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Arts & Life

What does the Doppler Effect sound like? We explore this history of Doppler Radar in South Dakota

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Courtesy Photo

This story is from SDPB's daily public-affairs show, In the Moment, hosted by Lori Walsh.

An upgrade in weather detection and prediction was set in motion on this day in 1995 when The National Weather Service in Sioux Falls commissioned a new Doppler Radar. It was hailed as a milestone for the National weather service.

Radars are critical and allow observers to "see" inside clouds and help understand what's unfolding. Doppler weather radars are remote sensing instruments capable of detecting particle types ... like rain, snow, hail, and even insects. In addition, they can detect intensity and motion. Doppler Radar data can be used to determine the structure of storms and help predict the severity of storms.

The current version of the National Weather Service Doppler Radar is called NEX-RAD or Next-Generation-Radar. The radar emits a burst of energy. If the energy strikes an object like a raindrop, snowflake, or hail, energy is scattered but a small fraction of the emitted energy is directed back toward the radar.

Computers analyze the strength of the returned pulse, the time it took to travel to the object and back, and phase, or doppler shift of the pulse. And this process happens up to 1,300 times each second.

The ability to detect what's called the "shift in the phase" of the pulse of energy makes NEX-RAD a Doppler radar. This Doppler effect was named after the Austrian physicist, Christian Doppler, who discovered it. The phase of the returning signal typically changes based upon the motion of the raindrops.

You have most likely experienced the "Doppler effect" from passing trains or cars. As a train or car passes your location, you may have noticed the pitch of the train whistle or car engine noise changing from high to a lower pitch. On the approach, the sound waves are compressed making the pitch higher. As the object moves away from you, the sound waves are stretched, lowering the pitch of the sound.

The same effect takes place in the atmosphere as a pulse of energy from NEX-RAD strikes an object and is reflected back toward the radar. The radar's computers measure the phase change of the reflected pulse of energy which then converts that change to a velocity and direction of the object.

The Sioux Falls National Weather services commissioned a new Radar system on this day in 1995.

Production help thanks to Brad Tennant, Ph.D., Professor of History at Presentation College.