urban heat island

Urban Heat Island
Effect
Causes, Effects and Mitigation Measures

What Is An Urban Heat Island?
• The term “urban heat island” (UHI) is used
to describe an urban area that is warmer than
the rural areas that surround it.

What Is An Urban Heat Island?
• Environmental Protection Agency has noted
that many cities have temperatures that are
up to 5.6°C warmer than the surrounding
areas - but a temperature increase of just one
or two degrees is enough to make a noticeable
impact.

Environmental Protection Agency has noted that many cities
have temperatures that are up to 5.6°C warmer than the
surrounding areas

Urban Heat Island
Effect
• An urban heat island (UHI) is an urban area
or metropolitan area that is significantly
warmer than its surrounding rural areas due to
human activities.
• The temperature difference is usually larger
at night than during the day, and is most
apparent when winds are weak.

Introduction
• The current climate crisis is having a
drastic effect on the planet.
• While it’s easy to place the blame on rapid
deforestation and unsustainable industry, one
of the largest contributors to the current
climate change phenomenon is much closer
to home: our cities.

one of the largest contributors to the current climate change
phenomenon is much closer to home: our cities.

Introduction
• According to estimates, our cities are responsible
for around 70% of the world's CO2 emissions.
• This is largely down to greenhouse gas emissions
from urban transport and industry, as well as
impractical waste management, and harmful building
practices.
• Despite being one of the biggest contributors to
the global climate crisis, cities are also expected to
be the biggest victims of it.

According to estimates, our cities are responsible for around
70% of the world's CO2 emissions

Introduction
• Urban life is estimated to be greatly
impacted by climate change.
• Extreme weather events, catastrophic flooding
from rising sea levels, drought, and the easy
transmission of disease due to population
density are very real threats to metropolitan
areas.

Introduction
• Today, cities are battling a number of climate-
related problems that are having devastating
impacts on urban services, city infrastructure,
employment, housing, and urban public health.
One of the most pressing issues facing
metropolitan areas is the Urban Heat Island
Effect: a self-perpetuating problem with
dangerous consequences

Urban Heat Island Effect: a self-perpetuating problem
with dangerous consequences

Urban Heat Island
Effect
• Urban Heat Island (UHI) effect, which is causing
discomfort to the urban dwellers in the summer time
is gaining much attention around the world because
the world is getting urbanized as it advances in
technology.
• Alterations of surface area, improper urban planning,
air pollution, etc. are causing this increasingly
growing phenomenon and it is accountable for human
discomfort, human casualties and decline of climate.

Urban Heat Island
Effect
• In most of the large cities, the temperature at
the heart or the center of the city is noted to be
higher than its surroundings or the suburban
area.
• The phenomenon is called Urban Heat Island
(UHI) effect

Urban Heat Island
Effect
• When a huge amount of natural land is
replaced by artificial built surface that absorbs
incoming solar radiation or heat and re-radiate
it at night, it causes heat Island effect.

Urban Heat Island
Effect
• The main cause of the UHI effect is urban
development.
• Natural landscapes are made up of a variety of
permeable surfaces, such as vegetation and soil
surfaces.
• However, in cities, these surfaces are covered over by
impervious materials such as bitumen and concrete.
• Unlike vegetation and other natural surfaces,
modern building materials absorb heat rather than
reflect it. This causes ambient temperatures to rise.

Urban Heat Island
Effect
• Other contributing factors include heat
pockets between tall buildings, waste heat
caused by industry and transport, and the
self-perpetuating problem of air
conditioners.
• As cities get hotter, citizens require greater air
conditioning to continue functioning, which in
turn produces waste heat that contributes to
the problem. It’s a cyclical pattern.

As cities get hotter, citizens require greater air conditioning to
continue functioning, which in turn produces waste heat that
contributes to the problem. It’s a cyclical pattern.

How Do Heat Islands Form?
• Heat islands form as vegetation is replaced by
asphalt and concrete for roads, buildings, and
other structures necessary to accommodate
growing populations.
• These surfaces absorb rather than reflect the
sun's heat, causing surface temperatures and
overall ambient temperatures to rise.

