The Global Precipitation Climatology Project (GPCP) was established by the World Climate Research Program (WCRP) in 1986 with the goal of providing monthly mean precipitation data on a 2.5 x 2.5 degree latitude-longitude grid for the period 1980-2004. The GPCP has accomplish this by combining infrared and microwave satellite estimates of precipitation with rain gauge data from more than 30,000 stations. Infrared precipitation measurements are obtained from Geostationary Operational Environmental Satellites (GOES) (United States), Geostationary Meteorological Satellite (GMS) (Japan) and Meteosat (European Community) geostationary satellites and National Oceanic and Atmospheric Administration (NOAA) operational polar orbiting satellites. Microwave estimates are obtained from the U.S. Defense Meteorological Satellite Program (DMSP) satellites using the Special Sensor Microwave Imager (SSM/I). Together these data sets will be used to validate general circulation and climate models, study the global hydrological cycle and diagnose the variability of the global climate system. Figure 1 describes mean annual global precipitation over an twenty-five year period measured in millimeters per month.
The average annual precipitation of the entire surface of our planet is estimated to be about 1050 millimeters per year or approximately 88 millimeters per month. Figure 1 indicates that actual values vary spatially from less than 10 millimeters per month or to a maximum of more than 300 millimeters per month depending on location. The reasons for these patterns are as follows:
|Table 1: Precipitation extreme weather records.|
|1-year Rainfall||Cherrapundi, India||26,470||1861|
|1-month Rainfall||Cherrapundi, India||9300||1861 (July)|
|Average Annual Rainfall||Mt. Waialeale, Hawaii, USA||11,680|
|24 hr. Rainfall||Belouve, La Reunion Island||1350||Feb 28, 1964|
|Lowest Annual Average Rainfall||Arica, Chile||0.8|
|Greatest 1 Month Snowfall||Tamarack, California, USA||9910||1911 (Jan)|
|Greatest Snowfall Single Storm||Mt. Shasta, California, USA||4800||Feb 13-19, 1959|
- 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. This can also result in a sharp reduction in rainfall in regions adjacent or on the leeward slopes of these areas. This phenomenon is commonly know as the rainshadow effect.
Table 1 describes some of the precipitation extremes recorded around the world.