As a hurricane approaches the coast or thunderstorms threaten to become severe, you're likely to hear about what Doppler, or Nexrad radar shows. Doppler refers to the principle the Austrian scientist Christian Doppler discovered in 1842. Doppler worked out his ideas using sound waves, but the same principle applies to radar's radio waves and to light arriving from distant stars. Nexrad refers to the Next Generation Weather Radars the National Weather Service has installed around the U.S., which use the Doppler principle.
All weather radars send out radio waves from an antenna.
Objects in the air, such as rain drops, snow crystals, hail stones or even insects and dust, scatter or reflect some of the radio waves back to the antenna. All weather radars, including Nexrad, electronically convert the reflected radio waves into pictures showing the location and intensity of precipitation. Doppler radars also measure the frequency change in returning radio waves. Waves reflected by something moving away from the antenna change to a lower frequency. Waves from an object moving toward the antenna change to a higher frequency.