Ask A Met: How Does Radar Work?

This week's question examines the details of a device that many of us take for granted when looking at our local weather. Morning Brief reader Mark asks, "How does radar work?"

Meteorologist Sara Tonks: I used to give school talks about meteorology and always enjoyed talking about this.

The way that I would begin to explain radar to students would be by asking them to imagine a ball feeder for tennis. Maybe you’ve used one? It’s like a cannon you load full of balls to practice your shot. You might have seen something similar in baseball batting cages.

In any case, I want you to imagine this machine going haywire, spinning in a circle and spitting out balls in every single direction. The shots aren’t truly random, but it is spinning so fast and shooting in so many different directions that it might look that way to you.

This is what radar does, except the tennis balls are radar beams. What’s a radar beam, you ask? It’s a pulse of electromagnetic energy with a wavelength that qualifies as a microwave.

Just the same way a tennis ball would bounce back after being shot at a wall and hit you in the face, radar beams do the same thing with water in the air. That little pulse of energy is traveling, it's having fun, it hits a water droplet and it bounces back. The radar receives that bounced back energy.

If there are multiple things out there, say more raindrops, more beams will bounce back. What radar can do is measure the time it takes between emitting that pulse and the pulse coming back. With enough bounces back, that creates the shapes you see on weather radar maps.

The same thing happens when radar beams hit a moving object. If that particle is moving towards the radar when the beam bounces off of it, it gets compressed. Moving away, the beam is stretched. Radar uses this to determine how fast something is moving towards or away from the radar. It’s how we can confirm a “radar detected tornado” without having eyewitnesses.

Radar gets less accurate as the distance increases to where these bounces are coming from. As you might know, the Earth is round and as that curvature happens, radar beams begin to go over the clouds and precipitation.

The radar map of our country is less complete than you might imagine. There are many gaps, particularly in the West.

Radar doesn’t necessarily know what it is bouncing back from, either. As I explained in a recent video, it could be a cloud of bugs or birds or even a military exercise. Radars are configured to sort out noise, but that doesn’t mean it only shows us the weather.

Do you have a question to ask the meteorologists at Weather.com? Write to us at [email protected] and we’ll pick a new question each week from readers to answer.