Earth Notes: Doppler Radar

Jun 28, 2017

The National Weather Service operates a widely spaced Doppler radar network across our region—from Blue Ridge Reservoir south of Flagstaff to Albuquerque, Las Vegas, and Grand Junction, Colorado. 

Doppler radar installation
Credit NOAA

Weather radar is like a giant ear “listening” for precipitation falling from the sky. For a few seconds every hour, this equipment sends out pulses of energy around a thousand times a second in frequencies close to the highest radio waves.

The rest of the time the radar detects returning signals that bounce off any particles—like raindrops or snowflakes—in the atmosphere. 

Energy reflected from approaching objects is compressed, making the pitch higher than from a stationary source, while stretching it out on objects that are moving away. This is the Doppler Effect, the phenomenon that causes a train horn to change pitch as it passes by.  

Weather radar uses that same effect to detect the motion of precipitation—why it’s called Doppler radar.

The radar converts the tiny bundles of reflected energy into position, speed, intensity, and size of rain, snow, and ice. Sometimes insect swarms or bird flocks are detected—signals that the system’s sophisticated software can weed out.

Updating findings every three to eight minutes, these “ears” can provide important early warnings about approaching storms, tornadoes, heavy snow, or excessive rain and possible flash flooding.