The Doppler Effect explained- a moment of science

A Moment of Science

Radar is quite spectacular in telling us exactly where rain is falling by pinging precipitation with radio waves! Our weather radar can determine more than just a storm’s location, but its movement as well thanks to the Doppler Effect!

Before I explain how doppler radar works, let me explain what the doppler effect is by showing you what it sounds like. 


You’ve heard that before- a car honking its horn or a firetruck with its sirens blaring sounds different as it rushes towards, then past you. That change in pitch is the Doppler Effect- describing “an increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move toward (or away from) each other.”

Let’s visualize this. 


Sound waves from a stationary source.

If the sound’s source is not moving, its sound waves look like this- uniform, perfectly spaced apart on all sides.

Sound waves if the source is moving

However if it starts moving- the waves “bunch up” in the direction of motion, leaving waves spaced further apart behind it.

We can call the number of waves the frequency, or pitch when talking about sound. Notice there’s a higher frequency, a higher pitch for someone listening to a sound coming towards them, and a lower frequency or pitch for someone listening as the sound passes.

The amount this pitch changes, or shifts, is based on the speed of the object. This principle lets police officers see if you’re speeding, and allows meteorologists to see a storm’s motion by measuring that doppler shift, not in sound waves, but radio waves. 

Doppler radar site, credit: NOAA

When doppler radar sends out blasts of radio waves, it makes sure to note the shape, form, and phase of these pulses because it may look different when some of that energy returns after hitting an object. A positive phase shift implies motion toward the radar and a negative shift indicates motion away from the radar- a direct result of the Doppler effect!

A trained meteorologist can use this velocity data to determine how fast a storm’s winds are, or if there’s rotation by looking for couplets of outgoing winds (red) vs incoming winds (green) within a storm.

This feature of modern radars is crucial in severe weather events, but this is just one application of the Doppler Effect! Using the same principles, astronomers have proven that our universe is expanding by looking at the shift of light waves from distant stars!

That’s a story for another time. 

Storm Team 2 Meteorologist David Dickson

Copyright 2021 Nexstar Media Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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