Monday, January 26, 2015

What Makes a Blizzard?

It's not snowing much in New York City right now. But later today, we're expecting a big blizzard. The plows are getting ready to shovel a lot of snow:

All this makes us wonder: What exactly is a blizzard, and how does a regular old snowfall turn into one? According to the National Weather Service, a blizzard is actually more about wind than snow. For a snowstorm to be considered a blizzard, it has to be accompanied by winds that blow at least 35 miles per hour. It also has to reduce visibility—how far you can see—to less than 1/4 of a mile (in New York City, that's about 5 regular-length city blocks) for at least 3 hours.

Here's how a blizzard forms: Temperatures in the air and on the ground turn freezing; if the temperature on the ground is too warm, then snow falling from the sky will turn to rain or freezing rain. To make snow, air has to blow across a large body of water, like the Atlantic Ocean or the Great Lakes. This causes some of that water to evaporate and escape upwards, into the air. Finally, for a storm to achieve blizzard status, warm air has to rise over cold air. This often happens when freezing winds blow down from the North Pole, and meet warm air being pushed north from the Gulf of Mexico. This is called a "front." As of 11:30 AM in New York City on Monday, the front was moving  from the midwest of the country towards the east. The temperature was 29 degrees—3 degrees below freezing—and the winds were blowing at 22 miles an hour. How long will it take before the blizzard here is raging?

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