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

Heavy rainfall often occurs around cities. In fact, cities themselves can affect the weather. Scientists have several different hypotheses that may explain how cities impact rain.

One hypothesis is that the urban heat island effect, which causes warmer temperatures in cities, creates unstable air which leads to rain. Air is unstable when it is warmer than the air around it. The warm, unstable air starts to rise. The air cools as it rises, which allows water vapor within it to condense and form 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. This makes unstable air. The unstable air flows upward and cools, allowing water vapor to condense, forming clouds, which can lead to rain.

Tall buildings might have another effect that causes clouds and rain. As wind approaches the buildings, it may be divided 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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