During the 1960s, scientists began experimenting with Doppler weather radars--radars that could detect the motion of raindrops within a cloud. During the 1980s and early 1990s, these radars became operational across the country.
HOW DOES IT WORK?
Doppler radars send out a radio signal which is bounced off of raindrops, hail, snowflakes, and even bugs or birds. Based on the frequency that comes back to the radar from a storm, the radar can tell if the raindrops/hail/etc. are moving towards or away from the radar and how fast (much like a police radar can tell how fast a vehicle is moving towards or away from it).
Currently, weather radars scan a single layer of the atmosphere: the level at which the radar dish is pointed. Research meteorologists are now working on a phased array radar that will scan the entire atmosphere in about a minute (a process that currently takes anywhere from 5 to 15 minutes, depending on weather conditions), similar to how Navy ships scan the atmosphere at sea for airplanes. You can read more about that research here.
322 Shipyard Boulevard