If you've misspent any part of your life watching TV you've undoubtedly seen meteorologists pointing at radar--often Doppler radar. Radar is short for radio detection and ranging. A blast of radio energy is sent out to bounce off the hydrometeors we mere mortals usually call raindrops. Measure the time it takes the radar signal to make the roundtrip, divide by the speed of light, add the processing time in the radar's electronics and you've got the distance to your rain. Factor in the direction the radar antenna is pointed and you know exactly where the rain is... well, not exactly. More on that in a second.

Doppler radar (capital "D" because it's named after Christian Doppler) is an added function of the radar that measures the speed those hydrometeors are moving by seeing how the frequency of the returned radio signal has shifted. If it pitches up, the drops are moving toward the radar, pitched down and they're moving away. Stand near a highway listening to cars drive by and you'll experience a similar Doppler shift. The pitch of a car's sound shifts down as it drives past you and away.