The science behind weather radar

Published 7:00 am Tuesday, March 7, 2017

By Skip Rigney

It may come as a surprise that Picayune is often the gathering site for amateur weather radar analysts.

If you don’t believe me, just drop by Friendship Park on a spring afternoon when dark clouds are building nearby. You are bound to see coaches, parents, and fans of Picayune Youth Athletic Association’s youth baseball teams staring intently at their cell phones. If you glance over their shoulders, you’re likely to see a map overlaid with moving green, yellow, and red blobs.

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Those folks will be using their favorite weather radar application to try to figure out if little Johnny’s game is about to go into a rain delay, be entirely cancelled for the evening, or luckily continue as showers skirt by Friendship Park.

Weather radar has revolutionized short-term weather forecasting and dramatically improved the issuance of severe weather warnings since the earliest network of weather radars became operational in the United States in the 1960s.

Meteorologists and engineers have greatly improved weather radar technology over the last half century. Television meteorologists introduced output from the radars in the 1970s, and over the past decade nearly instant access to radar imagery has become available via the Internet.

While these radars can be mounted on aircraft, the imagery that you see on television and the Internet are from radars on the ground.

Most of them are owned and operated by the National Weather Service.

The most basic components of weather radar are a microwave transmitter, receiver, and antenna.

As the antenna rotates the transmitter sends out pulses of electromagnetic energy whose wavelengths are about four inches long.

This is the optimal wavelength for weather radar. A shorter wavelength would not be able to penetrate heavy showers or clouds. If a longer wavelength were used, not enough energy would be reflected back to the radar by targets as small as raindrops.

In between bursts of transmitted microwave signals, the radar receiver is on the lookout for energy reflected back from raindrops, hail, sleet, snow, or other small objects. Even birds can show up on weather radar.

The radar’s computer software can then compute the distance to the precipitation. This processing combines the fact that the transmitted and reflected microwaves move at the speed of light along with the precisely measured times when the pulses are transmitted, received, and the interval between the pulses.

In addition to knowing where the masses of raindrops are, the radar can estimate whether it’s a light or heavy shower by measuring the intensity of the backscattered energy. That information is usually color-coded on radar maps with green, yellow, and red corresponding to light, moderate, and heavy rain.

Many radar maps now available to the general public on the Internet combine data from multiple radars. The National Weather Service radars nearest to us are in Slidell, Mobile, and Jackson.

Because the radars are collecting data every few seconds, imagery over minutes and even hours can be strung together to give an animation of where rain showers have been moving over time.

Whether you are practicing your radar analysis skills at the ballpark later this spring or during today’s showers and thunderstorms, remember that just because a blob of rain has been moving in a particular direction for the past hour doesn’t mean that it will continue to do so for the next hour.