Learning the life cycle of thunderstorms
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A thunderstorm's life cycle usually consists of three stages. In the cumulus stage (Figure 1, at right), the potential thunderstorm is a cumulus cloud. In the mature stage (Figure 2, center), falling rain drags down cool air, creating downdrafts. When warm air stops rising (Figure 3, far right), the storm dissipates. | ||
When you hear thunder you know that lighting is within about 10 miles. And lightning occurs only in clouds that hide a certain amount of violence, no matter how pretty they look from a distance.
Lightning is a huge electrical spark that flashes between areas of negative and positive electrical charge. Both charges could be in the same cloud, in different clouds, or one in the cloud--usually negative--and the other on the ground. Nature separates positive and negative charges with strong up-and-down winds carrying a mix of supercooled water drops, which are below freezing but still liquid; ice crystals; and maybe hailstones, balls of ice that can be as big and as hard as golf balls.
If you are in an airplane that's in a thunderstorm or too close to one, the airplane could be:
- hit by lightning.
- coated with ice that interferes with lift.
- battered by hailstones.
- shaken by violent turbulence.
Lightning is the least of your worries in such a case because today's airplanes are made to survive lightning strokes. Violent turbulence is the biggest danger.
Awareness of thunderstorm hazards goes back to the earliest days of aviation. By the 1930s pilots and weather scientists were paying serious attention to them as pilots became skilled in instrument flying and airlines were becoming a reliable way for ordinary people to travel.
In 1938, for instance, Assen Jordanoff wrote in his book Through the Overcast: The Weather and the Art of Instrument Flying that airline pilots "religiously avoided" thunderstorms because their severe turbulence "not only causes considerable personal discomfort, but places a severe strain upon the aircraft itself."
Data from aircraft--sometimes collected inadvertently--accounts for many things scientists know about thunderstorms. But, until after World War II most scientific thunderstorm knowledge came from ground observation and trying to figure out what had happened inside a thunderstorm to cause a crash. This wasn't enough.
Airline crashes in the 1930s and '40s and military crashes during World War II led Congress to appropriate $185,000 to begin a study of thunderstorms during the government's 1945 fiscal year--that would be around $1.6 million in today's dollars.
During the summer of 1946, researchers based in Orlando, Florida--and, the following summer, in Wilmington, Ohio--combined these ingredients in the Thunderstorm Project, the beginning of today's thunderstorm science. The Army Air Forces (it became the separate U.S. Air Force in 1947) provided Northrup P-61C Black Widow airplanes, which had been designed as radar-equipped night fighters and carried a crew of two or three. During the two years of the project the airplanes flew into 76 thunderstorms with a half-dozen planes flying through each storm at the same time at different altitudes.
On many flights in Ohio hail left two- to three-inch dents in some of the airplanes. Lightning hit airplanes 21 times with no serious damage. The strongest updraft--a wind going up--measured was about 58 mph, and the strongest downdraft encountered was about 38 mph. Updrafts and downdrafts can be faster than these.
Among the many things scientists learned was that thunderstorms are made of cells that go through a life cycle much like a living thing. The project's final report described this cycle, which is illustrated in figures 1, 2, and 3.
The cycle begins when warm, humid air rises and its moisture begins to condense into the tiny water drops of a cloud. If the air rises high enough it cools below freezing and the drops become supercooled, as shown in Figure 1. Eventually ice crystals begin forming. This is called the cumulus stage because the potential thunderstorm is a cumulus cloud. If conditions aren't favorable, the process can stop here as the cloud drifts with the wind, never dropping any rain before it evaporates.
If conditions are more favorable, ice crystals--or sometimes water drops--can grow large enough to begin falling through the rising air. During the summer ice crystals melt on the way down to fall as rain. As the rain falls it drags down cool air, creating downdrafts. Now the thunderstorm has reached its mature stage, shown in Figure 2. It's characterized by updrafts and downdrafts as well as falling rain and sometimes hail. If the storm is vigorous enough it begins to produce lightning and becomes a thunderstorm. A weaker cell could die after being nothing more than a rain shower, which could produce heavy rain but no lightning.
Finally, the warm air stops rising, often because the cold air coming down with the rain spreads out to strangle the updraft. If you look at the bottom of Figure 2 you can see how air coming down with the falling rain could spread (to the right in the figure) to block the rising warm air.
