Pulses, squall lines, and supercells
The development of practical instrument flying in the 1930s, which allowed pilots to safely fly through many clouds, made commercial and military aviation more useful--but it also exposed more aircraft to the dangers hidden in thunderstorms. In fact, thunderstorm-related aircraft accidents before and during World War II led the United States government to organize and fund the Thunderstorm Project in 1946 and 1947 that laid the foundations for today's thunderstorm science.
These foundations include the three-stage model of a thunderstorm's life. Figure 1 is a National Weather Service graphic showing the first of these, the towering cumulus stage when water vapor being carried aloft by updrafts (rising air) condenses to form a cumulus cloud.
| Figure 1: Towering cumulus stage |
Eventually the cloud's tiny water drops and ice crystals merge to form raindrops that begin falling. The falling rain drags air down, forming downdrafts, as shown in Figure 2. When this happens the storm has reached its mature stage, which is the most dangerous with updrafts and downdrafts, falling rain, supercooled water drops that turn to ice when they hit an airplane, and maybe the balls of ice (sometimes as big as softballs) known as hail.
| Figure 2: Mature stage |
The cloud's shadow, falling rain, and cool downdrafts can cool the air below the cloud, cutting off the supply of warm, humid air rising into the storm. When this happens, the storm moves into the dissipating stage, shown in Figure 3, and dies.
| Figure 3: Dissipating stage |
Knowing only a little more than these three stages about thunderstorms could save a pilot from an unpleasant and possibly dangerous surprise.
Imagine you're flying solo on an instrument flight plan soon after earning your instrument rating. Clouds don't completely cover the sky, and you figure that going in and out of the clouds will help increase your confidence about flying in restricted visibility. If you don't know much about how thunderstorms form and grow, you might fly into a towering cumulus cloud just as it's moving into the mature stage with a combination of updrafts and downdrafts, which can cause severe turbulence. Such turbulence, by definition, can cause temporary loss of aircraft control.
Avoiding thunderstorms begins with a good preflight weather briefing. Whether you talk with a briefer via 800/WXBRIEF or obtain the briefing via the FAA's Direct User Access Terminal service for pilots, you should always learn about thunderstorms now occurring anywhere near your planned flight route, and the forecast for thunderstorms near your route.
Meteorologists predict that thunderstorms are likely for a particular time and place, when:
- The air is humid or humid air is moving in.
- The atmosphere is or will be unstable.
- Some mechanism will give air near the ground an upward shove.
Humid air is needed because as air rises, it cools. If the air is humid enough the water vapor (the humidity) in the air will begin condensing into water drops and turning into ice crystals. Changes from vapor to water or ice release latent heat (see "The Weather Never Sleeps: Energetic Response," June 2006 AOPA Flight Training). Adding this heat to the rising air makes it less dense, which causes it to rise faster. The faster air rises, the stronger a thunderstorm will be.
When the atmosphere is unstable, air that's given an upward shove will continue to rise (see "The Weather Never Sleeps: An Air of Understanding," January 2008 AOPA Flight Training).
A boundary between air masses, such as a cold front where cold air is shoving under warmer air, can supply the initial upward shove that causes air to begin rising. In addition to cold fronts, such boundaries can include a dry line, which is a boundary between dry air and humid air often found on the western Plains where air from the Southwest deserts encounters air flowing toward the west from over the Gulf of Mexico.
Once a thunderstorm begins, air from its downdrafts that hits the ground and spreads out, like a miniature cold front, can trigger new thunderstorms.
Unequal heating of the ground by the sun can cause air to begin rising as thermals. For example, the air over a rocky area or a parking lot warms up faster than air over grass, forests, or lakes.
Some single-cell thunderstorms, as shown in Figures 1, 2, and 3, are pulse storms, which weaken only to grow stronger again. Such storms can sometimes go through a few such cycles before finally dying. At times pulse storms even become severe, which means they produce 55-knot or faster ground-level winds, or hail at least three-quarters of an inch in diameter.
The take-home lesson for pilots is that even a small thunderstorm with no other thunderstorms nearby can be dangerous. In fact, a rain shower with no lightning can be on the verge of becoming a thunderstorm. By definition, a shower becomes a thunderstorm when it produces its first lightning flash--lightning produces thunder, giving the storm its name. A towering cumulus that is growing rapidly is a good candidate to become a thunderstorm.
