Pulse weather radars measure the wind field by harnessing the Doppler effect. This is the shift in wave frequency due to the movement of the wave source relative to an observer. You have probably noticed this at a race track, railroad crossing, or busy highway where the vehicles make a high-pitched sound that immediately changes to a lower pitch once they have passed you. For an example, play the animation.

While the driver hears only a constant sound, the observer hears a change in pitch. This happens because, as the source of the sound waves moves toward the observer, each wave takes slightly less time to reach the observer than the previous one, producing a higher frequency sound. When the source passes, each wave is emitted from farther away, resulting in a lower frequency sound.