This cross section through a typical city shows how temperatures are usually lower at the urban-rural border than in dense downtown areas.
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Lisa Gardiner / Windows to the Universe, based on a figure from the Lawrence Berkeley National Laboratory

The Urban Heat Island Effect

It often feels a bit warmer in a city than it does in a nearby rural area. This is called the urban heat island effect.

What makes cities warmer? †Urban heat islands form because the buildings, roads, and parking lots in cities hold onto heat more than natural places full of trees and other plants. Trees shade the ground, preventing the Sunís energy from being absorbed. Without them, the ground surface heats up. Cars and trucks make heat from their engines and exhaust, which adds to the heat island effect.

An urban heat island can make a heat wave hotter and last longer in a city. City heat can influence the weather too - changing winds, clouds, and rain.

Today, many cities are making an effort to combat the urban heat island effect. White or reflective materials are being used for roofing and roads. Trees are being planted along city streets. And, in many areas, green roofs - living plants on rooftops Ė are being installed.

Last modified July 15, 2009 by Lisa Gardiner.

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