This happens in ordinary thunderstorms. But when conditions are right, the thunderstorm comes together in a way that prevents such strangulation. These thunderstorms, known as supercells, are characterized by a rotating mesocyclone, which helps the thunderstorm hold together. Supercells can last for hours and produce the strongest tornadoes as well as fierce straight-line winds and large hail.
While the Thunderstorm Project gave scientists their first real look at what happens inside thunderstorms, others had learned a lot before 1945 about the conditions that cause them.
For instance, nineteenth-century meteorologists knew that air cools as it rises and if it rises far enough the air's humidity condenses into cloud droplets. They also learned that as water vapor condenses it releases latent heat, which it gained when it originally evaporated. This heat works to warm the air, offsetting some of the cooling being caused by the fact the air is rising.
Air warmed by latent heat rises faster than it otherwise would. This means that the release of latent heat powers thunderstorms. The more humid the air, the more latent heat it will release, and the more powerful a thunderstorm can grow.
Air cooling as it rises is only part of the story. The air will rise only as long as it's warmer than the surrounding air. If the air aloft is relatively warm, air won't rise far from the ground and thunderstorms won't get going. On the other hand, if the air aloft is relatively cold, air will rise even faster from the ground, making thunderstorms even stronger. The greater the temperature contrast between the air at ground level and the air aloft, the better the odds that thunderstorms will form and that they will be strong. The greater the temperature contrast between air at the surface and aloft, the more unstable meteorologists say the air is.
Many times pools of cold air aloft, which you'll often hear television meteorologists refer to as upper air disturbances, move over an area, making the air more unstable and increasing the odds of thunderstorms.
Finally, something has to give air near the ground an upward shove to get things going. This could be as simple as the sun heating the ground during the day, which warms the air next to the ground, especially air over surfaces that absorb heat such as pavement and bare fields. When it becomes warm enough, bubbles of warm air can begin rising to create thermals.
Cool air can move into an area--the moving boundary between warm and cool air is called a cold front. Such cold fronts often trigger thunderstorms as they give warm, humid air an initial upward shove.
Wind forcing air up over mountains can also supply the shove needed to initiate thunderstorms. After a cumulus cloud forms over a mountain it can continue growing into a thunderstorm as the wind pushes the cloud away from the mountains.
On the Great Plains from eastern New Mexico and western Texas north into western Nebraska, a boundary called the "dry line" often forms between dry air to the west and humid air from the Gulf of Mexico to the east. Since dry air is denser than humid air, the boundary acts like a cold front when it moves toward the east with the dry air pushing the humid air up. Dry line thunderstorms can be vigorous.
Finally, thunderstorms can supply the shove needed to move air up and form new thunderstorms. If you live in a place where thunderstorms occur, you've probably felt the cool winds that sometimes precede a storm's arrival. This is the gust front caused as air coming down from a thunderstorm hits the ground and spreads out. As the gust front moves into warm, humid air it acts like a miniature cold front, triggering new pop-up thunderstorms miles from the original storm and a few hours after the original storm dies.
At times gust fronts can form new cells adjacent to the parent storm. These storms can be in multicell clusters or multicell lines, which are commonly called squall lines.
National Weather Service and private meteorologists look for potential combinations of thunderstorm ingredients to forecast the chance of thunderstorms. You can find these by going to the Weather Service's Storm Prediction Center Web site, which has "mesoscale discussions" and "convective outlooks." Mesoscale refers to middle-size weather systems from a few miles to tens of miles across, which includes thunderstorms plus clusters and lines of thunderstorms. Convective refers to the up-and-down movements of air, which are the key to all thunderstorms. In fact, meteorologists tend to use "convective" as a synonym for "thunderstorm."
The Storm Prediction Center issues watches for areas where the conditions are forecast to be right for severe thunderstorms, with winds faster than 50 mph or hailstones larger than three-quarters of an inch in diameter. Local National Weather Service offices issue watches and warnings for severe storms and tornadoes. The Weather Service's Aviation Weather Center issues convective sigmets to warn pilots of thunderstorms.
Unless you happen to be one of the few pilots with the weather knowledge, training, skill, and specialized aircraft designed for the job, the only sensible thing to do is to pay attention to the weather and stay far away from thunderstorms.
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.
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