Datalink weather has changed cockpit decision making in ways no one could have envisioned a few short years ago. Where before we struggled along in instrument or marginal VFR weather conditions, full of doubt, and unable to make educated tactical weather decisions, now we have a flood of data. METARs, TAFs, sigmets, airmets, and much more are available at the push of a few buttons.
For this reason, I believe datalink weather should be standard equipment in every airplane designed for serious cross-country flying — and probably will be before very long. Renter-pilots can buy a Garmin GPSMap 396 and have their own portable source of datalink weather. I've even heard Piper Cub pilots talk about how they used a 396 to fly around a line of thunderstorms.
But with the growing popularity of datalink weather comes a responsibility to better understand and interpret this huge volume of information. In short, you have to become your own weather briefer, and one who can work on the fly, so to speak. That goes double for the most popularly viewed of all the datalink weather products: radar imagery.
Radar imagery seems like the answer to every pilot's prayer. So much so that it's tempting to make it your whole focus. After all, there's no attenuation of datalinked radar signals; heavier precipitation returns don't block the view of more severe thunderstorms lurking behind them — a major safety issue for those relying solely on airborne radars for circumnavigating thunderstorm cells. The attenuation problem remains a huge drawback of airborne weather — especially the lower-powered units with smaller antenna dishes — and one that has tricked many a smug pilot into believing that an absence of returns guaranteed a safe passage. Datalinked radar imagery is beamed up to us from powerful, narrow-beamed, ground-based Nexrad radars that don't suffer from attenuation. The result: a true picture of the precipitation situation.
And besides, radar seems so intuitive. We've been watching radar on televised weather broadcasts for years. Even our ground-pounding friends can look at a radar screen and make some sense of it. It all seems so easy: green, light precipitation; yellow, heavier precipitation; red, the worst of the storm; purple, run for cover!
Those overgeneralizations usually hold true, of course. But there are subtleties, tricks, and traps to learn and avoid. As pilots we need to know a lot more about radar interpretation than simply associating colors with risk levels.
Bad shapes
When you study radar returns on your datalinked imagery, take a good, close look at them. Shapes say a lot! Here are a few that you should commit to memory, and avoid by at least 20 nautical miles lest you fly into extreme turbulence and risk loss of control.
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.
Not-so-bad signatures
Not all radar returns are deadly. Areas of light precipitation, for example, can generally be penetrated with a fair degree of safety — assuming you are instrument-rated, have kept your instrument flying skills both current and proficient, and have a way out if things turn bad.
Large swaths of light (light-green signature) precipitation usually pose no problems. In fact, if low-level air is dry enough, the precipitation you see on your display screen may not even be reaching the ground. It's been evaporated by the warmer, drier air below, so VFR flight beneath any clouds could even be possible. Darker-green returns usually mean that rain is reaching the surface, and along with it the risk of instrument meteorological conditions.
All of this assumes an absence of contouring cells (islands of yellow, red, or purple returns) within the green areas. Solid green swathes are generally associated with stratiform cloud conditions, which produce smooth rides and no convection.
As you fly in the summer season, it's typical for a few returns indicating light to moderate precipitation to crop up if conditions are right. Watch them closely for signs of change. Are they generating contours of precipitation — islands of yellow, red, or purple — within the swath of green? If so, you've got a building situation, and you've been forewarned. Prepare your avoidance plans, and don't rule out a precautionary landing at a nearby airport — or a 180-degree turn to better conditions.
You did check the...
...TAFs, METARs, convective outlooks, sigmets, airmets, and other storm warning sources (such as those mentioned in " Wx Watch: Convection Countdown," May Pilot), didn't you? If so, you should have few surprises in the weather department on your summer flights. Just don't let datalink weather become a substitute for good, old-fashioned preflight planning. It's a wonderful new tool for weather awareness and safe in-flight decision making, but it doesn't mean you have carte blanche to fly too close to convective weather.
E-mail the author at [email protected].
Links to additional information about datalink weather may be found on AOPA Online.
Wx Briefs
Tip of the month: Hurricane on the way!
You can use a rule of thumb to develop hurricane alerts of your own. Old salts know that if the surface winds are out of the easterly quadrants of the compass (i.e., from the northeast to the east) and the barometer/altimeter setting is 29.80 in Hg (mercury) — and falling rapidly — a hurricane may be in your future. The faster the winds increase in intensity, the closer you are to the storm center's position. The rule also works for coastal storms occurring along the eastern United States in the autumn and winter months, commonly called nor'easters; it's the northeasterly winds, in fact, that give these storms their name. Generally speaking, from the onset of the easterly winds and the pressure drop, the hurricane may arrive within 24 hours or so. If you detect these conditions, it's time for a weather briefing, and preparing to fly away from trouble.
Tamdar to the rescue
NASA's Tropospheric Airborne Meteorological Data Reporting (Tamdar, for short) program has been gathering data since September 2004. The program involves installing sensors for temperature, pressure, relative humidity, wind speed and direction, and ice accretion aboard Mesaba Airlines' fleet of 64 Saab 340 turboprop airliners. (The University of North Dakota's Piper Seminoles also have been fitted with Tamdar sensors.) The Tamdar sensors collect data on routes out of Mesaba's hubs at Minneapolis, Detroit, and Memphis. The goal is to boost numerical-model forecast accuracy by providing more data points. Today's network of radiosonde balloon launch sites is essentially the same as it was in the 1940s, researchers say — and a lot of weather happens between balloon sites. ACARS (Aircraft Communications Addressing and Reporting System) gear aboard many jet airliners also provides data to the forecast community, but Tamdar officials say that these reports don't cover small and medium-size cities. Tamdar's 100 daily soundings are fed to meteorologists at the Forecast Systems Laboratory for evaluation. For more information, visit the Web site. — TAH
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