Mean annual global precipitation 1988-1996. The average annual precipitation of the world is estimated to be 1050 millimeters per year or 2.9 millimeters per day. However, Figure 6g-1 indicates that actual values from vary from a minimum of 0 millimeters per day or to a maximum of 10 millimeters per day depending on location. The deserts in the subtropical regions occur because these areas do not contain any mechanism for lifting air masses. In fact, these areas are dominated by subsiding air that results from global circulation patterns.
Continental areas tend to be dry because of their distance from moisture sources.
Polar areas are dry because cold air cannot hold as much moisture as warm air. Areas near the equator achieve high rainfall amounts because constant solar heating encourages convection, and global circulation patterns cause northern and southern air masses to converge here causing frontal lifting . Mid-latitudes experience cyclonic activity and frontal lifting when polar and subtropical air masses meet at the polar front. Further, the air masses in this region generally move from West to East, causing levels of precipitation to decrease East of source regions. Mountain ranges near water sources can receive high rainfalls because of orographic uplift , if and only if the prevailing winds are in their favor.