The Weather Never Sleeps

It's where working pilots earn their pay

Studying weather for knowledge tests could leave student pilots thinking that weather systems come in two sizes-roughly analogous to a Cessna 152 and a Boeing 747. They cover thunderstorms, which might be only 10 miles across. At the other end of the scale are large synoptic-scale systems that can affect almost all of the contiguous 48 states during a winter's week with cold and warm fronts and widespread rain and snow.

Between those extremes are squall line thunderstorms. But they aren't the whole story of middle-size weather.

Radar imagery shows a bow echo as a squall line approaches Grand Rapids, Michigan (above). Large thunderstorms are seen forming over Iowa, Illinois, and Indiana (below).

A thunderstorm approaches--rain-cooled air from the storm moves out ahead and plows under the warm moist air, forming a flat shelf cloud (above).

In fact, squall lines are just one of the kinds of weather known as mesoscale convective systems. Meteorologists use the term mesoscale to refer to middle-size weather phenomena that are larger than an individual thunderstorm and can be as large as approximately 300 miles across or long. The word convective means thunderstorms are involved because they are convective storms with both rising and descending air currents. Calling phenomena such as squall lines systems shows that the thunderstorms involved are organized; they didn't just happen to form in the same general area.

As a squall line moves, often toward the southeast or east in the United States, some of its storms die out, but air that comes down from each storm spreads out along the ground to help trigger new storms in the line. The line, which can be from fewer than 50 miles to more than 150 miles long, continues moving as new storms form until after sunset. As the ground begins to cool, the squall line thunderstorms weaken and die.

Squall lines are commonly found along advancing cold fronts. "Prefrontal squall lines, which are often several miles ahead of and parallel to a cold front, are usually stronger than lines with a front, because they can draw in the warm, humid air that supplies thunderstorm energy from both sides of the line-while a squall line along a cold front has cool, dry air on one side.

If a squall line produces straight-line winds (as opposed to the rotary winds of tornadoes) of 57 mph or faster that hit several places along a path that's at least 240 miles long over a period of hours, the National Weather Service calls the line a derecho-pronounced deh-RAY-cho. (See "The Weather Never Sleeps: Derecho to the West," August 2006 AOPA Flight Training.) You might see that term in reports of current weather. If one of these is heading toward your airport, you should check the tiedowns on your airplane instead of thinking of going flying.

Squall lines, especially derechos, sometimes form bow echoes-named for the bow-shape of the line of thunderstorms seen on weather radar. This bow could be shooting or soon could shoot "arrows" of dangerous winds toward your flight since the most damaging squall line winds are often associated with a bow echo.

Squall lines normally begin to form in the late morning or early afternoon as most thunderstorms do, since as the ground warms under sunny or generally sunny skies, it warms the air next to it and the warmed air begins rising. If the atmosphere is unstable and humid enough, these currents of rising air can grow into cumulus clouds and eventually thunderstorms. A squall line can continue ripping its way across the countryside until sunset turns off solar energy.

Pre-frontal squall lines form because a mass of relatively dense, cold, dry air shoving into lighter warm, humid air disturbs the warm air far ahead of the front in ways that cause the air to rise and form thunderstorms in a line parallel to the front. The other kind of mesoscale convective system that forms over land in the middle latitudes is the mesoscale convective complex (MCC).

Before meteorologist Robert Maddox studied infrared images of clouds over the central United States at night in the late 1970s and early 1980s, meteorologists knew that widespread thunderstorms often occurred on spring and summer nights on the Great Plains between the Rockies and the Appalachians. But they couldn't find any patterns to these thunderstorms using data from ground stations, weather radar, and weather balloons.

Maddox saw that on especially stormy nights, areas of very cold cloud tops covered large areas. He used the size and temperatures of cloud tops to define what he called a mesoscale convective complex: Cloud-top temperatures of minus-25 degrees Fahrenheit or colder need to cover at least 100,000 square kilometers (38,627 square miles) or about the size of Iowa. A center area of cloud-top temperatures of minus-62 degrees for colder covering at least 50,000 square kilometers (19,313 square miles) is also needed. Such temperatures indicate that cloud tops are 40,000 to 50,000 feet above the ground-very tall, and thus powerful, thunderstorms. The area of cold, high clouds also must be nearly circular in shape, and the system must last at least six hours.

MCCs grow and continue after sunset because low-level jet streams bring in warm, humid air from the Gulf of Mexico. They begin blowing when the ground south of the thunderstorms cools after sunset. As this happens, an inversion- air aloft that's warmer than the air at ground level-forms because the air aloft doesn't cool as fast as air next to the ground.

The inversion shuts down the up-and-down air motions that make flights on sunny days bumpy at low altitudes. Meteorologists say that an inversion decouples the upper-altitude winds from the surface. Without slow-moving air rising from below and some of the fast-moving air aloft sinking, the air aloft is no longer "connected" to slow-moving air near the ground. In the morning as the sun heats the ground the inversion is erased and the low-level jet fades away. When this happens, the MCC's thunderstorms weaken and die.

But, the MCC itself isn't completely dead. During the night an area of low atmospheric pressure with the winds blowing counterclockwise around it forms 15,000 to 20,000 feet above the ground in the MCC. After the rest of the MCC fades away in the morning, upper air winds carry the swirl of low-pressure air toward the east where it can organize any thunderstorms that form under it into another MCC.

A student pilot in a Cessna 152 will certainly stay far away from squall lines and won't be flying at night when an MCC is causing heavy rain and thunder. But if the student moves up to larger air carrier airplanes, safely coping with squall lines and mesoscale convective complexes will become part of the job.

Jack Williams, a freelance science writer specializing in weather and climate, is the author of six books. He is an instrument-rated private pilot. His latest book, The AMS Weather Book: The Ultimate Guide to America's Weather, is forthcoming from the University of Chicago Press.