NASA engineers building the GPM Core Observatory
People have measured rain and snow for centuries. Basic instruments such as a container with markings on the outside can catch rain and measure how much has fallen. More specialized gauges can provide information about the size of raindrops and how wind may impact the measurements. Ground-based weather radar systems send out a pulse of energy into clouds and can tell us where and how heavy the rain, snow and hail is within our area. However, when we want to know what is happening outside of our city or town, we often rely on satellites orbiting above the earth to give us a bigger picture of rain and snow over the globe. Satellites like the Tropical Rainfall Measuring Mission (TRMM) and the Global Precipitation Measuring (GPM) mission use sophisticated instruments that can look at storms in 3-D and understand how rain and snow vary over both land and oceans around the world.