Wx Watch: Radar Basics

"I've got radar!" you're apt to hear some pilots boast. Like they're able to drive through the mightiest line of thunderstorms and take on weather that would leave the rest of us grounded. But don't be fooled. Radar isn't a surefire antidote for thunderstorm encounters and, by itself, it can't do a thing. It takes a formal course of instruction, practice, and lots of real-world experience to get the most out of weather radar. It's only by understanding the theory behind airborne weather radar — and its weaknesses — that you can safely maximize its potential as a storm avoidance tool. We won't discuss lightning detection equipment in this article (except to contrast it with radar); we'll save that for another day. Instead, let's touch on the most basic of radar basics.

Send and receive. Radar works by sending out energy and "listening" for any returns caused by ricochets off water droplets. The bigger the droplets, the more distinguishable the returns. On the cockpit display screen you then see areas of precipitation in green depicted in various colors — with heavier precipitation being shown in yellow, red, or magenta hues. No precipitation? Then the radar energy wicks off into space and you see nothing. Low-power radar unit? Radar energy can bounce back off the raindrops immediately ahead, filling your screen with solid green and blocking your view of the weather just up the road. This premature bounce-back is called attenuation, and it's a serious problem. A severe storm cell may be just ahead, but you can't see it; instead, it looks like you'll continue to experience greenish, Level one rainfall amounts indicative of lighter precipitation rates. More powerful radars with bigger antennas don't have as much of a problem with attenuation, but it's still a factor. This is one area where lightning detection equipment has an advantage over smaller weather radars.

Small dish, big beam; big dish, small beam. Most small general aviation weather radars use a 10- or 12-inch-diameter radar antenna, or dish. This produces a beam/cone of radar energy that is about 10 degrees wide. This cone has a 100,000-foot diameter by the time the radar beam reaches a point 100 miles ahead. An airliner's 30-inch antenna puts out a 3-degree beam; its cone has a 30,000-foot cone at the 100-mile range. Why is this important? A big beam may not detect precipitation at longer ranges: too much radar energy can sail over, under, and around a storm cell. It doesn't "fill the beam," as experts say. Also, multiple storm cells may be merged into one image; that's called beam smearing, and it's also a function of low power and small antennas. That's why small general aviation weather works best at shorter ranges — say, out to 40 miles. Assuming there's no attenuation, you'll be able to see storm contours (adjoining areas of different rainfall rates) and signatures much better at closer ranges, and the closer the better.

Radar shadows. Radar shadows appear on your radar screen like "clear" areas behind stronger returns. But they're not clear at all. What's happening is that the rainfall in the stronger returns is blocking any further progress of your radar beam. It's a severe form of attenuation that can trick the unknowing into thinking that there's a promised land just beyond that soft spot, or that thin line of returns. The truth is that the worst of the storm can lurk in the clear-looking area — something that more than a few airline crews have learned with tragic consequences. This gives us an ironclad rule for flying with weather radar: Never fly toward a radar shadow. Some newer radars have software that identifies radar shadows with a bluish color.

Doppler advantages. High-end radars used in corporate jets and airliners now use Doppler technology to detect turbulence. Turbulent motions are picked up by Doppler radar as raindrops move toward and away from the radar antenna. Software then paints the turbulent areas a certain color or symbol on the radar screen, giving pilots advance warning of stronger turbulence.

It's all in the technique. To combat radar's shortcomings and use the system safely, you need to take formal training. This is available from AJT Inc. ( www.av-wx-rdr.com) and Honeywell ( www.cas.honeywell.com/bcas/support/training/). The courses are conducted by Archie Trammel and Dave Gwinn, respectively — both radar experts. Gwinn is a retired captain for a major airline. These courses give you a thorough grounding in the principles and operation of weather radar, and anyone who flies with radar and hasn't taken one of these courses should enroll ASAP.

To give you a taste of what you'll hear in those courses, here's a brief rundown of how to best operate a weather radar. These steps are no substitute for the courses, experience, or the information in radar operation manuals, but they'll give you some idea why working a radar can be more technique than technology.

After turning the unit on and waiting for it to run through its test functions, let it warm up in the Standby mode. This prevents any transmissions from harming line personnel and others who may be near the front of the airplane.

Check the antenna's tilt function during the runup. Make sure the antenna is, in fact, tilting up and down. You'll see the radar imagery move up and down if it is.

After takeoff, set the radar's range control. For smaller radars, be practical. Use a range setting of no more than 100 miles or so. Remember that it's the close ranges that give you the best information. At long ranges you may see what looks like a no-problem speck of green. It's no problem now, but as you fly toward it and move the range setting down you could see a whopper of a storm. (At the longer ranges the beam was too big to detect the storm.)

Use the tilt control to tilt the antenna down. You want to tilt the antenna down so that ground returns fill the screen (ground returns follow the arc of the radar's sweep). Then tilt the antenna back up until just the outer third of the display screen has ground returns. Note the tilt setting that produces this return. Then tilt the antenna up some more until no ground returns appear. Now you're ready to look for any suspicious returns. This tilt setting depends on altitude, by the way. The higher you fly, the more down-tilt you'll need to get ground returns.

Work the range and tilt as you fly. As mentioned earlier, keep your eye on any nonground returns. As you fly closer to them, dial the range down and notice whether the storm grows or changes shape. Tilt the antenna up to see if the tops confirm the presence of a thunderstorm. (Ground returns disappear with up-tilt — unless you're flying in a valley ringed by mountains!)

Look for dangerous signatures. These include any hook-, pendant-, or scalloped-edge shapes, as well as steep precipitation contour gradients.

Stay at least 20 miles away from any contouring returns. By this, we mean returns that have green and yellow — or green, yellow, and red. These promise the most violent storms.

Stay away from radar shadows. Many times you'll see a thin line or area of radar returns with an area of black behind it. The back side of the returns may have a bow shape to it. This is almost certainly a radar shadow.

Stay visual, if at all possible. Visual avoidance of thunderstorms is, bar none, the best way to fly. That goes for those who have the fanciest or the humblest weather radar. By staying well clear of clouds and precipitation you'll keep a safe distance and avoid the dangers of attenuation.

Again, this information is highly simplified, and we can't stress the importance of formal training enough. Take a course from Trammel or Gwinn and you'll learn that even high-time jet jockeys have plenty of misunderstandings about the whys and hows of radar usage.

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