Urban Heat Island Effect

New York City

Effects: 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 increase

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.