
The Global Precipitation Climatology Project monthly product: satellites and rain gauges merged on a 2.5° grid over land and ocean since 1979, in millimetres per day.The Global Precipitation Climatology Project (GPCP) is an international effort to produce a long-term, global record of rainfall. It merges data from many satellite sensors and ground-based rain gauges into a single monthly dataset on a regular grid. The result is a consistent estimate of how much rain (or snow, expressed as liquid water) falls over every part of the planet, including oceans where gauges are scarce.
GPCP was established in 1986 under the World Climate Research Programme to address the need for a global precipitation climatology, with the first multi-year products released in the 1990s.
Satellites estimate precipitation indirectly—for example, by measuring microwave emissions from raindrops or infrared cloud-top temperatures. Each method has biases. GPCP combines these satellite estimates with gauge measurements using statistical techniques that weight each source by its reliability in a given region and time. The merged product is then averaged onto a 2.5° latitude–longitude grid for each month.
GPCP data are used to validate climate models, monitor droughts and floods, study the global water cycle, and calibrate other satellite rainfall products.
Because rain gauges cover only about 20% of Earth's surface, satellites are the only way to measure precipitation over the vast oceans, which receive most of the planet's rainfall.



