• Displacing trees and vegetation minimizes the
natural cooling effects of shading and
evaporation of water from soil and leaves (evapo-
transpiration).
• Tall buildings and narrow streets can heat air
trapped between them and reduce air flow.
• Waste heat from vehicles, factories, and air
conditioners may add warmth to their
surroundings, further exacerbating the heat
island effect

Displacing trees and vegetation minimizes the
natural cooling effects

• Heat islands can occur year-round during the
day or night.
• Other contributing factors to the heat island
effect include:

• Size and shape of cities—aerodynamically, cities
have a very different shape than rural areas.
• Tall buildings act as obstacles and reduce wind
speeds.

• Urban deserts—cities can be thought of as
virtual deserts with almost no vegetation and
materials that are almost completely
impermeable to rain.
• This combination leads to a lack of evapo-
transpiration which increases sensible heat.

• Urban canyons—the tall canyons formed by
city buildings trap radiant energy in their
walls.
• Comparisons of this "canyon effect” suggest
that areas with denser and taller buildings will
more rapidly develop heat islands.

• Humidity effects—although there is little
difference in the amount of water that cities
and countryside's retain in their atmospheres
(absolute humidity), the higher urban
temperatures effectively lowers the relative
humidity (since warm air can hold more water
than cold air).

• Urban haze—The haze of air pollution that
hangs over many cities can act as a miniature
greenhouse layer, preventing outgoing thermal
radiation (heat) from escaping from urban areas.
• Anthropogenic heat—The release of heat from
the burning of fossil fuels can also raise urban
temperatures.
• On a typical winter day, Manhattan releases four
times more energy from burning fossil fuels than
the amount of energy that comes into the urban
area from the Sun.

Causes of Urban Heat Island and Its Effects
the following are the causes of UHI:
• Low amount of evapo-transpiration because of
less vegetation
• Absorption of solar radiation due to low
albedo (Reflection)
• Hindrance to the flow of air because of higher
rugosity
• High amount of anthropogenic heat release

Low amount of evapo-transpiration because of
less vegetation

Absorption of solar radiation due to low
albedo (Reflection)

• However there are number of factors which
contribute to the formation of Urban Heat
Island Effect.
• Low Albedo( Reflective) Materials
• Albedo is ratio of the reflected solar energy to
the incident solar energy.
• It depends on the arrangement of surfaces,
materials, pavements, coatings, etc.

• If the albedo of the urban surface is low, it will
store more solar energy and the effect will be
increasing of urban temperature.

• Increased Use of Air Conditioner
• To provide comfort to the human beings at
summertime air conditioner are massively used
with a rising trend.
• Air conditioners keep a building cool inside, but
release the heat absorbing from inside to the
atmosphere.
• As a consequence, the outside environment is
warmed leading to the increasing of atmospheric
temperature.

Increased Use of Air Conditioner

• Destruction of Trees
• To meet the demand of various urban facilities,
forests are wiped out in a massive scale.
• Lesser trees means less cooling efficiency Trees
intercept the solar heat and also absorb CO2 for
their own photosynthesis, making the
environment cool With the destruction of plant
life, the efficiency of cooling system goes
radically down, causing creation of the process

• Urban Canopy
• In urban areas there are multilayer buildings.
The heat reflected by a building is trapped by
the nearby taller building which is known as
the urban canopy

• Human Gathering
• As the human gathering is prominent at the
city centers owing to the availability of various
facilities, emission of CO2 is also huge in these
areas. CO2 stores heat enhanced atmospheric
temperature.
• The ultimate effect is that it assists in the
formation of heat island effect.

• Wind Blocking
• Due to the presence of densely situated
buildings, wind velocity is reduced.
• As a result, the cooling effect by convection
lessens.
• So, the heat trapped, cannot be blown out,
resulting in intensification of the effect

• Air Pollutants
• In the urban areas, especially in the city
centers, air pollution is eminent.
• Exhaust gases from vehicles and industrial
pollutants released in the environment, trap
solar radiations.
• Thus, the temperature rises and microclimate
effect becomes stronger

Effects
• The effects are devastating in the summer time
in the tropical and arid regions.
• It causes discomfort to the people living in the
middle of the city excessive heat people with
little enduring capability undergo heat stress
and it causes illness as well as death.

Effects
• Furthermore, the increased temperature will
energy required to cool the buildings to
provide comfort to the people.
• This will augment the expenditure of the
people and government as well.
• For every 10 0C temperature increase, the
energy demand may go up by 2-4% in the
summertime.

