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Wx Watch: Tools of the TradeWx Watch: Tools of the Trade

Steering clear of cells, with a little helpSteering clear of cells, with a little help

Back when I learned to fly, small general aviation airplanes had no cockpit tools for avoiding thunderstorms. It was just you, your eyeballs, and, if you were lucky, information from flight watch.

STORM SIGNS

When you see Nexrad imagery, be on the lookout for these dangerous radar signatures.

Hooks.

These are narrow, curved extensions from the main body of a radar return. They indicate rotating winds at the periphery of the parent storm cell, and are markers for possible tornadic activity.

Fingers.

These are thin projections from a parent cell. Tornadoes begin when strong winds start to draw moisture away from a parent cell, then encounter shear zones that start the wind-driven rain rotating. So fingers can precede hooks — which are more certain signs of tornadoes.

Scallops.

These shapes are markers for strong winds flowing in, out, and around active, maturing cells.

Pendants.

Anytime a precipitation return deviates from a circular, nearly symmetrical shape (the classic signature of a "garden variety" air-mass thunderstorm) you've got the signs of one or more shear zones at work. As these shear zones begin to "stretch" the cell horizontally, the cell shape can bulge out in one dominant direction — another sign that the cell has very strong shear and turbulence.

Lines.

Thin lines containing heavy and extreme precipitation (coded with red or purple, respectively) are signs of squall lines — narrow, fast-moving zones of heavy and extreme precipitation with violent winds.

Bowed-out lines.

These "bow echoes" indicate strong downburst activity. Sometimes squall lines will bow out and then show curled precipitation signatures. These are signs of strong, rotating winds, the kind that can form tornadoes.

Steep precipitation gradients.

Steer well clear of any cells with these. If there's very little distance between red, yellow, and dark-green returns, this means that the storm is a heavy or extreme one, or on its way to becoming so. Updrafts and downdrafts will predominate, and so will severe turbulence. — TAH

Back when I learned to fly, small general aviation airplanes had no cockpit tools for avoiding thunderstorms. It was just you, your eyeballs, and, if you were lucky, information from flight watch. Come to think of it, those primitive tools for avoiding angry clouds still work today! But now, we're much more fortunate. Small-antenna radars that fit into wing-mounted pods — or within wing leading edges — came into vogue during the mid-1970s. At the same time, the first lightning-detection units came on the market. By the mid-1990s, technology for the ultimate in storm detection was developed: datalinked imagery from ground-based Doppler radars. This imagery can play on handheld GPS units or panel-mount multifunction displays, together with navigation and flight data. Talk about situational awareness! Now even the tiniest piston single can be equipped with weather information that rivals, and often exceeds, that found in some airline cockpits.

OK, so you plunked down thousands of dollars for this weather-avoidance technology. Does this mean you can take on summer's worst weather? Yes, if you follow three simple rules: 1) steer clear of all storm cells and convective buildups, 2) understand your equipment's limitations, and 3) understand your own, and your airplane's, capabilities and limitations. We'll be brutally brief because of space considerations, but here is an expansion on those rules.

Steering clear

We've said it before, and we'll say it again, because it's the only foolproof way of staying out of thunderstorms. File an IFR flight plan if you're instrument-rated, but remain VFR when it comes time to navigate around convective weather. The typical advice is to stay at least 20 nm away from buildups, though there's no magic yardstick in the sky to help you gauge distance. Just keep buildups well off your wing, don't try to shoot the gap between building cumulus (watch the tops for signs of vertical movement), and for Pete's sake don't try to top building cumulus clouds. You won't make it — unless you're flying an F-15, or something like it — and you'll soon be on the gauges flying an out-of-control airplane.

Radar

Let's say you're instrument-rated and your airplane is equipped with weather radar. Can you safely fly on instruments, using your radar to dodge storm cells embedded in a larger cloud mass? Don't count on it, for some good reasons. The small antenna dishes used in the wing-mounted installations don't send narrow or powerful enough beams to accurately depict the full outlines of heavy precipitation echoes. These small (say, 12-inch diameter) antennas send out weak, unfocused signals that can't adequately penetrate a storm cell. Instead, the signals bounce off the first layers of precipitation only, leaving the back side of the storm cell unpainted. This phenomenon, called attenuation, can mislead you. What looks like a thin band of rain could really be a huge area of the most extreme radar returns. The lesson? Use these radars to go around, not through, bad radar signatures. If you can't go around, turn around.

Airliners and other large aircraft use much bigger (e.g., 30-inch diameter) antennas that focus powerful beams capable of seeing through most storm cells and giving more accurate pictures of the trouble ahead. I say "most" because even these radars can attenuate. That's what felled a Southern Airways DC-9 in 1977, after hail smashed its windshields and a double-engine flameout caused a forced landing on a country road, with many fatalities. The crew thought it saw a "thin spot" in a line of storms ahead. Instead, it was the strongest part of the thunderstorms.

