Weather

To the astronauts aboard the Space Shuttle Challenger on April 9, 1984, the clouds in this photo were stunning but not scary, since the astronauts were orbiting 200 miles overhead. At the same time ordinary pilots, who are confined to the atmosphere, were staying far away from the line of thunderstorms in the photo, which were towering as high as 55,000 feet.

The view is from the northwest as the squall line, which was moving toward the southeast into a cloudy area, stretched roughly 450 miles from over the Gulf of Mexico south of Pensacola, Florida, to the Georgia/Florida border near the Atlantic Ocean. The low clouds in the bottom part of the photo are typical of the area of low stratus clouds and widespread light rain that often follows a squall line to create a few hours of instrument meteorological conditions (IMC).

This squall line’s thunderstorms brought strong winds, hail, torrential rain, and a few small tornadoes—followed by light rain—to central and southern Florida.

Squall lines in the United States usually form east of the Rocky Mountains and are most likely in the spring and early summer with the squall-line “season” moving north from the southern states with warm weather.

A gang of thunderstorms. Pilots learn to avoid thunderstorms because they are the atmosphere’s angry, strong bullies that can attack with extreme turbulence, rain heavy enough to drown engines, balls of ice (hailstones) as big as baseballs, structural icing, and winds that blast down from a storm to spread out when they hit the ground—known as downbursts and microbursts. Any thunderstorm can be more than a match for the most skilled pilot in the strongest airplane.

One thunderstorm is bad enough, but a gang of these bullies is worse because many times the only way to avoid a squall line’s thunderstorms on a cross-country flight is to land when the squall line is far away, tie down or hangar the airplane, and wait until the squall line passes. Some squall lines stretch hundreds of miles, and flying around them often is not an option.

Forget about trying to fly through what appears to be a gap between two of its thunderstorms, too. For one thing, any thunderstorm’s strong winds aren’t confined to its clouds. Also, in a squall line new individual storms are always forming as older ones die. The few puffy clouds below as you fly through a “gap” could grow into a new thunderstorm that catches your airplane in minutes.

Where squall lines form. Squall lines form most often in the warm sector of middle-latitude storm systems south of a warm front and to the east or southeast of an advancing cold front, as shown below. While the entire area south of the warm front and east of the cold front is warm, the tongue of warm, moist air from the south adds extra heat and humidity to feed thunderstorms. The upper-air trough shown in the diagram helps warm, humid air rise from the surface. As the line moves, generally toward the southeast or east, some of its storms die out as new ones sprout and grow. Most squall lines are one-day events, usually starting in the late morning or afternoon and fading some time after dark.

They are fighters. While all squall lines are dangerous, some are especially damaging because they are long-lasting and powerful. The National Weather Service uses the term “derecho”—pronounced day-RAY-cho—for a squall line that has winds stronger than 58 mph, and extends across an area at least 280 miles long. Sometimes you’ll see this term used in NWS severe weather outlooks, which means not only should you not fly, but make sure your airplane is in a sturdy hangar or tied down as securely as possible if a derecho may be headed your way.

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