Weatherman's Wrong? Don't Blame Doppler Radar

Doppler radars help us get ready in the morning. They tell us whether to pack an umbrella or leave the coat at home. And they help us navigate conversations with anyone — because we all have a strange fascination with weather. But Doppler radars also used for more than just weather tracking — doppler radars can be found in aviation, police speed guns, radiology, health care and missile systems.

How does this invisible phenomenon work? In short, Doppler radars send beams of electromagnetic signals toward a desired target. The signals use sound reflection to analyze how the returned signal's frequency has been altered. But there's more to it.

The Doppler Effect

The Doppler effect (or shift) is named after the Austrian physicist Christian Doppler, who in 1842 proposed that a wave's frequency changes for an observer moving close to its source. The frequency is higher during its initial transmission, identical at the moment of passing, and lower during the recession.

It's commonly experienced when a car siren or horn passes, which is why the sound is always higher when the vehicle is approaching than when it is receding. With the naked ear, you can get a general idea of how far and fast the vehicle is traveling.

Since light waves have no medium, we use the Doppler effect in terms of the motion of the source relative to its observer (the radar). Obviously, this requires a much more complex and calculated formula than what's audible to the human ear.

Pulse-Doppler Radar

True Doppler radar, in its simplest form, is found in police officer's radar guns, which determine the speed of a motor vehicle. The electromagnetic radiation is sent out at a precise frequency, then the radar uses the frequency of the wave on the return path to calculate a targeted object's velocity.

However, one of the most common known examples of Doppler use, meteorology, uses a different technique called Pulse-Doppler radar.

GIF courtesy of Tumblr, accuweather

This sends the beam in a swirling, circular motion, which we often see on TV, shown in the GIF above. When a beam strikes a raindrop, hailstone or other precipitation, the doppler can calculate the distance, intensity and direction from the reflected beam. This radar can be so sensitive that it can track wind-blown particles and bird migration.

While True Doppler sends a direct beam, Pulse-Doppler sends pulses of radiation that allow for calculations of radial velocity. It can be sent out with varying frequency and carrier cycles, using the formula below to track.

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The U.S. Navy used continuous-broadcast (or FM) radar during World War II so that aircrafts could perform night combat operation, such as approaching target ships and training guns on enemy aircraft. They were also used as navigation aids, helping pilots accurately determine wind speed.

These early Dopplers relied on analog filters to calculate velocity, which were riddled with inaccuracies, due to the extra weight. It wasn't until the 1970s, when modern microprocessors were invented, that Doppler radar became more practical for weather and air traffic control.

Today, this technology is so advanced that we can determine weather complications weeks in advance. So next time you're stuck at the airport due to adverse weather when the skies appear to be clear and sunny, you can thank your nearest Doppler radar.

Why the Weatherman Is Wrong

So if Doppler is right, why is the weatherman so often wrong? For one thing, he's not always wrong, we just remember the times when he is (see also: Snowquester). The other thing to keep in mind is that the weather is very dynamic and constantly changing. There's a lot of data in weather, and forecasts have actually gotten far better over the years, though they're clearly not perfect. Newer prediction models are incorporating these massive amounts of data to help meteorologists understand dynamic weather patterns, which will lead to more accurate forecasts to help us get dressed for day — and know whether to grab the umbrella.

Image courtesy of iStockphoto, Anthro