albedo and runoff
characteristics of the land surface. Surface and atmospheric modifications
resulting from urbanization have led to a localized thermal climate that is warmer
than surrounding non-urbanized areas called an Urban Heat Island (UHI) (Voogt
and Oke, 2003). From the last few decades researchers have begun to develop a
clear picture that how significantly UHI impacts the climate and weather of the
urban environments.

2. Chapter-1 Introduction
2
1.1 Urban Heat Island
The climate of an urban area often differs from that of the surrounding
countryside. One of the features of urban climate is the Urban Heat Island (UHI),
which appears as the relative warmness of a city (Oke, 1973). The UHI in simple
terms is defined as the urban area that is significantly warmer than its surrounding
rural areas due to human / anthropogenic activities (Figure 1.1).
Figure 1.1: Urban surface absorb and retain more heat compared to rural
area (Adopted from Urban Heat Island, EPA 2009).
This phenomenon was initially identified when temperature discrepancies
between urban areas in Singapore and surrounding rural areas were measured
(Solecki et al., 2005; Stewart, 2011). The increase in the urbanization,

3. Chapter-1 Introduction
3
modification of land surface and human activities over urban areas might be
expected to be the main cause that results in greater absorption, retention and
production of heat which amplify the effect of climate which results in the
formation of UHI (Witkowski et al., 2009). As a consequence of continuous
altering in natural land cover patterns to anthropogenic patterns, there is a
modification of the radiative fluxes, changes in the thermal properties of surfaces
and trapped fluxes due to multiple reflections (Figure 1.2).
Figure 1.2: Transformation of thermal heat inside urban areas (adopted
from Sailor et al, 2012).
Consequently, the solar radiation and hydrologic balances are dislocated,
which in turn increases urban-rural contrast in air temperatures and surface
radiance (Brazel et al., 2000). This UHI phenomenon has numerous studied stated
that it has a role in the climatic change reported at Houston (Streutker, 2002),

4. Chapter-1 Introduction
4
Phoenix (Hawkins et al., 2004), London (Jones and Lister, 2009) and Delhi
(Mohan, 2012; Pandey et al., 2014) around the world. It is also considered as one
of the climatic phenomenon that requires more investigation to address the
climatic change and environmental degradation (Cueto et al., 2009; Gaffin et al.,
2008).
The magnitude of UHI in terms of temperature difference between urban
and rural areas is called UHI intensity. UHI intensity tends to vary both hourly
and seasonally, and is influenced by many factors such as local topography,
climate zone, 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 (Santana, 2007; Stathopoulou et al.,
2005). Cloud cover and incoming solar radiation also affect UHI intensity but that
effect is less significance than LULC characteristics and the abundance of urban
vegetation.
1.2 Urban Heat Island formation
A close relationship is expected between the temperature radiated by the
land surface (Land Surface Temperature) and the temperature of the atmosphere
above the land surface (near surface air temperature), because of the release of
energy due to the retention from the surface. The energy retention can be achieved
from two sources: surface heat and atmospheric heat (Figure 1.3) and may create
different types of UHI which are discussed next.

5. Chapter-1 Introduction
5
Figure 1.3: Urban Heat Islands types (adopted from Urban Heat Island
Basics: Compendium of Strategies, U.S. EPA, 2008).
1.2.1 Surface Heat Island
Surface heating of an area is caused when the area is directly exposed to
solar radiation and gets heated up. The exposed urban surfaces become hotter than
the air, while shaded or moist surfaces (or rural surroundings) remain close to air
temperature causing a difference in the temperatures. The heat island calculated
from the surface temperature is known as the Surface Heat Island (UHIsurface). This
UHIsurface is also known as the remotely sensed UHI because the temperature data
is retrieved from the land surface using infrared channel. UHIsurface is present

