Under The Radar: Why It Rains When There Are No Radar Echoes


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A radar beam. Illustration courtesy of NOAA

Relying on the weather radar for a forecast? Image courtesy of NOAA

You have a hot date for dinner. You put on your best outfit and check your hair. You access the current weather on TV or the internet and look at the radar. No rain. You walk to meet your date. And guess what? It starts raining. So much for the hair.

And you wonder: How can it be raining when there’s nothing showing on the radar?

What Is ‘RADAR?’

RADAR is actually an acronym that stands for RAdio Detection And Ranging. The word has become so ubiquitous in the English language that the capitalization has been dropped and it is now a word that has taken on many meanings. There was even a character on the TV series M.A.S.H. called ‘Radar’ for his ability to ‘forecast’ incoming wounded soldiers in need of medical care.

The historical importance of radar blossomed during World War II, when the military used it to detect enemy aircraft and ships. Just as a human eye detects an object because the object reflects visible light, so can electromagnetic radiation at other wavelengths ‘see’ the object.

Radio waves can be focused so that a radar can detect the return of the reflected ‘pulse’ from very far away — much farther than than the eye can see, even with magnifying aids.

How Radar Works

When an electromagnetic wave strikes an object, some of the energy of the wave is reflected back towards where the wave came from. A flashlight beam, for example, lights up an object at night and we can see it.

For detection at long distances, radio waves (electromagnetic waves with long wavelengths) are most efficient. In World War II, what would now be considered crude radars were used to detect German and Japanese planes and ships. In fact, the Japanese fleet of aircraft that destroyed the American ships at Pearl Harbor was detected on a very primitive radar, but no one believed the image was real.

During the war, as equipment improved, radar was effective in identifying enemy water- and aircraft. But the images were plagued by anomalous echoes, many of which finally were identified as precipitation.

Weather Radar

For the purpose of detecting drops of water, a wavelength between one and ten times the size of the drops is most effective. This puts the wave in the microwave portion of the electromagnetic spectrum. Very little of the radiation is returned to the receiver, but modern radar receivers are sensitive enough to ‘see’ precipitation several hundred kilometers away.

The Radar Image Can Have Errors

Radio waves can be bent by various atmospheric phenomena, most notably a surface of temperature change such as an inversion, which occurs when cold air is trapped at the ground beneath warmer air above. As a result, precipitation may be in a different place than where it appears on the radar screen.

Objects that are not raindrops can also reflect the radar pulses: Insect swarms, mountains, and buildings can all look like precipitation echoes.

Ways That Rain Can Hide From Radar

On a perfectly circular earth, radars would be constructed on towers and the beams would travel uninterrupted until they reached something in the air. But the earth is not perfectly round – and natural and manmade objects can interfere with radar signals.

Bismarck, North Dakota radar image

A Bismarck, North Dakota radar image. Due to the mountains in the region, this may not be an accurate representation of precipitation. Image courtesy of NOAA

Mountains and buildings can block radar pulses in rural and urban settings, respectively. And very heavy rain can completely absorb or reflect the radar beam. so that precipitation behind the deluge is obscured.

Though echoes from buildings and mountains can be mistaken for precipitation, anyone with a watchful weather eye can recognize this ‘ground clutter.’

The signature of ground clutter — buildings and mountains — is that it never moves.

Even casual observers can mentally delete ground clutter from the radar display after viewing images from the same radar several times and seeing echoes in the same positions.

To compensate for the possibility of the radar beam being blocked by a building or mountain, the radar is generally aimed at a slight angle to the ground. Furthermore the earth is curved. So due to these two factors, the radar beam gets farther from the ground as it gets farther from the source.

When You See An Echo, But There’s No Precipitation

Because the radar beam becomes more elevated with distance from the signal, it can be some distance above the ground fairly far from the source. Rain can fall from clouds and evaporate before it reaches the surface of the earth. This precipitation is called virga. The radar can record the echo for the virga, but no precipitation reaches the ground.

Virga can occur anywhere when the air is relatively dry, but is most common in the United States in the desert southwest.

Rain Under The Radar And Invisible Raindrops

Since the radar beam becomes more elevated with distance from the radar site, rain from low clouds at some distance from the radar can be entirely beneath the radar beam. This is the most common cause of rain with no radar echo.

In addition, the radar wavelength is designed to maximize detection of average-size raindrops. Drizzle, which comprises raindrops of very small diameter, is a poor reflector of the radar signal and may not show up.

There’s no question that radar images disseminated on tv and the Internet are helpful to those who are making plans to be outdoors. But it’s a good idea to consider where the radar site is located, whether anything is blocking the beam, and whether the beam could be above the precipitation.

The next time you look at the radar echoes, remember: Seeing isn’t always believing, and what you see isn’t always what you get.

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