For every 10 0C temperature increase, the energy demand may
go up by 2-4% in the summertime

Effects
• As the demand for electricity soars , more fossil
fuel is also burned causing high emission of green
house gases to meet the demand tending to
worsen the condition and decline of climate.
• At the same time, increased use of air
conditioners leads to worsening of the effect even
more.
• However in the winter season, the UHI effect
tends to give people comfort owing to the
increased temperature

How Do Heat Islands Affect Us?
• Higher temperatures affect people’s health, air
and water quality, and the amount of energy
that we use for summertime cooling.
• People’s Health: Heat islands can intensify
extreme hot weather, which can cause
breathing problems, heat cramps, and heat
stroke, and may lead to illness or even death—
especially in vulnerable populations such as
the elderly.

Higher temperatures affect people’s health

• Air Quality: Heat islands raise energy demand
to power air conditioning, which in turn can
increase utility bills and increase power plant
emissions of carbon pollution that causes
climate change.
• Higher temperatures also accelerate the
chemical reaction that produces ground-level
ozone, or smog.

Higher temperatures also accelerate the chemical
reaction that produces ground-level ozone, or smog

• Water Quality: Hot pavements heat up storm
water runoff, which can hurt aquatic life in
local waterways.

• Energy Use: Heat islands are responsible for
5–10 percent of summertime electricity
demand, leading to higher electricity bills,
pressure on the electricity grid, and brownouts
and blackouts.

The Impacts of the Urban Heat Island Effect
• Rising temperatures certainly make lives more
uncomfortable for citizens but the additional
heat also brings a number of other harmful
consequences.

Impacts on Animals
• Most species need optimum temperatures to
colonize, utilize and thrive in their ecosystems.
When there is the existence of high
temperatures due to urban heat island (UHI),
harsh and cruel ecological surrounding is
created which limits the essential activities of
the organisms such as metabolism, breeding
and reproduction.

Other Factors
• Naturally, rising urban temperatures will cause
increased levels of discomfort and premature
deaths to citizens, but it will also take its toll on
the animal populations within cities too.
• Some animal populations are more likely to
struggle to find food, water, and shelter in hotter
cities.
• Or there’s the other side, where some animal
species may find cities more attractive than the
wilderness where they belong, and turn into urban
pests that carry disease and become a nuisance.

Strategies to Reduce Urban Heat Island
• High Albedo Roofing Materials
• Dark roofs absorb heat from the sunlight and
make houses warm.
• In contrast, light colored roofs with similar
insulation properties do not get warmed
significantly by reflecting solar radiation.

• So, the choice of roofing color can contribute to
temperature reduction.
• Roofing materials with low albedo absorb
solar heat and make the house warm which
results in high consumption of energy for air
conditioning.
• So, one of the mitigation strategies is to use high
albedo roofing materials. If cool surfaces are
achieved through changing color, it adds no extra
cost to the roof

• Light-colored concrete and white roofs
• City rooftops are typically black because the
traditionally-used asphalt and tar are waterproof,
tough, ductile and easy to apply to complex
rooftop shapes and designs.
• However, black and dull colors absorb lots of
solar heat, which results in warm (and
sometimes, scorching hot) surfaces. So, to
mitigate the problem of urban heat island,
more people are having bright, white
roofs installed

Light-colored concrete and white roofs

Light-colored concrete and white roofs
• The use of light-colored concrete and white
roofs is far more effective in reflecting up to
50% more light and lowering the temperature.
Additionally, light-colored concrete and white
roofs reduce the overall air conditioning
demands (and costs).

• High Albedo (Reflective) Pavements
• More solar radiation could be reflected if the
road and highway pavements were of high
albedo materials.
• So, proper selection of pavement materials
can also contribute to the reduction of UHI
effect.

High Albedo (Reflective) Pavements

• Green Roofs
• Roofs in the cities represent about 21% to 26%
of the city area.
• So, if the roofs are made green by vegetating,
it will act a major role in mitigating the UHI
effect.
• Green roofs absorb heat and filter the air,
keeping the temperature low.

Green Roofs
• Plants utilize heat energy to continue their
evapo-transpiration process, making the
environment cool.
• In addition to it, green roofs help to delay the
runoff duration which will keep the cities
cooler for a longer period
• Again, green roofing will bring energy
balance to the corresponding building by
keeping the energy demand low.

