Wx Watch: The Eyes Have It

Buy storm-avoidance gear, but trust your eyes

May 1, 2005

Now that thunderstorm season is upon us, it's time to recall the prime directive for guaranteed storm-cell avoidance. It's a very important rule, and one that should be kept firmly in every general aviation pilot's mind, whether he or she is a neophyte or a grizzled high-timer with a logbook bulging with actual instrument time. Can you guess what it is? Don't worry, the answer isn't complicated. Here it is:

Keep your distance from cumulonimbus clouds, and the clouds surrounding them.

This tactic, called visual avoidance, implies some additional warm-weather flying rules and considerations for flying around clouds. For example, it's not enough to simply avoid convective buildups. The rule of thumb is to avoid them by at least 20 nautical miles. If you intend to fly between two buildups, you should have 40 nm between the two.

This raises an issue. How in the world can you tell if a buildup is actually 20 miles away? Or 15? Or 10? The answer is you can't. Storm cells don't come with mile markers, so the 20-mile call is a subjective one. With experience, however, you can learn to roughly gauge distances based on your experience scanning ground references. But even a high-time pilot can have trouble making a mileage call, so in the end you'll just have to use common sense to keep a conservative distance from buildups.

Bear in mind that all convective build-ups are not alike, and you have to adjust your minimum safe distance accordingly. A batch of large air-mass cumulus clouds over Florida is one thing; a countieswide convective complex in the Midwest is something much more dangerous. Its effects can be felt over larger distances, so the clouds associated with it should be avoided by larger margins.

Circumnavigation of convective clouds is sometimes a safe option, assuming you'll be flying toward improving conditions. A good preflight weather briefing — followed by a last-minute weather check prior to takeoff — should give you enough information to make an initial determination of the extent and nature of any cloud cover along your route of flight. But once you're flying, you're the weather expert, the one with the front-row seat, and you have to trust your own judgment.

Flying above building cumulus clouds isn't a good idea. You run the risk of flying into rising cloud tops, and merging with the most aggressive updrafts and downdrafts of a maturing storm. Flying beneath thunderstorms is an equally risky avoidance strategy. Here the problem also centers around downdrafts and updrafts. Strong downdrafts (sometimes marked by rain shafts) can force you to the surface; updrafts can suck you into the storm cell.

Obviously, if you can't safely fly through, around, above, or below areas of convective activity, there is only one option left — make a 180-degree turn and rethink your strategy. Many times, a landing at a nearby airport with good VFR weather is the best alternative.

All of this advice can sound pretty sterile when the forecast didn't turn out as advertised, a convective sigmet has just been issued for your location, you're facing an onslaught of convective clouds, the pireps are old, and flight watch advises you of disappointing prospects for a cloud-free escape path. Even with an instrument rating and plenty of instrument currency and proficiency, the dangers of an inadvertent thunderstorm penetration rise, and so does the stress level.

Fortunately, light general aviation aircraft can now be equipped with three different types of thunderstorm-avoidance tools. All have their strong and weak points, but the information they convey can — when properly understood, interpreted, and applied — make for safer circumnavigation of convective cells.

Lightning detection. This equipment works by detecting and plotting the locations of electrical discharges of the type associated with lightning. The theory is that if you avoid the symbols marking lightning strikes, you avoid the thunderstorm. This is the simplest of the storm-avoidance instruments, and — when purchased on the used market — can be the least expensive (as low as $2,500). Besides its comparatively low cost, lightning detection's other strengths include:

  • Dense clustering of lightning plots and high update rates (the rate at which the plots reappear after clearing the display screen) can confirm the presence of the worst storms.
  • Ease of interpretation.
  • 360-degree views can advise you of discharge points all around the aircraft, at ranges up to 200 nm.
  • Added features can include checklist functions and heading stabilization. The latter obviates the need to manually clear the discharge plots after turning.

Weaknesses include:

  • False ranging, or radial spread. This causes strong lightning strikes to plot closer than they really are, and weak ones to plot farther away. More modern units are improved and aren't as susceptible to the problem.
  • Sensitivity to nonstorm electrical discharge points. Power plants and other ground-based sources of high-voltage electricity can show up on the display, as can poorly grounded aircraft strobe lights and other electrical components.

Weather radar. Airborne weather radar units can be expensive ($7,500 to $25,000) and their antennas and power ratings — crucial elements in the accuracy and resolution of in-cockpit radar imagery — are too diminutive for consistently reliable information in high-precipitation conditions. Small airplanes cannot accommodate antenna diameters much greater than 12 inches, and that's too small for long-range viewing. Radar works by transmitting a beam of energy, which then bounces off precipitation and reflects back to the radar antenna. The bigger the antenna and the bigger the radar's power rating, the better the radar signal and the more true the precipitation depiction on the airplane's display screen. One analogy likens small-airplane radar beams to a weak porch light; airliners, with their 3-foot-diameter antenna dishes, have beams like searchlights. Still, at closer ranges, small radars can be useful in tactical avoidance of contouring storm cells. (Contouring refers to adjoining gradients of precipitation echoes — the marks of thunderstorms with cores of high-precipitation returns, surrounded by ever-weakening ones.) Some feel that airborne weather radar is the gold standard in storm-avoidance technology, and it does have its advantages. These include:

  • Real-time updates of precipitation echoes.
  • At close-range (under 40-mile) settings, accurate depiction of storm contours nearest the airplane.
  • Utility as a tactical-avoidance tool in circumnavigating precipitation returns.

