Weather

The following questions on these pages are typical of the many I have been asked about thunderstorms in my nearly three decades of writing about aviation weather. Thunderstorms are not to be taken lightly. Here's why:

Where do thunderstorms come from? Thunderstorms form when and where a lifting force, such as a cold front, and an unstable atmosphere combine to push humid air aloft. Individual thunderstorm cells go through a life cycle of cumulus, mature, and dissipating stages.

During a thunderstorm’s lifespan, winds at the surface and aloft push it along. Humidity is needed because the latent heat released when water vapor condenses into clouds and precipitation supplies most of a thunderstorm’s fuel.

Why should I care about the three stages of a thunderstorm’s life? My wise-guy answer is: “Because this is a question on FAA knowledge tests.” The real answer is that a pilot who knows a little about how thunderstorms work has a better understanding of their dangers.

During the cumulus stage, a thunderstorm’s birth, air is rising into a growing cumulus cloud. While these updrafts are turbulent, they are just a small taste of what’s to come.

By definition, a thunderstorm reaches its mature stage when rain or hail (or in rare cases, snow) begins to fall. The falling precipitation drags air down, which creates downdrafts. Currents of air that are racing up—sometimes faster than 100 mph—adjacent to those coming down create extremely violent turbulence.

When the updrafts fade away, leaving only downdrafts, a thunderstorm has reached its dissipating stage. But a fading thunderstorm is still dangerous. For example, potentially dangerous downdrafts are possible anywhere around a storm.

I didn’t hear any thunder from the storm I saw in the distance while doing a preflight inspection. Why did the Weather Service call it a “thunderstorm”? A cumulus cloud becomes a thunderstorm when it begins creating lightning. Lightning always produces thunder, but you won’t always hear the thunder because it usually can’t be heard more than approximately 10 miles away from the lightning. You also might not see all of a thunderstorm’s lightning, even at night, because thick clouds hide it.

Lightning is a greater danger to pilots doing a preflight inspection on the ramp than to pilots in the air.

Since thunderstorms mostly occur during warm weather, ice isn’t a problem for pilots. Right? Wrong. While the temperature might be in the mid-90 degrees Fahrenheit on the ground, air in an updraft cools well below freezing so quickly that the cloud drops that form can remain liquid at temperatures well below 32 degrees F. These supercooled drops instantly turn into ice when they hit something, such as an airplane’s wings or another piece of ice.

Supercooled drops hitting other pieces of ice freeze to form hailstones, which can grow as large as softballs. Even golf-ball-size or smaller hunks of hail can crack windshields and make dents in any forward-facing part of an airplane.

Just because no hail is reported as hitting the ground doesn’t mean there’s none in a thunderstorm. A great deal of hail melts into raindrops on the way down. Hail that does make it all of the way down often falls in narrow swaths that can easily miss weather stations. For more about hail, see “Thunderstorm Hazards: Hail” on the NWS Web site.

Since widespread, generally flat clouds form in a stable atmosphere, I don’t think I need to worry about thunderstorms when flying on instruments through such clouds. A pilot who thinks this is a victim of the often-simplistic presentations of weather in basic texts. Stable and unstable atmospheric conditions aren’t like black and white; one can shade into the other. Also, the atmosphere can be stable at one altitude and unstable at another in the same region.

Thunderstorms can be embedded in widespread clouds like raisins in your oatmeal. When you’re flying in clouds you are, so to speak, swimming in the oatmeal, not looking down on the bowl. By the time you see a thunderstorm you could be in it. While such a thunderstorm isn’t likely to be a monster like a Great Plains supercell that’s busy stirring up tornadoes, your flight will become more exciting than you want it to be.

What should I do if I blunder into a thunderstorm? Pilots with experience flying in stormy weather, such as those who fly into hurricanes, say you should forget about trying to hold altitude—let the airplane ride up and down in the updrafts and downdrafts—and concentrate on maintaining a level attitude while keeping the airspeed below maneuvering speed (VA).

Since keeping the wings level is important, you shouldn’t try to turn around to escape the storm. When you are sure you have control, contact ATC, which might be able to offer slight course changes that would get you out of the storm quicker.

Am I safe as long as I stay in the clear air around a thunderstorm? No. The FAA’s Aeronautical Information Manual recommends staying 20 miles from thunderstorms. This distance might seem a little conservative, but I wouldn’t advise testing it unless you are one of the few pilots who are paid to fly into storms.

At higher altitudes thunderstorms can create clear-sky turbulence much like that formed when wind flows over mountains. Close to the ground, thunderstorm downdrafts can create especially violent changes in wind speed and direction—wind shear—that even large airliners can’t handle. Even dissipating thunderstorms can cause such conditions. And, they can occur anywhere around a thunderstorm.

What’s the most dangerous type of thunderstorm? The most dangerous kind of thunderstorm for everyone, including pilots, is a supercell. These are especially strong, long-lived thunderstorms that can spin out the strongest tornadoes as well as other hazards including large hail (balls of ice); strong, gusty winds; extreme turbulence; and flooding rain.

For pilots who need to get somewhere, the most dangerous thunderstorms are those that come in bunches, such as a multicell cluster or a squall line, which might stretch for a few hundred miles across your planned route of flight. As the name says, a multicell cluster includes more than one thunderstorm cell in what might look like a single storm. Such a cluster includes cells in all three stages of growth and new cells can quickly form and grow in an area that a few minutes ago had been clear air.

Squall lines sometimes tempt pilots to fly through gaps between cells, sometimes with the help of air traffic control (ATC). Such gaps can close rapidly and catch an airplane inside a thunderstorm with little warning. Also, ATC’s first job is to separate instrument flight rules (IFR) aircraft from other IFR aircraft, not help you blunder through a thunderstorm. While today’s ATC radar shows precipitation, it isn’t as good as weather radars, which are designed to see water drops, hail, and ice crystals.

To see how relying on ATC to guide you through a hole in a line of thunderstorms can turn out, take the AOPA Air Safety Foundation’s online Weatherwise: Thunderstorms and ATC course and click on “Accident” on page 3. See how much you’ve learned by taking the AOPA Air Safety Foundation Safety Quiz on thunderstorms.