June 2003Departments

The Weather Never Sleeps

Ill Wind


Understanding wind shear and microbursts

Success, sometimes, can be dangerous. For instance, a pilot who can't recall her last bad landing could be heading for complacency that spells trouble.

Take low-level wind shear, for example. During the 1980s, people who couldn't tell you the difference between an aileron and an elevator talked with some knowledge about low-level wind shear, microbursts, and Terminal Doppler Weather Radar.

These topics were in the news because scientists, the aviation industry, the FAA, the National Transportation Safety Board, academic scientists, and others were busy trying to find an answer to commercial aviation's biggest killer: low-level wind shear - specifically, wind shear caused by microbursts.

Their efforts were successful, which means the topic is no longer newsworthy. General aviation pilots hear little about wind shear today, except maybe during a few minutes of a ground-school lecture on weather. (AOPA members can peruse an AOPA subject report on thunderstorm avoidance online.

From 1970 through 1994, low-level wind-shear accidents killed 563 people. Major crashes such as that of a Delta L1011 at Dallas/Fort Worth International Airport on August 2, 1985, which killed 137 people, spurred the effort to solve the problem.

No one has died from a low-level wind-shear airline accident in the United States since July 2, 1994, when a USAir DC-9 crashed at Charlotte, North Carolina, killing 37 people. (An AOPA Air Safety Foundation analysis of this accident is available online.

This isn't to say that low-level wind shear will never again cause an airline crash. Even the best safety systems can break down. And we know one thing for sure: The phenomenon that causes microbursts, the most dangerous kind of low-level wind shear, certainly has not gone away. This is why all pilots should understand the basics of low-level wind shear.

Meteorologists use the term wind shear in many ways. For instance, you might hear a hurricane forecaster say that wind shear has weakened a hurricane. This refers to winds that are generally from the east at sea level, and winds high above - at perhaps 30,000 feet - from the west pushing the top and bottom of the storm in different directions, trying to rip it apart. This is directional shear.

High-flying jets encounter turbulence as they fly into a jet stream to pick up a nice tailwind because of the speed shear between the outer edge of the jet stream, where the wind might be 50 knots, and the core, where it could be 150 kt.

Low-level shear, as the name says, occurs near the ground and can be the most dangerous because pilots have less time to recover from the loss of airspeed and lift that such shear can cause. This brings us to the microburst, the phenomenon that causes the most dangerous kind of wind shear - the kind that caused so many air carrier accidents. Microbursts are associated with thunderstorms, and sometimes even showers that have not yet grown into thunderstorms.

Rising warm air - updrafts - cause cumulus clouds to grow into thunderstorms. When rain begins to fall, it begins to drag cool air down to the ground, creating downdrafts. Most of the time these are benign. The cool air reaches the ground and spreads out as relatively gentle breezes that bring welcome cool air on a sticky day. This advancing cool air is the gust front. When it arrives it can cause wind shifts and make life harder for a pilot who happens to be taking off or landing, but these shifts are not normally violent.

Microbursts bring the violence. This happens when wind blasts down from a thunderstorm or even a shower. By definition, a microburst is less than 2.5 miles in diameter. When the wind hits the ground, it spreads out, as is shown in Figure 1. Microburst winds have been measured at speeds of 150 kt. As the winds spread out, they can create a vortex ring. Figure 1 shows rain coming down with the wind, but not all of the rain is hitting the ground. Rain that doesn't hit the ground is known as virga, and it can create what is known as a dry microburst. As the rain evaporates into dry air below the cloud, it cools the air, creating what amounts to a cold air balloon that plunges to the ground.

Dry microbursts occur most often in the West, where the air has little humidity. Sometimes a microburst's vortex ring can be seen when it hits dry ground and kicks up dust. In fact, this is one sign to look for if you happen to be flying out West. Flying under virga is a bad idea; you could encounter a dry microburst, especially when close to the ground taking off or landing.

In the East, where the air is usually much more humid, wet microbursts are more common. They occur when dry air is pulled into the thunderstorm and some of the rain evaporates in it, creating the cold-air balloon. But since only some of the rain evaporates, the microburst can be in the middle of a downpour.

The wind's downward movement is part of the danger when an aircraft flies into a microburst. But, the biggest danger is a sudden shift in wind direction. Figure 2 shows what can happen. On the left side, the airplane is descending at the proper airspeed, maybe with a headwind. When it runs into the edge of the microburst, the wind shifts suddenly to a tailwind, which causes the wing to lose lift, and the airplane pitches down and begins descending. Unless the pilot quickly applies power and pitches up, the airplane can hit the ground. This is why the first rule of staying out of trouble with microbursts is "avoid, avoid, avoid."

Thanks to the work done in the 1980s, Terminal Doppler Weather Radar has been installed at large air-carrier airports in parts of the United States and some foreign countries where microbursts can occur. This radar is designed to alert controllers to microbursts, allowing them to warn pilots.

But, most general aviation pilots are not likely to use these airports very often. They are mostly on their own when it comes to avoiding microbursts. You need to know that a thunderstorm doesn't need to be big and obviously menacing to produce a killer microburst. The microburst that caused the 1985 Dallas/Fort Worth crash came from a thunderstorm that was more of a shower; its highest cloud was at only 23,000 feet. Several larger, more dangerous-looking thunderstorms were in the area at the time, including one with a top at 50,000 feet.

Microbursts are most likely to occur on warm or hot days when convective clouds are around. Any forecast of thunderstorms should alert you to the possibility of microbursts. Heavy rain or rings of dust on the ground could be signs of microbursts. Fortunately, microbursts are relatively small and last only 10 or 20 minutes.

Jack Williams is the weather editor of USAToday.com. An instrument-rated private pilot, he is the author of The USA Today Weather Book and co-author with Dr. Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.


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