Cities are hot places; in fact, they can be islands of heat in the midst of a cooler regional environment. "Urban heat island" is a term that many of us may have heard of, but not fully understood. The term comes from the fact that urban areas, with their many buildings, expanses of pavement, and other impermeable surfaces, are warmer than surrounding rural and suburban areas, forming "islands" of higher temperatures, both in the atmosphere and at the surface.
Surface heat islands tend to be warmest during the day when the sun is shining and surfaces are reflecting heat, with exposed surfaces often as much as 50 to 90 degrees warmer than the air! Atmospheric heat islands are generally hotter during the night as buildings and roads radiate the heat they absorbed during the day back into the air. On a calm, clear night, all this stored heat can make it as much as 22 degrees warmer in a city than in surrounding, non-urban areas.
Why do we care if our cities are warmer? Higher temperatures increase the demand for electricity for cooling, leading to increased emissions of pollutants from power plants. They also promote the formation of harmful ground-level ozone. The heat and increased pollution make life more unpleasant and sometimes more dangerous for city dwellers. And when the heat from impermeable surfaces is transferred to water running off it, this raises the temperatures of the waterways receiving the runoff, causing harm to aquatic ecosystems.
What is one of the best ways to combat the urban heat island effect? Plant trees, of course! Increasing the area covered by trees and other kinds of vegetation helps to keep cities cooler. Trees in the urban landscape provide many benefits. They offer city dwellers beauty and a connection to the natural world and improve the urban environment by reducing storm runoff and cleaning the air. Just as important, city trees also help to reduce the negative impacts of the urban heat island effects both through the cooling shade they provide to surfaces and the evaporative cooling that occurs as the trees transpire moisture into the air.
To find out more about the heat island effect and its impact on energy use, water and air quality, and human health, and learn more about strategies for reducing urban temperatures and heat island mitigation activities in your state or community, check out the informative resources from the U.S. Environmental Protection Agency at Heat Island Effect.