The Making of an Ice Storm

The only winter storm that might be worse than a blizzard is one that brings tons of freezing rain—and that is what's hitting the Southeast

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Photo by Davis Turner/Getty Images

An ice storm will pelt the Southeast with devastating freezing rain

How severe is the combination snow and ice storm whiplashing the Southeast today? At 7 AM today a weather balloon was launched from Atlanta, to aid meteorologists in determining just how badly screwed the Peach State was. (Answer: very.) Heavy ice began to accumulate on the balloon, until it was finally lost at about 12,500 ft. (3,810 m) above the ground. They decided not to send another one up.

Georgia and much of the Southeast is experiencing a classic ice storm of catastrophic consequences. In eastern Georgia a half-inch of ice had already formed on trees and power lines, with up to an inch of accumulation expected. That’s likely to take down parts of the electrical grid, knocking out power for hundreds of thousands of people and virtually shutting down transportation. As Slate‘s Eric Holthaus notes, the ice storm is also likely to destroy as much as a quarter of Atlanta’s trees, as thick layers of ice cause them to simply shatter.

In many ways, the Southeast might be better off if it were simply facing a full snowstorm—not that officials in the region have proven all that great at handling snow, either. But the weather conditions are largely producing freezing rain instead—and here’s why. Precipitation in the region is starting off as snow, high in the sky where temperatures are cold. But as the snowflake falls, it enters a warm mass of air—between about 2,500 ft. (762 m) and 5,000 ft. (1,524 m) above the ground—where temperatures are above freezing. The snowflake melts into a raindrop, but then hits a layer of cold air—closer to 24 F (-4.4 C)—just above the ground. The rain drop doesn’t freeze—if it does, the precipitation would fall as sleet, which happens when that layer of cold air is larger—but instead becomes supercooled, meaning its temperature falls below freezing, but it remains a liquid. When that supercooled raindrop hits the surface, where temperatures are below 32 F (0 C), it freezes immediately wherever it falls. The result is a thick sheet of ice that coats roadways, cars, power lines, trees—just about anything it touches.

And then the shattering occurs. Just a quarter inch (0.63 cm) of ice accumulation can add 500 lbs. of extra weight to power lines—and much of the Southeast will get far more ice than that. We won’t know the full toll of the damage until later in the week, but even the worst predictions are unlikely to rival the great ice storm of February 1994, which knocked out power to millions, caused $4.7 billion in damages (2013 dollars) and led to the deaths of nine people. Though for the millions of people throughout the Southeast who will be shivering through the storm—not to mention the countless trees that simply won’t survive it—that’s likely to be cold comfort.