6. Chapter-1 Introduction
6
typically at both the daytime and nighttime, but it is stronger at the nighttime
((Oke, 1981). The average difference in daytime surface temperatures between the
urban and rural areas is 5 to 10°C, whereas the differences during nighttime is
typically 10 to 15°C, which is typically higher than the daytime differences
(Voogt and Oke, 2003). UHIsurface exhibits a strong seasonal variation, and is
usually greater in summer time, due to changes in the incoming solar radiation,
difference in land and because of drier weather conditions associated with summer
in most regions.
1.2.2 Atmospheric Heat Island
In urban areas the warmer air compared to cooler air in nearby rural areas
defines the Atmospheric Heat Islands (UHIair). UHIair is highly dependent on the
weather, time of the day and wind conditions in the urban environment. Typically
UHIair effect is weaker in the early morning hours when the heat source has
expanded all the heat, whereas it is stronger after the sunset when the heat source
keeps the urban area warmer (Memon et al., 2008). This UHIair is again classified
in to two types.
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. UHI measured in the
canopy-layer show significant spatial and temporal variability within a city,
generally with a peak associated with the city center and a large temperature
gradient at the city periphery.

7. Chapter-1 Introduction
7
Boundary layer Urban Heat Islands:
This layer begins from the top of canopy layer 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 (Velázquez-Lozada et al., 2006).
1.3 Factors that influences Urban Heat Island
The factor that influences the formation of the UHI can be separated into
two groups as controllable and uncontrollable Figure 1.4.
Figure 1.4: Factors that influence the intensity and formation of UHI
(adopted from (Memon et al., 2008).
i. Controllable (vegetation cover, urban material, city structure and
anthropogenic heat)

8. Chapter-1 Introduction
8
ii. Uncontrollable (season, wind and cloud)
A detailed discussion on various factors from the controllable and uncontrollable
groups is provided next:
1.3.1 Vegetation
Vegetation plays a major role in controlling the temperature of an
environment (Mirzaei and Haghighat, 2010; Zhang et al., 2009, 2010). In most of
the rural and sub-urban areas the typical dominating landscape is the vegetative
land which helps in maintaining the temperature of the environment. However,
albedo and emissivity of these areas are different from those of urban areas. The
green vegetation cover over the rural and sub-urban areas helps to lower the
surface temperature by its shade and also lower the air temperature through the
evapotranspiration (Figure 1.5).
The shade of the vegetative fraction lowers the surface temperature by
reducing the amount of incoming solar radiation that hits the ground below the
vegetation, because the leaves have higher reflectance rate than compared to the
other materials (Klaus et al., 1999).
During evapotranspiration, vegetation releases water vapour into
surrounding air that contributes in 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.

9. Chapter-1 Introduction
9
Figure 1.5: Highly developed and naturally vegetated land with the surface
runoff (adopted from EPA, 2009).
1.3.2 Urban Materials
The composition of the materials present in the urban areas plays an
important role in interaction with the incoming solar radiation from the sun
reaching the earth in the visible and infrared range of the spectrum (Figure 1.6).
The urban materials have higher heat storage capacity than the rural materials, as
a result the urban areas of a city have more efficiency than the surrounding rural
areas to store the heat energy from the sun inside their infrastructure (Christen and
Vogt, 2004; Golden and Kaloush, 2006; Kikegawa et al., 2003).

10. Chapter-1 Introduction
10
Solar reflectance or albedo also influences the development of heat island.
The materials with high solar reflectance reflects most of the incoming heat
energy and hence less energy is stored in the material, whereas light coloured
materials reflects more energy compared to that of dark coloured materials (Coutts
et al., 2007). For example a white roof reflects more of the incoming energy from
the sun and while a black roof will absorb more incoming energy from the sun.
Figure 1.6: Surface reflectance or Albedo of different surfaces in urban area
(adopted from EPA 2009).
In urban areas most of the surface materials are paved roads, building
roofs which have lower effective albedo and compared to that of rural area. As a
result, built up communities generally reflect less and absorb more of the sun’s
energy. This absorbed heat increases surface temperatures and contributes to the

11. Chapter-1 Introduction
11
formation of surface and atmospheric urban heat islands. Another important
property that influences heat island development is the material’s heat capacity,
which refers to its ability to store heat. Many building materials, such as steel and
stone, have higher heat capacities than rural materials, such as dry soil and sand.
As a result, cities are typically more effective at storing the sun’s energy as heat
within their infrastructure. Downtown areas can absorb and store twice the
amount of heat compared to their rural surroundings during the daytime (Christen
and Vogt, 2004).
1.3.3 City Structure
The urban geometry or the city structure refers to the dimensions and
spacing of buildings within an urban area. The urban geometry influences the
energy absorption, wind flow, and the surface’s ability to emit long-wave
radiation back to the sky. This factor influences UHI development, particularly
during the night time, since in most of the urban areas, structures and surfaces are
abstracted by other objects such as neighbouring buildings. As a result, urban
areas cannot release their heat readily during the night and becomes a large
thermal mass due to the obstructions. The effects of urban geometry on urban heat
islands are often described through the “sky view factor” (SVF), which is the
visible area of the sky from a given point on a surface. For example, an open
parking lot or field that has few obstructions would have a large SVF value (closer
to 1). Conversely, an urban canyon in a downtown area that is surrounded by
closely spaced, tall buildings, would have a low SVF value (closer to zero), as
there would only be a small visible area of the sky. The reduced SVF of many

