Presentation on Urban Heat Island. An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surrounding rural areas. Main Cause of the UHI is Modification of the land surface by urban development which uses materials which effectively retain heat. As population centers grow they tend to modify a greater and greater area of land and have a corresponding increase in average temperature. The Effect of building materials on Albedo The Effect of Building Materials on Storage of Internal Energy Generation of Internal Energy By Human Activity Reduced Evaporation

1. Urban Heat Island …
ROLL NO .17141

2. Introduction
An urban heat island (UHI) is a metropolitan area which is significantly
warmer than its surrounding rural areas.
Dark materials absorb solar energy and release it back to the surrounding air
as heat. This condition causes the temperature in urban areas to be much
hotter than rural areas.
The temperature difference usually is larger at night than during the day,
and is most apparent when winds are weak.
Seasonally, UHI is seen during both summer and winter.
The air in an urban heat island can be as much as 20°F (11°C) higher
than rural areas surrounding the city.

3. CharacteristicsofHeatIslands
Surface temperatures vary more than atmospheric air temperatures during the day, but they are generally similar at night. The dips and spikes in
surface temperatures over the pond area show how water maintains a nearly constant temperature day and night because it does not absorb the
sun’s energy the same way as buildings and paved surfaces. Parks, open land, and bodies of water can create cooler areas within a city.
Temperatures are typically lower at suburban-rural borders than in downtown areas

4. Causes:
 Modification of the land surface by urban development which uses materials
which effectively retain heat.
 As population centers grow they tend to modify a greater and greater area of land
and have a corresponding increase in average temperature.
 The Effect of building materials on Albedo
 The Effect of Building Materials on Storage of Internal Energy
 Generation of Internal Energy By Human Activity
 Reduced Evaporation

5. Albedo (al-bee-doh) is a measure of how much light that hits a surface is reflected without being absorbed. Something that
appears white reflects most of the light that hits it and has a high albedo, while something that looks dark absorbs most of the
light that hits it, indicating a low albedo.
The Effect of building materials on Albedo
lower albedo absorbs
most solar radiation
higher albedo reflects
more solar radiation
cooler

6. Building materials have higher heat
capacities and store more internal energy
Temperature higher at night Vegetation has lower heat capacity and
can’t store much internal energy
Temperature decreases more quickly in
rural areas cooler at night
The Effect of Building Materials on Storage of Internal Energy
Building materials generally have higher heat capacities than soil and Vegetation. This means that buildings, streets and parking
lots tend to retain internal energy longer and stay warmer than surrounding rural areas especially at night.

7. Generation of Internal Energy By Human Activity
Many human activities generate internal energy as a waste byproduct (e.g. power plants, industrial processes, energy loss from
buildings). The much higher population densities in cities mean that these processes generate much more energy than in surrounding
rural areas.
Internalenergygenerated as
aresultof humanactivity
Higher Temperatures
very little internal energy generated
as a result of human activity
Lower Temperatures

8. Reduced Evaporation
The reduced evaporation in the city means that the relative humidity is lower than it is in surrounding rural areas. Since water absorbs
energy when it evaporates, the reduced evaporation in the city also contributes to higher temperatures than in the surrounding rural
areas.
The relative humidity tends to be lower
over a city and
The temperature higher
The relative humidity is higher and
The temperature is lower

9. Types of heat island
Temperatures are different at the surface of the earth and in the atmospheric air, higher
above the city. For this reason, there are two types of heat islands: surface heat islands and
atmospheric heat islands.
Surface Heat Islands –
 Form because urban surfaces such as roadways and rooftops absorb and emit heat to a greater
extent than most natural surfaces.
 Surface heat islands tend to be most intense during the day when the sun is shining.
Atmospheric Heat Islands –
 form as a result of warmer air in urban areas compared to cooler air in outlying areas
 Atmospheric heat islands vary much less in intensity than surface heat islands.

10. HeatIslandImpacts
Elevated temperatures from heat islands can affect a community’s environment and quality of life in
multiple ways
Increased Energy Consumption
Elevated Emissions of Air Pollutants and Greenhouse Gases
Reduced Precipitation in Urban Areas
Compromised Human Health and Comfort
Impaired Water Quality

11. IncreasedEnergyConsumption
Heat islands increase demand for air conditioning to cool buildings
In an assessment of case studies spanning locations in several countries, electricity demand for air
conditioning increased approximately 1–9% for each 2°F increase in temperature.
Countries where most buildings have air conditioning, such as the United States, had the highest increase
in electricity demand.[1] This increase demand contributes to higher electricity expenses.
Heat islands increase both overall electricity demand, as well as peak energy demand.
During extreme heat events, which are exacerbated by heat islands, the increased demand for air
conditioning can overload systems.

12. Elevated EmissionsofAirPollutantsandGreenhouseGases
Heat islands raise demand for electricity in summer and electricity typically rely on fossil fuel power plants
to meet much of this demand, which in turn leads to an increase in air pollutant and greenhouse gas
emissions.
These pollutants are harmful to human health and also contribute to complex air quality problems such as
the formation of ground-level ozone (smog), fine particulate matter, and acid rain.
Increased use of fossil-fuel-powered plants also increases emissions of greenhouse gases, such as carbon
dioxide, which contribute to global climate change.
Elevated temperatures can directly increase the rate of ground-level ozone formation.
Due to the thermal and topographical properties of buildings they create effective heat traps for solar
radiation

13. CompromisedHumanHealthandComfort
Contribute to heat-related deaths and heat-related illnesses such as general discomfort, respiratory
difficulties, heat cramps, heat exhaustion, and non-fatal heat stroke.
Excessive heat events, or abrupt and dramatic temperature increases, are particularly dangerous and can
result in above-average rates of mortality.
It exacerbate the impact of naturally occurring 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 particularly at risk during these events
From 2004 to 2018 the Centers for Disease Control and Prevention recorded 10,527 heat-related deaths in
the United States, an average of 702 per year.

14. ImpairedWater Quality
High temperatures of pavement and rooftop surfaces can heat up stormwater runoff
Rapid temperature changes in aquatic ecosystems resulting from warm stormwater runoff can be
particularly stressful, and even fatal, to aquatic life.
One study found that urban streams are hotter on average than streams in forested areas, and that
temperatures in urban streams rose over 7°F during small storms due to heated runoff from urban
materials

15. UrbanHeat IslandMitigation
action to reduce urban heat islands using four main strategies:
1. Increasing tree and vegetative cover,
2. Installing green roofs (rooftop gardens or eco -roofs),
3. Installing cool—mainly reflective—roofs, and
4. Using cool pavements

16. Increasing tree and Vegetative cover
Trees and other plants help cool the environment, making vegetation a simple and effective way to reduce urban
heat islands
They act as nature's air conditioners.
Leaves help reduce air pollution by "capturing" airborne particles, such as Nitrogen dioxide, Nitrogen oxide and
Sulfur dioxide, SO2
Trees not only helps to reduce the urban heat island effect, but also it reduces air pollution, higher demands on
cooling systems and health problems related to heat and pollution.

17. Installing Green Roofs
Green roofs provide shade and remove heat from the air through Evapotranspiration, reducing temperatures of the roof
surface and the surrounding air. On hot summer days, the surface temperature of a green roof can be cooler than the air
temperature

18. Cool Roofs
A high solar reflectance or albedo is the most important characteristic of a cool roof as it helps to reflect sunlight and heat away from a
building, reducing roof temperatures. High thermal emittance help roofs to absorb less heat and stay up to 50–60°F (28–33°C) cooler
than conventional materials during peak summer weather.