But there are big drawbacks:

  • Attenuation. Precipitation can cause radar signals to be absorbed and scattered, giving false indications of a cell's shape and nature. In some cases, the areas of heaviest precipitation can show up as the weakest.
  • Beam smearing. The smaller the antenna, the wider the transmitted radar signal. This causes poor resolution of storm shapes and contours. "Painting a nail head with a mop" is a good analogy.
  • Image interpretation. The pilot must learn the telltale shapes that indicate dangerous conditions, such as tornadic activity and strong up- and downdrafts.
  • Tilt management. There's a lot to learn about properly operating a weather radar. Correct antenna tilt is important in aiming the radar beam for optimum viewing of a cell. Too low an angle, and the radar will paint ground returns. Too high, and you could miss radar returns at lower altitudes. Bottom line: Take a course on weather radar theory and operation.

Datalink. This is the latest in weather-avoidance gear, and while it can be as expensive as weather radar there are numerous advantages that make these systems wise choices for general aviation aircraft. With datalink, composite radar imagery from ground-based Nexrad weather radars is sent to the cockpit via three methods: satellite broadcast, terrestrial broadcast, and request-reply. The broadcast methods use satellites or ground stations to continuously send out Nexrad and other weather information. With request-reply, the pilot must select the information he or she wants, send out the request, and then wait for the data to return. The radar imagery gives an excellent big-picture view, and can be scaled up or down to help examine storm or precipitation patterns. There are plenty of other datasets that round out data-link's strong points, which include:

  • No attenuation or beam smearing of radar returns. Nexrad radars are powerful enough to generate very accurate thunderstorm contours.
  • Thunderstorm tops, and speed and direction of cell movement, also can be shown on the cockpit display.
  • METARs, TAFs, pireps, sigmets, airmets, and other weather information also can be part of a datalink subscription package.
  • Value as a strategic thunderstorm-avoidance tool. With no false returns or attenuation, it's easy to steer well clear of cells, lines, or clusters of thunderstorms.
  • Adaptability to multiple display systems. Datalink can be made to run on electronic-flight-bag (EFB) displays and personal-digital-assistant (PDA) formats, as well as on panel-mount multifunction displays.

Datalink downsides are few:

  • The radar imagery's resolution can make datalink risky as a tool for safely threading your way around or through precipitation echoes. The imagery is made up of pixels with varying dimensions — from 2 to 4 kilometers square, depending on the manufacturer. When zooming in for close-range views of nearby weather, the pixels can't provide the fine resolution of an airborne weather radar image. This makes datalink more of a gross-avoidance tool than a tactical, weave-around-the-edges display.
  • Update rates vary, and can leave you with imagery that's anywhere from one to 10 minutes old. A lot can happen in a thunderstorm in a few minutes, and believe me, even a minute flying near a thunderstorm can feel like an eternity to a stressed-out pilot.

While having the right equipment can certainly bring you peace of mind when clouds start to build, remember the prime directive at the beginning of this article. Having storm-avoidance gear is great, but none of it can see clouds — something your eyes do very well. Providing you stay clear of them.


Links to additional information about thunderstorm avoidance may be found on AOPA Online ( www.aopa.org/pilot/links.shtml).


E-mail the author at tom.horne@aopa.org.


The American Meteorological Society (AMS), in its February 2005 bulletin, reported on a virtual tornadic thunderstorm graphical user interface developed by meteorologists at Iowa State University. It allows students to sample data such as temperature, pressure, and vertical and horizontal wind velocities, as well as a three-dimensional visual representation of a representative thunderstorm. A Linux version is available to download online ( www.vrac.iastate.edu/research/sites/tornado). A Windows version and a CD-ROM should be available soon. CD-ROM requests should be sent to wgallus@iastate.edu.

One- and two-day aviation weather courses are offered for $85 and $150, respectively, by Kenneth McCool, author of Aviation Meteorology Unscrambled: For VFR and IFR Operations. The course qualifies for credit in the FAA's Pilot Proficiency (Wings) program. Contact mccool@ntin.net for further information.

Freezing drizzle is more of a hazard to idling turbofan engines than freezing rain, according to a study by Roy Rasmussen of the National Center for Atmospheric Research (NCAR). Rasmussen studied two cases of freezing drizzle at Denver International Airport and found that freezing rain falls past engine intakes, while freezing drizzle — frequently underreported — is ingested into the engine. There it freezes on the engine's spinner, later to shed damaging ice particles into the fan blades once the engines are revved up for taxi. For more information, visit the Web site ( www.ucar.edu/news/releases/2004/drizzle.shtml). — TAH