Weather

Meteorological spring—March, April, and May—is when pilots should begin reviewing what they know about thunderstorms.

While thunderstorms sometimes occur during cold weather—“thundersnow” occasionally falls from winter thunderstorms—warm, humid weather greatly increases the numbers of thunderstorms.

WHERE THUNDERSTORMS OCCUR. The map shows the average number of days with thunderstorms in each 10,000 square miles across the contiguous 48 states. (A square with 100 miles on each side is 10,000 square miles.)

As the map shows, the warmer the average yearly temperature and the closer to a warm ocean, the more likely you are to encounter thunderstorms. Even though the West Coast states average fewer than 10 thunderstorms per 10,000 square miles, a few do occur, especially in the mountains during spring and summer.

Thunderstorms form and grow when and where a lifting force (such as an arriving cold front) and an unstable atmosphere combine to push humid air aloft and keep it rising. Basically, an unstable atmosphere refers to conditions when the air near the surface is relatively warm and humid, and the air aloft is relatively cold. This enables air to rise further and faster than it would when the atmosphere is stable.

The result is that as the rising air grows colder, its humidity condenses to form clouds and, eventually, rain and hail. The more unstable the atmosphere, the faster the air rises and the more violent the thunderstorm will be. (See “Weather: Stable or Unstable?” April 2014 Flight Training.)

Pop-up. You might hear a television meteorologist refer to the probability of “pop-up” thunderstorms. Obviously thunderstorms don’t just pop up out of nowhere. The air needs to be relatively humid and conditions have to be right to make the atmosphere unstable. Meteorologists can predict a day or two in advance when the atmosphere is likely to be humid and unstable enough to create thunderstorms. They are also becoming better at forecasting two or even three days in advance when conditions will be ripe for widespread or fierce thunderstorms. In other words, most of the time you should have a good idea the evening before you go flying whether thunderstorms could be a concern.

Pop-up thunderstorms refer to those that don’t form in clusters or lines, such as along an advancing cold front. While forecasters can say a day or two in advance which parts of the country are likely to have thunderstorms, they can’t say exactly where individual storms will form until weather radar detects thunderstorms actually forming.

If a cold front is heading toward a region where the atmosphere is primed for thunderstorms, forecasters can do a pretty good job of saying when the thunderstorms are likely to begin. It’s easier to predict the movement and timing of relatively large weather feathers such as fronts than it is to predict smaller-scale events such as individual thunderstorms.

While a forecast made a few hours or even a day before a planned flight can give you some idea of what to expect, you need to obtain a preflight briefing shortly before takeoff to ensure a thunderstorm isn’t likely to catch you in the air.

If you’re flying away from your airport you need to keep up with the weather by checking with Flight Watch, on 122.0 MHz, for en route weather advisories or with onboard weather on the instrument panel or a tablet.

THREE STAGES. All thunderstorms go through three stages, and all the stages should be avoided. A thunderstorm begins as a growing, puffy cumulus cloud, which forms as humid air rises and cools.

Pilots should be weather watchers; watching from the ground as clouds grow can be instructive, especially when the cloud is growing rapidly. Think about what it would be like inside a fast-growing cloud. You can see from far away that it is turbulent there.

When rain or hail begins falling from a growing cumulus cloud, it has reached its mature phase, which is the most dangerous of the three phases. The falling precipitation drags air down, which creates downdrafts. Extremely violent turbulence results from currents of air that are racing up—sometimes faster than 100 mph—adjacent to air that’s coming down at roughly half the speed of the updrafts.

When the updrafts fade away, leaving only downdrafts, a thunderstorm has reached its dissipating stage. Even so, a fading thunderstorm is still dangerous, since potentially dangerous downdrafts are possible anywhere around a thunderstorm—even when it’s dying.

WATCH FOR GROWING CUMULUS CLOUDS. Turbulence is likely to be fierce before a cumulus cloud produces its first lighting stroke, which officially makes the cloud a thunderstorm.

By the way, lightning by itself poses little danger for anyone in an airplane. Pilots are more likely to be struck by lightning while standing outside performing a preflight inspection. (See “Weather: Too Hot to Handle,” June 2013 Flight Training.)

A thunderstorm’s strength depends on the temperature difference between air rising into the thunderstorm and the temperature of the surrounding air at all altitudes. Patterns of winds aloft also help to determine wh

 ICY UP HIGH. While the temperature might be in the mid-90s at the ground, air in an updraft cools well below freezing so quickly that the cloud drops which form can remain liquid at temperatures well below 32 degrees Fahrenheit. These supercooled drops instantly turn into ice when they hit something, such as an airplane’s wings or another piece of ice.

Supercooled water 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.

WHAT SHOULD YOU DO? Flying VFR, you know you are required to remain at least 2,000 feet from any clouds if you are more than 1,200 feet above the ground. Never try to fly through a gap in a line of thunderstorms. Such gaps can quickly close. A better strategy is to avoid all thunderstorms by 20 miles or more.

Pilots flying IFR work with air traffic control to stay clear of thunderstorms, but sometimes they come too close. 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.

At higher altitudes, thunderstorms can create clear air 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.