A squall line is a fast-moving system of thunderstorms in a long, narrow line formation. They can stretch for hundreds of miles (typically arranged north-south) but are usually less than 32km (20 miles) wide. As they sweep across the landscape, they deliver a short, violent burst of weather that can include dangerous straight-line winds, lightning, hail, and torrential rain.
Squall lines tend to form in the mid-latitudes, where they move from west to east, or the tropics where they move east to west. They are most common in the US, east of the Rocky Mountains, especially during the spring.
Squall lines are usually seen along, or ahead of, the leading edge of a cold front. As it advances, the cold, dense air forces warmer, moister air in its path to rise. As the warm air rises, it cools, and the moisture it contains condenses into a cloud. Meanwhile, a downdraught of cold air from the top of the storm cloud rushes in to take its place, slamming into the ground and spreading out in all directions.
Ahead of the storm, this frigid ‘gust front’ intensifies the lifting effect and causes wind shear at the boundary of the two air masses, and can generate an ominous-looking shelf cloud. When the winds propelling a squall line forwards are strongest at the midpoint of the line, a feature called a ‘bow-echo’ can develop. On a radar image, this appears as a band of heavy rain shaped like an archer’s bow.
Occasionally, tornadoes spring up at the trailing tips of the bow. At their most extreme, bow echoes can become self-sustaining storms known as derechos, which maintain their structure for several hours, produce gusts as strong as a Category 1 hurricane (120km/h), and inflict wind damage along a path of 400km (250 miles) or more.
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Ceri Perkins is a New York City-based writer and editor who covers the environment, science, nature and human behaviour. As a freelancer, she has lived around the world, from Madrid to the Scottish Highlands. Before going freelance, Ceri was based in Geneva, Switzerland, as a staff writer/editor at CERN, home of the Large Hadron Collider. Later, she was News Editor at NYC-based magazine Spectrum, where she edited news and opinion stories about the neuroscience and genetic underpinnings of autism. In her spare time, Ceri is typically either outdoors in nature or curled up inside with a stack of books and a pile of things to make or fix. She holds a Bachelor’s in Atmospheric Science, a Master's in Science Communication, and you can read her work in TED Ideas, BBC Earth, The Guardian, Physics World, New Scientist, and more.