• Green Vegetation
• Increasing the amount of vegetation is one of
the most effective strategies to mitigate the
effects this can be achieved by tree plantation
both in residential and municipal tree
plantation programs.
• Trees contribute to reducing the heat island
effect by their evapo-transpiration

• Urban relief - planting trees in cities
• The practice of tree planting within and around cities
is an excellent way of reflecting solar radiation, while
also decreasing the urban heat island effect.
• Trees provide shade, absorb carbon dioxide, release
oxygen and fresh air, and provide a cooling effect.

Urban relief - planting trees in cities

• Shade Trees
• Shade trees are those with a huge canopy and
can provide protection to houses and
pedestrians from direct sunlight keeping them
comparatively cool.
• Shade trees also help to cut down the
temperature by evapo-transpiration process

• Pervious Pavements
• Impervious pavements do not allow water to
infiltrate and cooling effect by evapo-
transpiration is not significant.
• If the impermeable pavements are replaced
with pervious pavements which will allow
water to infiltrate, it can be expected that it
will be able to reduce the temperature to a
reasonable extent.

• Green parking lots
• Green parking spaces utilize sustainable
infrastructure strategies to limit the impacts of
urban heat island effect.
• More specifically, it protects against the
elevation of pavement temperatures which can,
in turn, prevent pollution resulting from storm
water runoff.

• Using harvested rainwater for cooling
• When rainwater is harvested you can re-use that
water in many ways during dry spells:
• Irrigation of lawns and green areas
• Flush toilets with retained water instead of
using town water that might become sparse
• Cool pavement by sprinkling water on it
• Use it on roof surfaces to cool buildings
• Firefighters can use the water if needed

Using Harvested Rainwater For Cooling

• Water Bodies
• Increased amount of water bodies may reduce
temperature due to their evaporative action and
enhanced wind speed is the opinion.
• Again, as the heat absorption capacity of
water is high, it will help to reduce the urban
temperature

• Urban Planning
• Proper urban planning can also play a vital role
in the mitigation of the UHI effect.
• In cities, it is expected that if there is sufficient
amount of free space and channel to circulate
the wind, it will help to minimize the effect of
the urban microclimate

• Awareness and implementation of heat
reduction policies and regulations
• Government environmental policies, such as low
carbon fuel standards and the uses of renewable
energy, can significantly regulate (and mitigate)
the problem of urban heat island effect.
• With fewer emissions, the level of green house
gases in the atmosphere can be reduced. This, in
turn, decreases the effects of climate change and
global warming.

Awareness and implementation of heat
reduction policies and regulations

• Education and community outreach can also
help to ensure that communities are made
aware of the economic and social benefits of
sustainable practices such as planting trees,
eco-roofing/paving.

Conclusion
• Among all the mitigation strategies, green
vegetation seems to be the most effective
measure to encounter UHI effect.
• Also it’s effectiveness is well proven and
widely accepted by the experts as a very much
efficient mitigation measures At the same time,
it has a few drawbacks.

green vegetation seems to be the most effective
measure to encounter UHI effect

Conclusion
• However, cities where tree plantation in a large
scale is not possible use of high albedo materials
and pervious pavements
• Adopted Green roofs may play an important role
also.
• On the other hand, as a small scale mitigation
measure, shade trees can be used where there is
enough space in the house yard considering its’
limitations.
• In growing cities, proper planning must have
to be done to minimize the effect economically.

In growing cities, proper planning must have to be
done to minimize the effect economically

References
Causes of Urban heat island
https://www.conserve-energy-future.com/effects-solutions-urban-heat-island.php
Heat Island Effect
https://www.epa.gov/heatislands
How The Urban Heat Island Effect Is Harming Our Cities
https://hub.beesmart.city/en/solutions/how-the-urban-heat-island-effect-is-harming-our-cities
Urban heat island
https://en.wikipedia.org/wiki/Urban_heat_island
Urban Heat Islands
https://scied.ucar.edu/learning-zone/climate-change-impacts/urban-heat-islands
Heat Island: Causes, Effects and Mitigation Measures
https://www.wits.ac.za/media/wits-
university/conferences/documents/UrbanHeatIslandCausesEffectsandMitigation.pdf
What Is an Urban Heat Island?
https://climatekids.nasa.gov/heat-islands/
What You Can Do to Reduce Heat Islands
https://www.epa.gov/heatislands/what-you-can-do-reduce-heat-islands

Wake Up Before It's Too Late !

urban heat island

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