To stick, snow requires consistent cold air all the way from where flakes form in the clouds to the ground. If temperatures remain below freezing throughout that column, snowflakes never melt and fall as snow.
The situation changes where warm air exists in layers between cold zones — a condition more common farther south. “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,” said Judah Cohen, a research scientist at MIT.
This process creates sleet: rain droplets or partially melted flakes that refreeze in the lower cold layer before striking the ground, producing bouncy ice pellets. Freezing rain occurs when the lower cold layer is so shallow that rain doesn’t have enough time to freeze in the air, instead freezing on contact with the ground.
Freezing rain is the most devastating of winter precipitation, according to Cohen, because it turns roads into skating rinks and can be heavy enough to bring down power lines. Snow can be plowed, and at least that offers some remediation. But the ice in sleet makes it much more difficult to move.
Between snow and sleet lies graupel, a rarer precipitation type. “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.”
Hail, which some people confuse with sleet, is distinct and uncommon in winter storms. Real hail usually forms in summer, requiring warmer air closer to the surface that creates an updraft. That updraft allows rain to move up, freeze, fall, and move up again, forming layers of ice similar to an onion’s concentric rings.
Snow presents its own hazards. Sufficient accumulation can send cars skidding into ditches and create whiteout conditions that are life-threatening. Latitude affects which precipitation reaches the ground. “The further north, the deeper that Arctic layer is, the more likely to support snow,” Cohen said.
When the National Weather Service determines that expected temperatures and wind chills are low enough to be dangerous, it issues alerts to warn the public. A cold weather advisory means dangerous weather is likely. An extreme cold watch signals that life-threatening weather is possible. An extreme cold warning means life-threatening weather is likely.