A radar sends out repeating pulses of electromagnetic radiation, using a high-powered transmitter. The radar is also equipped with a receiver that listens for these pulses to return, once they've struck an object. The return signal is called an echo. Processors then interpret the return echoes and paint a picture of any detected objects, usually over a map of the surrounding country that's drawn across the radar's screen. Radars usually rotate 360 degrees every minute (sometimes less) as they send and receive their signals. The movement of the radar beam is often displayed on the radar screen as a rotating white line called the scan.
---D.M.
