How big winter storms create snow, sleet and freezing rain

When big winter storms move in, they can bring nasty weather that ranges from snow to sleet and freezing rain — or maybe extreme and dangerous cold.

Here's a look at some weather conditions and how they vary from place to place.

How snow might become ice or sleet on the ground

To stick, snow needs consistent cold air all the way from where flakes form in the clouds to the ground. If it's below freezing the whole way, the snowflakes never melt, so nothing turns to ice.

“The further north, the deeper that Arctic layer is, the more likely to support snow,” said MIT research scientist Judah Cohen.

Farther south, the atmosphere may include a sandwich of warm air between cold layers. That's how sleet and freezing rain happen.

“The snowflakes form, they fall and then they meet a warm layer, a layer above freezing, and they will melt. But then there's another layer near the surface that's below freezing again, so they will refreeze before they hit the ground,” Cohen said.

Sleet requires the lowest layer to be cold enough that raindrops refreeze when they hit the ground, creating bouncy ice pellets. If that lower cold layer is shallow, the rain doesn't have enough time to freeze in the air. So it hits the ground as raindrops that freeze when they make contact.

Then there's graupel, which is a rarer mix between snow and sleet. Not quite fluffy, and not quite hard.

“It's snow that has tried to melt on its way down, but not quite melted,” said David Robinson, New Jersey state climatologist at Rutgers University. “It's out of that six-point crystalline shape and has begun to look more like a cotton ball. So it hasn't gotten to the point of full melt that it could then refreeze as sleet.”

There's also hail, which Robinson said some people mistakenly use to describe sleet. But real hail probably isn't happening in a winter storm. That usually happens in the summer because it requires warmer air that's closer to the surface. That creates an updraft that allows rain to move up, freeze, fall, and move up again, forming layers of ice similar to the layers of an onion.

Different precipitation, different hazards

Snow can be dangerous — sufficient to send cars skidding into ditches and be life-threatening in whiteout conditions. But at least it can be plowed.

The ice in sleet makes it much more difficult to move.

But the most devastating moisture is freezing rain, Cohen said, because it turns roads into skating rinks and can be so heavy it has the power to bring down power lines.

And then there's extreme cold.

When the National Weather Service deems expected temperatures and wind chills are low enough to be dangerous, they issue alerts.

A cold weather advisory means dangerous weather is likely. An extreme cold watch means life-threatening weather is possible. An extreme cold warning means life-threatening weather is likely.

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