Precipitation Along the Tropics of Cancer and Capricorn

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In the tropics and subtropics (between about 30 North and 30 South) there is a distinct tendency for the eastern sides of continents to be wet and the western sides to be dry. This can be clearly seen in the graphs at the right and also in the global map. There are several reasons for this. Among them:
  • The primary source of water vapor which eventually falls as rain is evaporation from the warm tropical and subtropical oceans.
  • In the tropics and subtropics, the wind usually blows from the east (the Trade Winds), which means that moist ocean air impinges on the east side of continents. (Note)
  • On the east side of the continent, heating by the land or flow over elevated terrain causes the air to rise, forming clouds and precipitation.
  • Air flowing over the west side of continents and over the adjoining eastern oceans comes from the east side of the continent; it has been depleted of much of its water vapor and therefore of its precipitation potential. This process is evident even for islands such as Taiwan and Madagascar.

There are interesting variations on this basic mechanism caused by atmospheric circulation. One example of this is the precipitation maximum at 90 East. Here southerly winds from the Bay of Bengal impinge on the Himalayas, causing heavy rain.

Asia, Africa, and Europe form the world's largest land mass. Because the Sahara and Middle East are in the subtropics on the western side of this supercontinent, they are exceedingly dry.

Now, what about clouds and water vapor? Are they consistent with the precipitation? Click Next Page to find out.

Tropics Precipitation

Annual precipitation along the Tropics of Cancer and Capricorn. The global mean annual precipitation (943 mm) is shown as a dashed line.
Annual Precipitation

For comparison: mean annual precipitation (mm).

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