How does weather radar work?

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How does weather radar work?
Crane lowers huge dome over large concrete structure within which people stand
Crane moves radome into position during installation of radar antennae at NOAA’s National Severe Storms Laboratory in Norman, Oklahoma. NOAA

Radar, an acronym for RAdio Detection And Ranging, was invented during World War II to detect aircraft, but precipitation frequently got in the way. The military’s noise is meteorology’s signal.

A radar unit consists of a transmitter and a receiver. The transmitter emits pulses of microwaves, a type of radio waves, outward in a circular pattern. Precipitation scatters these microwaves, sending some energy back to the transmitter, where it is detected by the radar’s receiver. The intensity of this received signal, called the radar echo, indicates the intensity of the precipitation. Measuring the time it takes for the radio wave to leave the radar and return tells us how distant the storm is. The direction the radar is pointing locates the storm.

Uniquely, Doppler radar can measure wind speed in precipitating regions. A Doppler radar receiver “hears” waves of a higher frequency if precipitation particles are moving toward the radar, and a lower frequency if particles are moving away. This allows Doppler radars to identify severe weather. For example, if particles switch from moving toward and then away from the Doppler radar over a small distance, the source may be a tornado.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.