1. Urban Heat Island (UHI)
What is “Urban Heat
Island”?
• Urban areas are warmer, like
an “island” of heat surrounded
by cooler rural areas
• Large horizontal temperature
gradient exist at the urban/rural
boundary, could be as large as
4C/km
• Urban areas appear like a “plateau” with weaker increasing gradient. This
pattern is influenced by distinct intra-urban land uses: open areas, parks,
lakes (cool), dense buildings, commercial areas or industrial areas (warm)
• Intensity of the UHI depends on the urban-rural temperature difference

3. Urban Heat Island Madison County Growth 1984 – 13% Developed
LEGEND
Developed Land
Undeveloped Land
Change: 1984 – 1990
Change:

4. Spot the city

5. Salt Lake City 7/13/98

6. Population size v. heat island effect

7. New York City

8. Chicago is 101 degrees F, surrounding country is in the mid 70’s.

9. Urban Heat Island What happens to the Sun’s energy
at the Earth’s surface over a city?
• An acre of forest or cropland receives as
much sunlight as an acre of buildings
• Green space will be cooler because of
transpiration and shading of the ground
• Urban surfaces get much hotter than
vegetated surfaces during the day.
• They release this energy at night, creating
a dome of warmer air at night

10. Urban Heat Island Atlanta Urban Heat Island
• Atlanta Central Business District (CBD)
- May 2007
Midtown
Residential
CBD

11. Urban Heat Island Atlanta Urban Heat Island (Con’t)
• Atlanta cross-city
temperature
Midtown
Residential
CBD

12. Climate Elements Urban Heat Island Intensity
• Factors below contribute to UHI, relative roles are not certain
• Factors combine to make the urban area store sensible heat during the day,
and inhibit this excess heat from dissipating rapidly at night.
Altered energy budget terms
Features of urbanization underlying energy budget change
Increased absorption of short wave radiation
Canyon geometry - increased surface area and multiple reflection, dark materials (e.g., asphalt
Increased long wave radiation for sky
Air pollution - greater absorption and emission
Decreased long wave radiation loss
Canyon geometry -- reduction of sky view factor
Anthropogenic heat source
Waste heat from city buildings and traffic
Increased sensible heat storage
Construction materials - increased thermal admittance
Decreased evaporative cooling
Construction materials - increased water-proofing, less vegetation, shrubs, and other plants
Decreased total turbulent heat transport
Canyon geometry - reduction of wind speed

13. Urban Heat Island Urban Heat Island Effects
• UHI has several direct and indirect
effects; including biological, economic
and meteorological effects
- Increases demand for cooling
energy
- Increases electricity generation
which leads to higher emissions
of SO2, CO, NO, PM, CO2,
accelerates the formation of
harmful smog, higher O3
- Increases chemical weathering
of building materials

14. Urban Heat Island Urban Heat Island Effects (Con’t)
• UHI has several direct and indirect effects; including biological, economic
and meteorological effects (Con’t)
- Temperature affects ozone amount
> Increases the thermal
destruction of some
hydrocarbon-nitrate
molecules
(peroxyacetyl-
nitrate or
PANs)
that free NO2
> Increases the
vegetative emission
of VOCs and
evaporation
of VOCs

15. Urban Heat Island Urban Heat Island Effects (Con’t)
- Induces convergence over city
- Initiates convective activity downwind of city
- Precipitation and cloud cover enhancement occur downwind of city
- Trends of global warming are higher in urban areas; most weather
stations are located near urban areas, which could bias the trends of
global warming.

16. Changes over time?
Night temperatures in Phoenix are 7 to 13 degrees F warmer than before 1970
Winds over cities decrease by 20-25%

17. Effects:
Can raise temperatures over cities to more than 10 degrees 10C over that of surrounding areas. Montreal and Paris are degrees C warmer.
Can increase peak energy demand
Air conditioning costs increase
Air pollution increase
Heat-related illness and mortality increase

18. As of 1990, 45% of people live in cities
UN study says that by 2025, 80% will live in cities

19. Urban Heat Island
What can be done to mitigate the effects of urbanization?
• NASA has been flying aircraft over
nine cities to determine their thermal
characteristics that drive the
development of the urban heat
island
• Working with city officials, urban
planners, architects, schools and
other groups to educate and affect
change
- Color of roofs
- More trees
- Better community planning

20. Urban Heat Island Urban Heat Island Summary
• Urban areas are usually warmer than rural areas, especially at night. Under
ideal conditions, UHI generates its own circulation, radially inward flow at
street level, radially outward flow above.
• Formation of UHI and its magnitude results from urban and geographical
characteristics, city size (population, height-to-width ratio), meteorological
conditions (wind speed, cloud cover).
• UHI has distinct seasonal behavior, usually greatest in the summer, weakest
in the winter.
• UHI not only affects temperature, it affects cloudiness, precipitation and air
quality. It also impacts global warming.

22. Space-based rain radar on NASA satellites show precipitation downwind of cities is an average of 28%, and up to 51% greater than upwind

23. And max. rainfall rates are on average 48-116% greater