The combatant nations of World War II developed radar to detect enemy airplanes. They also noticed that they were picking up signals from raindrops, hailstones, and snowflakes. After the war these signals were used for weather forecasting. From 1948 weather radar was carried on aircraft to help the crew detect potentially hazardous cumulonimbus cloud systems.
Weather radar stations transmit a directional, narrow beam of pulsed microwaves up toward the clouds, using an antenna that rotates and scans the sky. These microwaves typically have wavelengths of 1-10 centimeters, these lengths being about ten times the size of the raindrops, hailstones, and snowflakes they are aiming to detect. The falling particles scatter the microwaves and a return pulse is then picked up back at the radar station.
Five things are measured. The-time taken for the pulse to travel to and from the scattering body gives the distance to the precipitation. The Doppler shift of the return pulse gives the direction and speed in which the cloud is moving. Variability in the Doppler signal can indicate the turbulence of the rain drops and thus gives clues as to whether thunderstorms or even tornadoes might develop. The strength of the return pulse is a function of the amount of rain in the specific cloud, and the shape of the return pulse distinguishes between rain, snow, and hail. To get a complete 3D moving weather picture, together with cloud heights, requires an interlinked series of weather radars dotted over the land. Meteorological weather radar usually provides a complete radar rain picture every half hour.