12. Chapter-1 Introduction
12
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 affect 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 (Zhang et al., 2012).
1.3.4 Anthropogenic Heat
Another important factor that affects the UHI recognized recently is the
anthropogenic heat (Déqué, 2007; Ferreira et al., 2011; Taha, 1997).
Anthropogenic heat contributes to atmospheric heat islands and refers to heat
produced by human activities. It can come from a variety of sources and is
estimated by combining all the energy used for heating and cooling, running
appliances, transportation, and industrial processes. Anthropogenic heat varies by
urban activity and infrastructure, with more energy-intensive buildings and
transportation producing more heat. Anthropogenic heat typically is not a concern
in rural areas and during the summer. In the winter, over rural areas, and year
round in dense, urban areas, anthropogenic heat can significantly contribute to
heat island formation.
1.3.5 Additional Factors
Besides the above controllable factors there are some additional
uncontrollable factors such as weather and geographical location that influence
the formation of UHI.

13. Chapter-1 Introduction
13
Weather
There are two primary weather characteristics such as wind and cloud that
affects UHI development. A calm wind with clear sky condition favours the
formation of UHI, because at these conditions the maximum solar radiation
reaches the ground and less amount of convection takes place. Conversely, strong
winds and cloud cover suppress urban heat islands.
Geographical location
Urban areas geographical location is partly determined by the climate and
topography over the area, which influence UHI formation. For example, large
water bodies’ moderates temperature and can generate winds that convect heat
away from the areas. Nearby mountain ranges can either block wind from
reaching a city, or create wind patterns that pass through a city. Local terrain has a
greater significance for heat island formation when larger-scale effects, such as
prevailing wind patterns, are relatively weak.
1.4 Impact of the Urban Heat Island
The raise in the temperature concern over the urban areas particularly
during the summer, can affect a community’s environment and quality of life.
While some heat island seem to have positive impacts, such as lengthening the
plant-growing season, whereas most of them have negative impacts which
includes

14. Chapter-1 Introduction
14
1.4.1 Economic
The increase in the temperature over the urban areas might increase in the
energy consumption demand for cooling and add more pressure to the electricity
due to most of the houses and offices are running air conditioners against the heat
gradient to optimise indoor air temperature particularly during extreme heat
events (Ca et al., 1998). This peak urban electric demand increases 1.5 to 2
percent for every 1°F (0.6°C) increase in summertime temperature. Steadily
increasing downtown temperatures over the last several decades mean that 5 to 10
percent of community-wide demand for electricity is used to compensate for the
heat island effect (Akbari, 2005).
1.4.2 Air Quality
To accommodate the increase in the temperature during a hotter and dry
summer day within urban area, more energy is consumed for cooling over the
area. Fossil fuels are the most common source for the production of electricity
worldwide (Chow et al., 2003). The pollutants emitted from the power plants are
mostly include sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter
(PM), carbon monoxide (CO), and greenhouse gases particularly carbon dioxide
(CO2) which are harmful to human health and participate in complex air quality
problems and contribute to global climate change (EPA, 2009).

