We’ve got a mixed bag of wintry precipitation in the forecast over the next couple days. If you’ve looked at the temperatures forecast, you may be thinking “wait, temperatures will be below freezing. Why isn’t this all snow?”
We look at the atmosphere as a whole, from the surface to several hundred thousand feet in the air. ALL precipitation begins as snow. The temperature at different levels of the atmosphere dictates what type of precipitation will actually hit the ground.
We’ll keep things simple. The snow will remain snow, as long as the entire column of air is below freezing (32°F).
For sleet, most of the air is below freezing, until a shallow layer of warmer air several thousand feet above the ground (greater than 32°) melts the snow to a rain drop. That rain drop then refreezes as it reaches the colder air near the ground.
Freezing rain happens when the snowflake falls through a thicker layer of warm air above the surface. The snowflakes becomes a raindrop through that layer of warm air, but doesn’t have enough time to change back to sleet or snow as it hits the below freezing air at the ground. Therefore, the rain freezes on contact causing travel headaches.