1. Urban Changes & Climate
Urban Heat Island
Urbanization
Effects
Mitigation Strategies
Science Concepts
Surface Types
Surface Energy Budget - Cities
Ozone - Temperature
Relationship

2. Urban Heat Island How has urban Atlanta growth changed the
climate system
over cities?
• Landsat 7 images
from the 1970s,
1980s and 1990s
show trends in
urbanization
- Note changes in green, i.e., forested area
- For example, Atlanta, GA gobbled up
380,000 acres of trees between 1973 and
averages about 55 acres of trees per
day
Atlanta

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

4. 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

5. Urban Heat Island 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

6. Urban Heat Island Atlanta Urban Heat Island
• Atlanta metro region 9/28/2000
Landsat-7 image
• True-color image (top)
- Urban areas are gray
- Wooded suburbs and open fields
are green
• Surface temperature (bottom)
- Yellow areas are relatively cool
- Red areas are hot
• Note correlation between hot areas and
roads, etc.

7. Urban Heat Island Atlanta Urban Heat Island (Con’t)
• Atlanta Central Business District (CBD)
- May
1997
Midtown
Residential
CBD

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

9. 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

10. 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

11. 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

12. 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.

13. 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

14. 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.
• First noticed ~170 years ago. Even with numerous studies since the 1960’s,
it is still not well-understood.
• 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.