Urban Heat Island Effect

Urban Heat Island Effect
New York City
• Can raise temperatures over cities 1 to
more than 10 degrees F over that of
surrounding areas. Montreal and Paris
are 10-24 degrees C warmer.
• Can increase peak energy demand
• Air conditioning costs increase
• Air pollution increase
• Heat-related illness and mortality
• Night temperatures in Phoenix are 7 to
13 degrees F warmer than before 1970
• Winds over cities decrease by 20-25%
Chicago is 101 degrees F, surrounding country is
in the mid 70’s.
Salt Lake City 7/13/98
Satellite photo of earth at night
Space-based rain radar on NASA satellites show
precipitation downwind of cities is an average of 28%,
and up to 51% greater than upwind
And max. rainfall rates are on
average 48-116% greater
Population size v. heat island effect
• As of 1990, 45% of people live in cities
• UN study says that by 2025, 80% will
live in cities
U.S. Weather Service meteorologists
Kalnay and Cai (Nature, 2003)
• Human influence on climate may be twice as large as the urban heat
island effect factor assumed by the climate models. Past estimates of
urbanization and land use changes have been based on rising
population counts or satellite measurements of nighttime urban lights.
They do not include changes in reflectance and soil moisture
produced by forest clearing, the shifting of land from pasture to crops,
adding irrigation- which have impacted millions of acres of land.
Therefore, climate models may overestimate surface temps by 40%.
If the Kalnay-Cai trend is correct, the U.S. temp trend for the twentieth
century drops from 0.45 degrees C to 0.25 degrees. C. This is not
statistically significant.
Their research highlights the problem of separating real warming from
effects of land use changes and city-building. It also confirms that the
Medieval and Roman Warm periods were warmer than the Modern
Warm period.