15. Chapter-1 Introduction
15
1.4.3 Human Health and Comfort
Every year, around the world there are thousands of deaths related to
extreme heat events. The increase in the temperature leads to the extreme heat
which intern affect human health by contributing to general discomfort,
respiratory difficulties, non-fatal heat stroke, sun burn, heat cramp, dehydration
and heat related mortality (Argaud, 2007; Loughnan et al., 2010). Urban heat
islands can also exacerbate the impact of heat waves, which are periods of
abnormally hot, and often humid, weather. Sensitive populations, such as
children, older adults, and those with existing health conditions, are at particular
risk from these events.
1.4.4 Water Quality
Pavement and high rooftop surface temperature heats the storm water
runoff and degrades the water quality mainly by the thermal pollution.
Experiments showed that runoff from the urban areas was about 11o
C – 17o
C
hotter than runoff from the rural areas on a summer hotter days where the
pavement temperatures were 11o
C – 19o
C hotter than the air temperature (Roa-
Espinosa et al., 2003). Water temperature affects all aspects of aquatic life,
especially the metabolism and reproduction of many aquatic species. Rapid
temperature changes in aquatic ecosystems resulting from warm storm water
runoff can be particularly stressful. Brook trout, for example, experience thermal

16. Chapter-1 Introduction
16
stress and shock when the water temperature changes more than 1o
C – 2o
C in 24
hours (EPA, 2009).
1.5 Reduction of Urban Heat Island Impacts
The knowledge of UHI phenomenon is disputed for decades in the
literature but the concern and community interest regarding the UHI but it got
more attention in the recent times. For the reduction of UHI effect, in many cities
around the world, some strategies like planting trees, improving vegetation, green
roofs and cool roofs (EPA, 2009) have been implemented.
1.5.1 Improving Vegetation and Planting Trees
One of the controllable factors that mitigates the influence of UHI is
improving the vegetation over the area which was adopted in most of the
developed areas (Akbari et al., 2001; Klaus et al., 1999; Sandifer, 2002; Zhang et
al., 2010). Planting trees in the urban area increases the greenness over the area of
the urban environment. Thus leaves and branches, participate in reducing the
surface temperature of urban areas through shading, however they also reduce the
air temperature by evapotranspiration and improve the water quality by draining
the water in to the soil and reduces the smog and dust pollution.
1.5.2 Green Roofs
This method involves in growing the trees on and around the man-made
structures (Akbari et al., 2001; Sandifer, 2002). This method has been adopted by

17. Chapter-1 Introduction
17
many developed countries around the world like USA, China, UK, Australia etc.
this involves in growing the plants on the rooftops of the houses and buildings so
as to increase the solar reflectance and reduce the heat that is absorbed directly.
Regardless of rooftop moisture content, they also change the albedo to a certain
extent. The surface of a vegetated rooftop can participate in cooling the ambient
air, particularly on hot days, during daytime (Liu and Baskaran, 2003;
Vandermeulen et al., 2011).
1.5.3 Cool Roofs
Cool roof schemes are designed to reduce the amount of heat that transfers
directly in to the buildings. A dark roof absorbs most of the incoming solar
radiation and gets heated. Heating the roof would heat the building as well as the
air around the building. Cool roof scheme mainly focus on the materials of the
roof that have a high albedo. Those products with high albedo reflect 65% of
energy back in to the space and only 35% is absorbed and transferred in to the
buildings. Furthermore, these materials reflect radiation across the entire solar
spectrum, particularly in the infrared and visible wavelengths (EPA, 2009).
Emissivity or thermal emittance is a very important consideration when
selecting materials for installing a cool roof. Any surface exposed to radiant
energy become 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

18. Chapter-1 Introduction
18
more readily because of the surfaces with high emittance reach thermal
equilibrium at a lower temperature than surfaces with low emittance (Lilly Rose
and Devadas, 2009).
1.6 Gaps in understanding the formation of Urban Heat Island
and its association with air quality for the metropolitan
cities of India.
Rapid progress in industrialization and urbanization has resulted in the
concentration of economic growth and social functions in the urban areas.
Unplanned and hasty urbanization in modern cities, especially during the last
three decades, has caused environmental problems including an increase in energy
consumption, an alteration of the local climate, and higher amounts of air
pollution. One of the consequences is a consistent rise in temperature in the urban
atmosphere (Oke, 1973). The noticeable temperature rise in the urban atmosphere
is known as the Urban Heat Island (UHI).
Oke (1982) lists a number of factors contributing to the urban heat island,
including altered energy balance terms leading to a positive thermal anomaly;
multiple reflection of short-wave radiation between the canyon surfaces and
decreasing the effective albedo of the system; increased long-wave radiation from
the sky due to air pollution; decreased long-wave radiation loss because of the
reduction of the sky view factor, anthropogenic heat sources, increased sensible
heat storage and decreased evapotranspiration due to construction materials, and
decreased total turbulent heat transport due to wind speed reduction caused by
canyon geometry.