What meteorologists call glaciation is one sign that a cumulus cloud could become a thunderstorm. A rapidly growing cumulus cloud has a hard-edged, cauliflower-like appearance. In this area, tiny cloud drops are liquid, even those that are colder than 32 degrees Fahrenheit. Such cold drops are supercooled. If you fly into this part of the cloud the supercooled drops will freeze when they hit your airplane, possibly coating the airplanes with dangerous structural ice.
Meteorologists say the smoother-looking part of a cumulus cloud is "glaciated" because it's made of ice crystals. These are larger than cloud drops and are not packed as closely together. This means that sunlight reflecting from ice crystals doesn't show as much cloud detail. Glaciation is often a sign that a cloud's downdrafts will soon grow stronger.
A single-cell thunderstorm will often grow into a cluster of cells known as a multicell thunderstorm or into an even larger collection of thunderstorms known as a mesoscale convective system. In fact, multicell thunderstorms are more common than single-cell storms.
A multicell thunderstorm begins with downdrafts undercutting warm, humid air near the ground, and pushing it up. That lifting causes new cells to form.
Unlike a multicell cluster, which is considered to be a single thunderstorm with more than one cell, a mesoscale convective system is a collection of individual thunderstorms. Mesoscale refers to weather systems that are from a few miles to maybe 100 or so miles across. Convective refers to the up-and-down air motion needed for thunderstorms. System indicates that the thunderstorms are interacting with the atmosphere around them and each other--they are not merely several thunderstorms that happen to be occurring in the same area.
A multicell line, which is usually called a squall line, is more likely to disrupt aviation than any other kind of thunderstorms. At the height of the spring and summer thunderstorm season, a squall line could stretch from Illinois to Louisiana with the tops of the storms more than 40,000 feet high. Such a line will force even the highest-flying jets to divert hundreds of miles from their planned routes.
Take the course!The AOPA Air Safety Foundation has produced an interactive online course, Weather Wise: Thunderstorms and ATC, which is designed to help you learn how to effectively communicate with air traffic control when convective activity affects your flight. In this course you will learn effective pilot/ATC communication, how ATC describes precipitation, and the weather-radar equipment controllers can use to help pilots avoid convective activity. The course qualifies for the FAA Wings program. Visit the Web site. The April 2008 edition of AOPA Pilot magazine features several articles on thunderstorm avoidance, including an analysis of the thunderstorm-related accident that claimed the life of famed test pilot A. Scott Crossfield, warning signs that signal convective activity, and thunderstorm avoidance tools available in the cockpit. See the thunderstorm avoidance articles on AOPA Pilot Online. |
A mesoscale convective complex is a roughly round area of thunderstorms as big as a state such as Iowa. These complexes reach a peak overnight with heavy rain, frequent lightning, and severe winds and hail. A complex usually dies down early in the morning, but leaves a large swirl of air that moves toward the east to trigger another complex the next night.
The most dangerous kind of thunderstorm is a supercell. Even though these storms are made of a single cell, they aren't classified with ordinary single-cell storms because they are much more complex. The main distinguishing feature of a supercell is that the main updraft feeding the storm is a swirl of rotating air, maybe 10 miles across, called a mesocyclone. This rotating mesocylone is not a tornado--even the strongest tornadoes aren't this wide. But it helps to pump enough humid air high into the atmosphere that it can create the strongest tornadoes, fierce straight-line winds at the ground, and large hail.
A supercell's organized structure allows it to stay together for hours as it tracks across an entire state, such as west to east across Oklahoma, while spinning out tornadoes, downbursts, large hail, and drenching rain. Fortunately for pilots and people on the ground, supercells are easy to track with today's National Weather Service Doppler weather radars. Meteorologists can also do a good job of forecasting where a supercell will travel in the next hour or so. While they can't say exactly where and when it will create a tornado or microburst, forecasters can issue warnings well ahead of time that a dangerous supercell is on the way.
Unlike pilots of the 1930s and 1940s who flew into thunderstorms without realizing all of their dangers, today's pilots can call on good, but not perfect, information about where thunderstorms are now and forecasts of when and where thunderstorms will occur. Using this information requires knowing what the forecasters are talking about.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
