If there were a groundhog for predicting the start of winter instead of just the end of it, it would be chortling happily at the kind of December we’re having. As if the below-freezing days and nights aren’t enough, we’ve had a bouillabaisse of Jack Frost’s nastiest stuff.
In honor of our imaginary woodchuck, here’s a brief glossary of some of the messier precipitation we’ve been on the receiving end of:
Freezing rain
is precipitation that falls in liquid form — that is, as rain — but freezes on contact with the surface. This occurs when temperatures hover around the freezing point and there is a layer of colder air at the surface. Freezing rain can be the worst winter hazard. It may form an icy glaze that can make roads and sidewalks extremely slippery, and which is capable of felling trees and utility lines.
Black ice
on roadways can be a product of freezing rain, or of rain or melted snow that refreezes. As drivers know, black ice is especially dangerous because it can be nearly impossible to detect. It is not really black, but rather transparent and thus the color of the surface on which it sits.
It may seem obvious, because we’ve all seen the signs, but ice usually forms on bridges before other road surfaces, so it would be wise to heed those signs. The reason for this is that air flows freely above and below a bridge, lowering the temperature faster than on other surfaces.
Sleet
differs from freezing rain in that it reaches the ground as small, transparent or translucent ice pellets, which may be spherical or irregular in shape. Sleet is formed when rain, or sometimes briefly melted snow, descends through a substantial layer of subfreezing air, thus freezing (or refreezing) before it hits the ground. Sleet is also capable of producing very slippery surfaces, though it has the virtue of being visible.
It is interesting to know that
hail and sleet, though both are precipitation in the form of ice, are defined differently and have different origins. It would be rare to see true hail in winter, because hail is formed exclusively in thunderstorms (technically speaking, in cumulonimbus clouds). Hail is created when ice crystals in the cloud circulate in the cloud’s powerful updrafts, each time freezing and refreezing and accumulating more ice as they do. That’s why the pellets of hail are generally (though not always) larger in size than those of sleet. When the hailstones get heavy enough, they fall to the ground.
As for
snow — well, that’s just fine with me. It’s why we choose to live as hardy New Englanders, right?
Fred Baumgarten is a naturalist and writer. He can be reached at fredb58@sbcglobal.net. His blog is at thatbirdblog.blogspot.com. 
