Thunderstorm Avoidance

Mobility Forum, Jul/Aug 2004 by Meintel, Julie

Mother Nature is quite a scary lady. When the skies are blue and clear and the sun is shining brightly, you can forget just how scary she can be. But just ask anyone who has had the bad fortune of being stuck in a convertible or on a motorcycle during a violent thunderstorm or anyone who has lived through a tornado. Although there are wonderful advances in meteorological technology, and weather patterns can be forecasted with increased accuracy, Mother Nature still catches us by surprise now and again. The whims of the weather can be unpredictable and change very suddenly. Here in southwest Ohio, there is a joke about the weather that is probably repeated all over the country: if you don't like how the weather is now, just wait ten minutes - it'll change. Keeping on top of those changes is essential to safe operations in the aviation world. Among the most dangerous weather conditions for flyers are wind shear, ice, volcanic ash, arctic conditions and of course, thunderstorms.

In fact, according to AFI 11-203, Vol I, Weather for Aircrews, thunderstorms contain the most severe weather hazards to flight. This is due in no small part to the presence of very strong and sometimes violent winds, turbulence and severe icing, lightning, heavy rain and wind shear. If all those are not enough to discourage even the most willing risk-takers, toss in potential for large, damaging hail, microbursts, which have been responsible for several major aircraft accidents, and even tornadoes.

The best course of action is, of course, to always avoid flying near or into thunderstorms. However, that is one of those things that is sometimes easier said than done. Thunderstorms happen every day, in every climate, all over the world. It is estimated that approximately 44,000 thunderstorms occur daily all around the world. With that kind of frequency, it is not a matter of if, just a matter of when your crew will encounter one.

In addition to knowing what types of weather conditions contribute to thunderstorms, it is wise to have some working knowledge of how to deal with trying to fly safely around or even through them.

First of all, we should touch on the necessary ingredients to whip up a thunderstorm. There are three basic requirements: unstable air, moisture, and lifting action. The presence of these three conditions does not automatically guarantee a storm, but it sure increases the chances. It is possible for air to be lifted to a point where it begins to condense and form clouds, but those clouds do not always result in storm activity.

Condensation is the moisture part of the equation. Once condensation occurs and a cloud forms, it releases heat in the process of changing from a vapor state to liquid. That heat makes the cloud itself warmer than the air that surrounds it, thus creating an unstable environment. The warmer air rises quickly and forms tall towers of cumulus and maybe eventually cumulonimbus clouds, which are the origin of a thunderstorm.

Lastly, the brewing storm needs some sort of lifting action to complete its creation. Lifting might typically result from mountainous terrain, weather frontal systems, warmth from underneath the cumulonimbus cloud, or convergence. Convergence can be defined as upward vertical motions correlating to air coming together from different directions.

The life cycle of a thunderstorm varies widely. Some less severe storms may last only half an hour or so, while others may take several hours to complete the three steps in the cycle. These three steps consist of the cumulus or growth stage, the mature stage and then the dissipation stage.

During the growth or cumulus stage, updrafts occur and extend from the surface of the earth or very near it, to several thousand feet above the visible tops of the clouds. These updrafts get more powerful the higher they get and can exceed 3000 feet per minute. There is usually no precipitation or rain during this period.

The mature stage tends to be characterized by updrafts as well as downdrafts. When the updrafts lose strength and cannot support the ice particles and moisture in the cloud, downdrafts develop and it is at this point when rain begins to fall. Sometimes the precipitation includes hail as well as raindrops. When the downdraft gets close to the ground, it spreads out and creates a sharp decline in temperature along with strong and gusty winds. The gust front often precedes the actual storm, sometimes by several miles. The mature stage is the most intense part of the thunderstorm; in other words, it's as bad as it's going to get.

As the storm dissipates, its energy supply is cut off since updrafts are necessary to produce condensation and the related heat energy. The updrafts continue to lose strength and the downdrafts force the air to become stable once again through the cooled air moving into the previously warm moist air.

The dissipation stage is also seen when the anvil shape at the top of the storm cloud forms as the strong winds higher up force the cloud apart.