19. Chapter-1 Introduction
19
The intensity of the UHI phenomenon is dependent on several factors the
most important one is the time of the day, this phenomenon is striking at night and
non-existing during the day. Weather condition is another factor modulating UHI
intensity, when the atmosphere has cloudless skies and light wind conditions.
The other factors in addition to the above two are urban topography
including the canyon effect which increase absorption of the incoming shortwave
radiation and reduces the outgoing longwave radiation. As a result, the surface
roughness is increased which reduces the boundary layer winds and increases the
sensible heat loss to the atmospheric environment. Another phenomenon is the
expansion of the urban area by the build-up area which causes a decrease in
surface albedo.
The most important factor causing formation of UHI is the replacement of
the open lands i.e., the soil and vegetative land is emerging with the areas of the
residential, commercial and industrial periphery with improper urban materials
such as concrete, asphalt, corrugated roof, bricks, stones etc., which affect the
albedo and runoff characteristics of the land surface causing a change in local and
regional land-atmospheric energy exchange process.
The higher temperature in the urban region compared to its surrounding
rural areas have adverse effect on the air quality over the region. The emission of
anthropogenic heat, pollution, etc., emitted from the motor vehicles, power plants,
and other sources of combustion of fossil fuel affect the UHI intensity with
various impacts depending on the seasons (Oke et al., 1991). The ground level
ozone is produced by the volatile organic compounds (VOCs) in the presence of
the nitrogen oxides (NOx) and sunlight by a process called photochemical smog

20. Chapter-1 Introduction
20
mechanism (Cardelino and Chameides, 1990), which is a public health hazard that
causes respiratory and cardiovascular illness. Additionally, the UHI is known as a
positive contributor to increase instantaneous human mortality during the heat
events.
There are enormous studies made available on urban heat island effect in
the recent decades and is exhibited in many major cities around the world. During
night under clear and calm conditions, a large temperature gradient develops
between the city and its surroundings (Oke, 1982): such urban heat islands (UHI)
were already documented for instance in Montreal (Oke and East, 1971), Paris
(Escourrou, 1991), Toulouse (Estournel et al., 1983), Mexico (Oke et al., 1999)
and Atlanta (Bornstein and Lin, 2000). During the day, even if the UHI can also
be observed, the dynamic processes are preponderant compared to the radiative
and thermal effects. The dynamic roughness of the city favors the production of
turbulence and the development of the urban boundary layer (UBL) (Dupont et
al., 1999). With respect to the thermal impact induced by human activities, the
anthropogenic heat flux in the city is a factor that is often ignored in atmospheric
models (Troude et al., 2001) but has recently been shown to be important in the
development of the UHI (Sailor and Fan, 2004). Anthropogenic heating is the
combination of waste heat released from vehicle fuel combustion, building and
industrial energy consumption, and human metabolism. The heat intensity varies
with climate, population density, and intensity of industrial and commercial
activities. When averaged over a city locally it may be an order of magnitude
higher (Sailor and Fan, 2004). While the average anthropogenic heat flux is small
compared with summertime mid-day solar radiation, it can play a major role in the

21. Chapter-1 Introduction
21
urban energy budget during the night and winter, when the urban heat island
effect is most prominent and the relative solar forcing is low.
Many studies have reported the influence of wind speed and cloud cover
on UHI, the results show that the UHI is negatively correlated with wind speed
and cloud cover (Kim and Baik, 2005). Pongracz et al., (2006) reported that
anticyclone conditions increase Urban Heat Island Intensity (UHII). The role of
the evaporation and photosynthesis is extremely important when examining the
UHI effect mechanisms. Different urban land uses participate in proliferating or
alleviating the urban heat island effect, and influence temperatures in cities.
Finally, anthropogenic activities result in a significant (Hinkel and Nelson, 2007),
but currently poorly quantified, heat emission (Krpo et al., 2010; Souch and
Grimmond, 2006). Urban heat island has wide range of impact on city
environments like Human comfort, Energy use, Air pollution, etc., (Sharma and
Bharat, 2009). The higher UHI around the time of peak temperatures in the city
both in the afternoon hours and night hours increases the energy demand resulting
into generation of more anthropogenic heat and thereby increasing the UHI
(Mohan et al., 2009). Urbanisation and the consequent loss of lakes has led to
decrease in catchment yield, water storage capacity, wetland area, number of
migratory birds, flora and fauna diversity and ground water table. As land is
converted, it loses its ability to absorb rainfall. Also, increased urbanisation has
resulted in higher population densities in certain wards, which incidentally have
higher LST due to high level of anthropogenic activities (Ramachandra and
Kumar, 2010).

