Is That a Thunderstorm?
Weather radar is a great tool, but we need all of our senses for severe weather avoidance.
By J. Mac McClellan
May 2009
There is little excuse for any pilot flying in the United States not to have seen a recent weather radar picture when thunderstorms are possible along the route. Any computer can link you to Nexrad radar sites before takeoff, and satellite downlink of more current weather radar is available at a price that makes sense to any pilot who travels in an airplane.
But the radar image is only one ingredient necessary to formulate a severe weather avoidance plan. Radar is the single most important tool, but it needs to be combined with other information to get a true picture of the weather.
Radar, as you almost certainly know, functions by transmitting a short, powerful burst of radio energy. The energy is focused tightly into a beam and aimed by a moving antenna. When the radio energy strikes an object a small amount of the energy is reflected back to the radar antenna. By timing the round trip of the radar pulse to the target and back, the range to the target can be calculated with precision. The angle of the antenna when the pulse is transmitted and echo then received provides bearing to the target.
The first use of radar was to detect enemy aircraft and ships at ranges earlier technology could not accomplish. Radar pioneers quickly discovered that the radio energy transmitted by the radar would reflect back from precipitation as well as aircraft. This was a huge obstacle to the mission of target acquisition, and massive efforts were made to find radio frequencies and pulse lengths that could penetrate any precipitation and find the threat.
But the serendipity of radar detecting precipitation as well as ships and airplanes was soon realized and new types of radars were developed that were optimized to locate, measure and track precipitation. For the first time weather forecasters could locate precipitation at great distances, and measure its intensity and movement.
Another fortunate coincidence is that larger raindrops, and heavier concentration of those drops, are the best radar reflectors and thus show up as the strongest echoes on the radar. And, big drops and heavy rain are also a signature of thunderstorms. The powerful vertical currents that are required to create a thunderstorm are also part of the mechanism that creates large drop size. So, when a weather radar shows a strong echo we can conclude that there are large drops, heavy rain, and there could be a thunderstorm.
Modern digital radars like the Nexrad images that we see from the satellite in the cockpit, or on our computer, or on the TV weather report, rank the strength of a returned echo and code it by color. Nexrad has more than a dozen different levels of echo strength with slight color change to show the intensity of each target area. Light greens are weak echo strengths moving to intense reds, and then magenta and a sort of purple for the strongest.
So, what could be easier than to avoid severe weather with a color radar map of the precipitation. Just dodge the areas that are anything but green and you will miss all thunderstorms, right? There is a little truth in that. But you will not necessarily miss turbulence, and you may find yourself flying around radar targets that are nothing more than rain in smooth air.
So you need more than weather radar to do a good, safe, efficient and comfortable job of staying out of the nasty stuff. To help complete the picture you need to understand the general weather system you are considering, and you want to have information on lightning.
An essential characteristic of a thunderstorm is, well, thunder. In the old days when humans made weather observations and reports, if the observer heard thunder there was a thunderstorm nearby and that's what he reported. He didn't need to see visible lightning, just hear the thunder to make the report. And there was no need for rain. That, you may remember, is reported as TSRA, or thunderstorm with rain.
 |
|
|
Avidyne has introduced a new lightning detector, the TWX670, that is able to map lightning activity and present the data in a way that looks much like a weather radar display. Like the radar image, the colors show higher levels of activity with red being the area of most lightning strokes.
|
Thunder is created by lightning that rapidly heats and expands the air it travels through, and it is that expansion that we hear as thunder. So thunder is really the audible evidence of the presence of lightning, so it is actually lightning that makes a thunderstorm. Or the other way around, actually. It is the friction of the rapidly moving air currents in the forming storm that create the lightning that makes the thunder.
As pilots we want to do our best to avoid lightning strikes, even though airplanes are designed to withstand them. Lightning can do physical damage to the airframe, particularly to a trailing edge where it exits and can blow part of the structure away. Lightning can also knock out electrical and avionics systems. Airplanes do get hit with surprising frequency, and damage is not always caused by the strike, but it can and does happen.
Though lightning is a real threat, it is actually the powerful air currents in the most intense thunderstorms that cause the lightning that can tear an airplane apart. So, just as we use the radar picture of precipitation to try to identify a thunderstorm, we do the same with lightning. Avoiding very heavy rain and lightning is a good idea, but the real enemy is turbulence, and perhaps hail that is found in many severe storms.
The weather providers that send information to the cockpit or computer offer lightning strike information collected by national networks. This data is very useful because if there is lightning, there is a thunderstorm. And the accuracy of the displayed lightning location from the networks is good.
Discuss this article in our forums
| 
Copyright @ 2009 Bonnier Corp. All Rights Reserved. Reproduction in whole or in part without permission is prohibited.
