Precipitation (falling rain and snow) that occurs around the globe is the source of fresh water on which all life depends. Even in the tropics, much of the precipitation begins as snow in the clouds high above the surface, then generally melts as it falls into warmer temperatures near the surface, except when the surface temperature is below freezing. The heaviest precipitation is found in the persistent band near the Equator, known as the Intertropical Convergence Zone (ITCZ). Just outside the ITCZ, more noticeably in the Northern Hemisphere, the tight swirls of tropical storms (including hurricanes and typhoons) can be seen occasionally in the northern summer. At subtropical latitudes in both hemispheres there are persistent dry areas associated with subtropical high pressure centers. The major deserts of the world occur in the subtropical highs, but these highs also cover vast regions of the oceans. At midlatitudes, the storm tracks display the swirls of low pressure systems and bands of frontal precipitation. [Note the opposite directions of circulation in the Northern and Southern Hemispheres!] Important year-to-year and shorter-interval variations cause strong modifications to local conditions, bringing droughts and floods.
Precipitation changes the naturally occurring microwave energy emitted by the Earth's surface and atmosphere. These signals are detected by satellite instruments that are attuned to specific microwave frequencies. In addition, a few satellites carry weather radars, providing data to adjust the estimates of precipitation from the microwave instruments. Finally, sometimes it is useful to estimate precipitation from information about cloudiness that is provided by infrared sensors flying on geostationary satellites (geo-IR).
This NASA GPM IMERG dataset is provided to Science On a Sphere in near-real time, about six hours after the observation. This dataset uses information from as many satellite microwave sensors as possible, and includes satellite radar data for calibration and geo-IR for additional information. The satellite data provide observations at each location about every three hours or less, and smooth transitions from one observation to the next are computed to give estimates every half hour.