A squall line is a long, thin line formation of rapidly moving thunderstorms. Although they are normally less than 32 km (20 miles) broad, they can extend for hundreds of kilometres (often oriented north-south). They produce a brief, intense burst of weather as they move across the terrain, which can include hazardous straight-line winds, lightning, hail, and heavy rain.
Squall lines often develop in the tropics, where they travel from east to west, or in the mid-latitudes when they move from west to east. East of the Rocky Mountains in the US is where you’ll find them most frequently, especially in the spring.
Squall lines are typically observed along or in front of a cold front’s leading edge. The cold, dense air pushes warmer, moister air in its path upward as it moves forward. The moisture in the warm air condenses into a cloud as it cools and rises. A downdraught of frigid air rushes in to take its place, crashing into the earth and spreading out in all directions, from the top of the storm cloud.
This chilly “gust front” that precedes the storm increases the lifting effect, generates wind shear at the interface of the two air masses, and can produce a shelf cloud that has a foreboding appearance. A phenomenon known as a “bow-echo” can form when the winds driving a squall line ahead are highest near the middle of the line. This shows an area of heavy rain like an archer’s bow on a radar picture.
Sometimes, tornadoes will appear at the bow’s trailing tips. At their worst, bow echoes may develop into Derechos, which are self-sustaining storms that can last for several hours, create wind gusts as high as a Category 1 hurricane (120 km/h), and cause wind damage along a span of at least 400 km (250 miles). (Sciencefocus)
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