When you’re watching summer weather forecasts on TV, it’s a little unnerving to look at the map and see a massive line of thunderstorms seeming to band together and move in a straight line toward your city or town.

But squall lines, as meteorologists call them, are actually a fairly common phenomenon during the summer season — in the last couple of weeks, we’ve been seeing them everywhere, stretching from Ohio and the South to the the Northeast and Mid-Atlantic states to Indiana to Winnipeg, Canada.

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The moist, rising air masses in the atmosphere that cause thunderstorms are enormous, so it’s not really that surprising that multiple storm cells would form at once. Squall lines form along the wedge of colder, higher-pressure air that bumps up against a warm low-pressure area, driving the air upward like a car going up a ramp.

According to the National Weather Service, these squall lines can stretch for hundreds of miles and persist for many hours, with updrafts forming new cells along the edge of the system, and rain and hail following just behind.

Tornadoes sometimes form along the leading edge of a squall line, where they slam into whatever’s in front of them with brutal power, driven by the horizontal spreading effect of the thunderstorm’s downdraft as they descend.

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But the worst-case scenario is for a squall line to generate an extremely powerful, fast-moving straight wall of wind called a derecho, which can move at 100 miles per hour over a stretch of several hundred miles.  Back in late June 2012, for example, the Ohio Valley/Mid-Atlantic derecho caused protracted, widespread power outages in cities from Cincinnati, Columbus and Dayton to Atlantic City, Baltimore and Washington.

Photo: A shelf cloud forms in front of a squall line of thunderstorms in the Netherlands. Credit: Jumbo0 via Wikimedia Commons