Urban areas can help make clouds and rain fall within or downwind of the city.
Click on image for full size
Courtesy of the University Corporation for Atmospheric Research
Weather in Urban Areas
Cities can affect the weather. Looking at satellite data, scientists have observed that heavy rainfall often occurs around cities. They have several different hypotheses that may explain how cities influence weather.
One hypothesis is that the urban heat island effect starts air moving in a way that eventually leads to rain. The urban heat island effect can cause air temperatures to climb higher in a city than in the surrounding area. Air is unstable when it is warmer than the air that surrounds it and it starts to rise. The rising 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 storm clouds.
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 rising, unstable air cools, causing its water vapor to condense, forming clouds, which can lead to rain.
Tall buildings might have another effect on the wind which leads to clouds and rain too. As wind approaches the buildings, it may be divided with some of it going around one side of the city and some of it around the other side. Past the city, the wind comes back together. It collides, causing unstable air to rise, cool, release water vapor and form clouds.
Air pollution in cities may also affect cloud formation and rain. Water vapor condenses upon tiny particles in the air pollution called aerosols, forming the droplets that make a cloud. The aerosols in air pollution are often smaller than natural aerosols. This allows water to form many tiny droplets instead of fewer, larger drops. In some cases, skies are cloudy but less rain falls because the droplets are small. In other cases, pollution aerosols may cause more rain.
Last modified July 15, 2009 by Lisa Gardiner.
Shop Windows to the Universe Science Store!
The Fall 2009 issue of The Earth Scientist
, which includes articles on student research into building design for earthquakes and a classroom lab on the composition of the Earth’s ancient atmosphere, is available in our online store
You might also be interested in:
Rain is precipitation that falls to the Earth in drops of 5mm or greater in diameter according to the US National Weather Service. Virga is rain that evaporates before reaching the ground. Raindrops form...more
An urban heat island is a metropolitan area that is significantly warmer than nearby rural areas. Many cities have air temperatures 2 - 5°C (3.6 - 9°F) warmer than the surrounding natural land cover. This...more
Condensation is the process by which water changes its state from a vapor or gas to a liquid. Condensation is responsible for the formation of clouds. Common examples of condensation are: dew forming on...more
Wind is moving air. Warm air rises, and cool air comes in to take its place. This movement creates different pressures in the atmosphere which creates the winds around the globe. Since the Earth spins,...more
A cloud is composed of tiny water droplets or ice crystals that are suspended in the air. A series of processes have to happen in order for these water droplets or ice crystals to form into clouds in the...more
What do smog, acid rain, carbon monoxide, fossil fuel exhausts, and tropospheric ozone have in common? They are all examples of air pollution. Air pollution is not new. As far back as the 13 th century,...more
Aerosols, also called particulates, are tiny bits of solid or liquid suspended in the air. Some aerosols are so small that they are made only of a few molecules – so small that they are invisible because...more