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In dry regions the timing of precipitation is critical. On this page, we explore the seasonality of precipitation.

The image at the top shows the percentage of the annual precipitation which falls in the rainiest three-month period. The range of this parameter is 25% (for locations in which every month receives the same amount of precipitation) to 100% (for locations in which all precipitation falls in one three-month season). The higher this number, the more effort must be put into storing water for the dry season. Areas which are green to red, including most of the Middle East, have highly seasonal rainfall. Of note are the red areas, including the Sahel and the Great Indian Desert, which have extremely seasonal precipitation.

For agriculture, a problem related to seasonality is how much rain falls in the growing season. The middle figure shows the fraction of precipitation which falls in the cold season, that is in the nongrowing season. For the Northern Hemisphere, the cold season is defined as October through March. For the Southern Hemisphere it is defined as April through September. In the tropics, this parameter is not useful because temperatures are always high enough to grow crops. Outside the tropics, however, if rain falls in the cold season, it may be useless for watering warm-season crops unless extensive storage projects have been constructed.

Most of the Middle East outside of the tropics has the dry summer climate typical of the Mediterranean. It shares this climate also with western Australia, western portions of South America and southern Africa, and especially with the west coast of the United States. What lessons can be learned from the way these places have coped with their water problems? Click Next Page for our conclusions.


Fraction of rain which falls in the rainiest three months (%).

Cold-Season Precipitation

Fraction of rain which falls in the cold season (October-March, Northern Hemisphere; April-September, Southern Hemisphere).

Annual Precipitation

Annual Precipitation (mm).

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