22. Chapter-1 Introduction
22
It has also been reported that the temporary effect variables in certain
ways. The effect was anticipated, the correlation between the biological
activity and land surface temperature (Gábor and Jombach, 2010). Exploration
of the UHI uses, such as calculating cooling degree-days or monitoring heat
waves, the impact of urban development on runoff and soil surface moisture have
also been successfully demonstrated (Dousset et al., 2011; Herb et al., 2008;
Petropoulos et al., 2009). Urban areas experience a substantially different
meteorology than their rural counterparts. The physical properties of cities and
buildings result in a modified surface radiation and energy budget, which need to
be accounted for in further studies. First, building clusters interact with solar
radiation such that multiple reflections between buildings and roads occur before
the solar radiation is reflected to space. Therefore, cities typically have a smaller
albedo than crops or grasses. In addition, buildings limit the sky view of the
surface, and therefore emission of thermal radiation to space is limited. Also,
building configurations provide additional friction to the flow, which affects wind
speed and turbulence intensity not only in the cities, but also downstream from
them. Moreover, fabric, concrete and asphalt have a higher heat capacity than
rural areas, which limits rapid cooling after the evening transition. This land cover
results in less shade and moisture to keep urban areas cool. This is because built
up areas have less moisture contents, and therefore evaporate less water into
the air causing high surface and air temperatures.
Urban heat island effects were found to be most dominant in areas of
dense built up infrastructure and at commercial centers. The heat island intensity
(UHI) was observed to be higher in magnitude both during afternoon hours and

23. Chapter-1 Introduction
23
night hours (Mohan, 2012). Urbanization has led to a highly significant decline of
annual cold nights at and a highly significant increase of annual warm nights.
Although urbanization effects are significant for cold and warm days, they are
relatively smaller, and the smallest absolute values of annual-mean urbanization
effects for most of the indices series are found to dominantly appear during 1966–
76, a well-known deurbanization period resulting from the Cultural Revolution. It
is therefore clear that urbanization effects on the long-term trends of the
frequently used extreme temperature indices series of the observational data
commonly applied in mainland China are of significance, despite the fact that they
are generally smaller compared to the regional background changes, probably
resulting from other factors and processes. The larger and more significant
urbanization effects are especially notable for the indices related to daily
minimum temperature in relatively developed areas of the country (Ren and Zhou,
2014).
One of the less studied aspects of the UHI is its peak development during
the monthly and diurnal period. Utilization of satellite and SAFAR network
observations may provide useful quantitative information about structure of the
maximum UHI intensity by employing urban and meteorological parameters. The
main purpose of this work is to study the effects and interactions inside the city on
the surface air temperature under all-weather condition except rain at the time just
a few hours after sunset when the UHI effect is most pronounced. The second
purpose is to determine quantitative influences of anthropogenic and natural
factors on the urban-rural temperature differences during the period of the study.

24. Chapter-1 Introduction
24
1.7 Outline of the present work
The thesis is divided in seven chapters. An outline of the contents of each
chapter is given below:
 Chapter 1 in the current context providing the introduction to the subject
and outlining the thesis layout.
 Chapter 2 deals with the data and methodology.
 Chapter 3 includes the study of Comparative study of Urban Heat Island,
by using Satellite and SAFAR observations over Delhi, India.
 Chapter 4 discuss the seasonal variation of Urban Heat Island and its
impact on air-quality using SAFAR observations at Delhi during the hotter month
May and Colder month December 2013.
 Chapter 5 describes the spatio-temporal Evolution of Urban Heat Island
over Metropolitan cities under SAFAR network, during 2015.
 Chapter 6 deals with the comparative study of Urban Heat Island and its
impact on air quality during the colder month December of 2013 and 2015 using
SAFAR Network over Delhi.
 Chapter 7 summarize the conclusions of the study and future scope