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Urban areas can help make clouds and rain fall within or downwind of the city.
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Courtesy of the University Corporation for Atmospheric Research

Weather in Urban Areas

Scientists have found that it rains more around cities than in other places nearby. They suspect that cities can affect the weather and have several different ideas about how this might happen. Each of these ideas is called a hypothesis. Scientists are studying each hypothesis to figure out why it is extra rainy in cities.

One hypothesis is that the urban heat island effect is to blame for all the extra rain. The heat island effect warms air in cities. Warm air rises. It cools as it rises. This causes the water vapor in it to condense and make clouds. If the warm, rising air was carrying enough water vapor, those clouds can grow into rainclouds.

Another hypothesis is that when wind hits the skyscrapers and other tall buildings in a city it is pushed up higher in the atmosphere. As the air flows upward it cools, allowing water vapor to condense, forming rainclouds.

Tall buildings might have another effect that causes clouds and rain. Wind may divide to get around a city with some of it blowing around one side of the city and some of it blowing around the other side. Past the city, the wind comes back together. It collides, flows upward, cools, and releases water vapor forming clouds.

Air pollution in cities may also affect cloud formation and rain. Water vapor condenses on tiny particles in the air pollution, forming the droplets that make a cloud.

Last modified July 15, 2009 by Lisa Gardiner.

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