Weather

Radar—Scanning the Skies

The first radar (radio detection and ranging) device was developed in 1935 by a team of British scientists led by Robert Watson-Watt. Beams of radiation are sent out from the device, reflect off something, and return as an echo. It didn't take long for this instrument to be adopted by meteorologists who wanted to detect both the location and the intensity of precipitation.

The emitted radiation is in the microwave range. The pulses, or waves, are sent out at the speed of light. The distance between crests is the wavelength, and the frequency is the number of waves that go by every second. The period is the time interval between the passage of the crests. The following figure shows the classic waveform.

Weather-Speak

A radar transmitter sends out pulses. When they encounter an object, a small fraction of microwaves bounce back to the radar antenna. The object shows up as a pattern on the radar screen. This on-screen image of a radar target is called an echo.

The speed of the wave is constant—it is the speed of light, 186,000 miles per second. In mathematical terms, the frequency multiplied by the wavelength is equal to the speed of light. If the wavelength decreases, the frequency must increase, so the speed of the wave stays fixed at the speed of light. The frequency is controlled by the generator of the signal and can be adjusted. As it is regulated, the wavelength will also adjust.

A classic wave.

A classic wave.

Now, if the outgoing wavelength is very small compared to what's in the air, it will be scattered by everything it encounters. If the wavelength is too large, the beam will only be reflected and scattered by the largest of objects. So it's important to use the right wavelength for what you're monitoring. If the radar is focusing on aircraft or buildings, a much larger wavelength is needed than if it were to focus on raindrops.

Radar was first used in the military, to keep an eye out for large objects (like German bombers). The principle and nature of military and meteorological radar systems are the same. Only the target is different. Meteorologists modified radar signals for their own use so the raindrops would serve as the reflecting and scattering surfaces. The required wavelength needed for detection must be small enough to bounce off that size object, but also large enough not to be completely reflected and absorbed by a good number of small drops.

Weather Words

"Others will … measure the pathways of the sky, or forecast the rising stars."

—Virgil

A radar beam's bounce back to the receiver detects both the presence of precipitation and its distance. The strength of the echo is indicative of the intensity of the precipitation and can be color-coded. On television, the bright red echoes on radar displays show the heaviest precipitation, the yellows and oranges include moderate to heavy activity, and the greens show lighter activity. All of this is called reflectivity. Because the time elapsed between transmitting the signal and receiving the echo is known, and because the signal speed is fixed by the speed of light, the distance to that precipitation can be determined.

In general, weather radar not only provides a horizontal scan of precipitation distribution, it also shows the vertical distribution of moisture—from which cloud elevation can be measured. The radar can be tilted up and down, as well as in a horizontal circle. This vertical scan is very helpful in determining thunderstorm intensity because severe storms have high cloud tops. Cumulonimbus clouds are mountainous. When their tops exceed 30,000 feet, you know the weather's going to be interesting. Beyond cloud tops of 40,000 feet, the weather becomes severe with damaging winds and torrential downpours. Beyond 50,000 feet, the weather becomes absolutely violent. Tornadoes are most frequently spawned in these tall thunderstorm clouds. Radar operators also look for hook-shaped echoes, which often show up during tornadoes. These actually appear as hooks on the radar screen.

Weather radar does have its limitations. Because of the problem related to beam wavelength and raindrop size, lighter precipitation may not be detected. If it is, other clutter will show up on the screen. Also, near the antenna, buildings and hills can block the signal. This ground clutter is always a problem, although radars now have clutter-suppression devices. These devices work on the idea that echoes, which do not move or change, are clutter and are suppressed. Sometimes precipitation is falling but evaporating before reaching the ground. The echo will look impressive, but nothing is happening at ground level. This is common during the winter, when a dry layer prevails in the lowest layers of the atmosphere and prevents the onset of precipitation. Rain or snow might look like it's falling for hours before it's actually observed on the ground. Eventually, after two or three hours, the moisture will usually penetrate the dry layer.

Weather-Speak

Reflectivity is the measure of radiation reflected by a given surface.

Speaking of snow, radar has more trouble detecting it than rain. That's because snow doesn't reflect as well, and when it does show up strongly, there's usually rain or sleet mixed with it. Sleet, frozen droplets or ice pellets, is very reflective. When the winter radar screen shows pockets of red, there's usually sleet. In addition, the radar beam sometimes gives false echoes if atmospheric conditions force the signal to bend toward the ground. This happens when a temperature inversion exists, causing the radar screen to be filled with clutter. This is called anomalous propagation. Sometimes, too, a strong storm outside the normal range of the radar will cause an echo that mixes with the regular signal. The radar becomes confused. So radar may allow for valuable observations of the atmosphere, but sometimes the picture is foggy.

book cover

Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.