One popular analogy compares a small-antenna radar to a 20-watt porch light that dissipates its feeble energy far and wide in just a few feet. Large-antenna radars are like spotlights, with strong beams that see far ahead. Please keep this in mind, no matter what kind of radar you may have. "Radar-equipped" may look very macho stenciled on your fuselage, but it won't be of much help if you ignore the ways radar can lie.

Lightning detection

Lightning detection is a great, relatively low-cost method of identifying trouble spots. But if it does a great job of plotting lightning, it's less useful as a means of finding the perimeters of any storm cells. Yes, there's a strong correlation between intense, frequent lightning and the centers of strongest convective activity. This means that lightning detection should be used as a gross avoidance tool, good for flying well clear of any strikes.

Spurious electrical activity can produce false returns. One pilot told me a story about two "lightning strikes" at his 9 and 3 o'clock positions that followed him around every time he flew. Turns out his wingtip strobes were improperly grounded, triggering his lightning-detection gear. Electrical power plants on the ground, I'm told, can do the same thing.

To identify the worst strikes and storm activity, first let a few strikes plot. Then press the Clear button to clear the strikes from the display screen. Now watch to see how quickly the strikes reappear. The more, and the faster, strikes appear, the stronger the thunderstorm. Check to see if your lightning detector has modes for choosing between painting individual strikes and clusters of strikes. Some newer units have a "cell" mode that lets an algorithm display the strongest lightning activity in tighter clusters. This makes the worst areas of a storm more readily identifiable.

Lightning detectors show both cloud-to-cloud and cloud-to-ground lightning. Their update rates — every two seconds — are fast enough to warn you of any growing storms in short order. This combination of attributes is a big advantage, because research has shown that most lightning tends to occur in the building and mature stages of a thunderstorm's life cycle.

Datalink

Datalink weather is the ultimate when it comes to in-cockpit weather information. Using it, you can call up METARs, terminal aerodrome forecasts and — most helpful of all — Nexrad weather radar. Optional, extra-cost products such as sigmets and airmets can be ordered with some service packages.

Nexrad imagery does not suffer from attenuation problems because it's generated by the nation's network of huge, powerful ground-based radars. This means that you get a truer picture of precipitation contours, and therefore better guidance for safe circumnavigation.

Nexrad update intervals can come as far apart as every five to 15 minutes, so be sure to watch your display screen for the age of the displayed information. Also, be on the lookout for the worst radar signatures (see "Storm Signs," this page). Know also that Nexrad does not show clouds! It shows precipitation only. To be more specific, it shows wet hail, hail, and the heavier downpours best. In general, the bigger the water droplet, the better the detection. It does a poor job of detecting drizzle and snow.

Depending on your datalink service provider and your subscription's level of service, you may be able to call up satellite imagery that will show clouds, and let you combine the cloud and Nexrad imagery in one view, giving you the so-called "satellite-radar mosaic" picture, which many pilots prefer. Most often, the cloud imagery is in the infrared spectrum. This imagery is great at night, when visible imagery is, of course, unavailable, but there are limitations to infrared. For example, it has trouble seeing low clouds and fog at night and during the cooler months.

Some datalink service packages are quite extensive, and show winds aloft, echo tops, freezing levels, precipitation types (at the surface), and lightning. Datalink lightning imagery, however, comes from the nation's ground-based lightning-detection network, and depicts only cloud-to-ground lightning. So you won't see any cloud-to-cloud strokes.

You and your airplane

Let's step away from the fancy gear and concentrate on the most important part of your storm-avoidance equipment: you. Are you instrument-rated and current? Do you have significant experience flying in actual instrument conditions? Are you familiar with the route of flight? Have you been following weather patterns in the days before your flight? Have you taken advantage of the many Internet sources covering convective weather? Do you fully understand the operation and interpretation of all your weather-avoidance equipment? Have you obtained a quality weather briefing? And finally, do you feel comfortable and confident with the weather situation you'll face after takeoff? If the answer to any of these questions is no, then the safest alternative might be to postpone the flight.

And how about your airplane? Is it a low-powered piston single? A high-performance, turbocharged piston single or twin with comparatively high cruise speeds? Does it have all the weather-avoidance gear we've discussed? Is it a turboprop single or twin? A business jet? Your answer here makes a difference too. It's one thing to be tooling along at Flight Level 410 doing 450 knots in the clear, but quite another to be plodding along, weaving around clouds at 100 knots and 7,000 feet, with no weather detection to help you out. Either way, the first rule still holds: Steer clear of buildups, and use everything at your disposal to make that happen.

E-mail the author at [email protected].

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