3.13 - Global Distribution of Precipitation

Average Annual Precipitation

Tropical latitudes contain most of the World's wettest areas. The warm easterly trade winds carry enormous amounts of moisture, and where air is forced to rise, very heavy rain falls. These are the Trade Wind Coasts. In Equatorial regions warm, moist, unstable air is uplifted in the ITCZ causing consistent, high precipitation. 

West coasts in North and South America between 40° and 60° of latitude also have high annual precipitation. These areas are in the path of the westerlies. Air blows ashore and is forced to rise over the many  north-south coastal mountains on both continents causing significant precipitation. To the east of these zones, there is a pronounced rain shadow effect and consequently, little rainfall (think Nevada).

The dry areas of the world include the western sides of continents between 25° and 30° latitude. The Sub Tropical highs dominate here year round. There are also dry areas in the Western U.S., north central Mexico, southwestern Asia,  and the center of North Africa and Australia. In each case, dryness is due, at least in part, to lack of access to moist air masses. Continentality plays a role, as all these areas are in the center of continents far from a moisture source. Among the driest places on the planet are the high latitudes, Antarctica and the Arctic, where water surfaces are scarce and cold so that there is little evaporation.  They are essentially cold deserts.

Seasonal Precipitation Patterns

In most places in the World, most precipitation falls in Summer. Over the interior of continents,  strong summer heating causes instability and the potential for greater convective activity.  Coastal areas often have more balanced seasonal precipitation though again, most places still get most of the moisture in the summer.  California is not the norm. We share our summer drought with other Mediterranean climate locations around the world (central Chile, around the Mediterranean Sea, the Cape Town area of South Africa and southern and south western Australia). During our summers the Sub Tropical high moves north, blocking precipitation. In winter, it moves south and storms are able to move in.