Why Is The Sahara So Dry?

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Let's simplify the problem. The annual precipitation image on the preceding page is complicated. Precipitation varies with latitude (in the north-south direction), with longitude (in the east-west direction), and with time. [Click here to see a movie of mean monthly precipitation.]

Let's remove the longitudinal variation by averaging the data all the way around the Earth at each latitude. The result is called a "zonal" average. (Zones are areas of constant latitude.)

Now we see that precipitation falls in preferred zones:

  • The greatest precipitation falls in the deep tropics, within about 15 of the equator, where intense solar radiation causes rising air, clouds, and precipitation.
  • Near the Tropics of Cancer and Capricorn, which bound the sun on its annual journey, are areas of low precipitation--low, that is, in comparison with the global average precipitation. Here the air which rose near the equator sinks, which evaporates clouds and suppresses precipitation. (Note)
  • Poleward of the subtropics are the mid-latitude precipitation maxima caused by eastward-moving storm systems.
  • Finally, the polar regions are very dry, but, due to low temperatures, evaporation is also low, and the precipitation which does fall stays as ice (at least in Antarctica).

The Sahara and the Middle East are dry in part because they straddle the the Tropic of Cancer.

But the Tropic of Cancer isn't dry everywhere. Click Next Page to see what this means.

Zonal Precipitation

Zonally averaged precipitation from the GPCP data set. The area-weighted global mean annual precipitation is 943 mm.

Annual Precipitation

For comparison: mean annual precipitation (mm).

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