Why is it so icy?
Ice storms happen when there's a warm layer high up that melts falling snow into rain, and a thin layer of freezing air at the ground. The rain is too shallow to refreeze in the air, so it freezes the moment it touches roads, trees and power lines.
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The warm layer in the middle
In winter a warm front can push mild air up and over cold air that's stuck near the ground. Snow falling from the clouds melts in that warm layer. If the cold air underneath is thin, the drops reach the ground still liquid but colder than 32°F (supercooled), and freeze on impact. That's freezing rain, and it can coat everything in a glaze of clear ice.
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Sleet and black ice
- Sleet is the same setup with a deeper cold layer, so drops refreeze into ice pellets before landing. It bounces and piles up like sand.
- Black ice is a thin, clear sheet that forms when water on a road freezes, often on bridges, which cool from above and below, and in shady spots.
Why ice storms are so damaging
Ice is heavy: a half inch on a tree or power line adds hundreds of pounds, which is why ice storms bring down branches and cause long power outages.
Questions people ask
- Why do bridges freeze first?
- Air flows under and over a bridge, so it loses heat from both sides, while a road on the ground is warmed a little by the earth below.
- Is freezing rain the same as hail?
- No. Hail grows inside summer thunderstorms; freezing rain is liquid rain that freezes on contact in winter.