I went out Saturday evening into the freezing rain because I was out of vermouth. Actually I was out of good vermouth, which is as bad as being completely out. So a trip to the good wine/spirits store ensued despite the worsening weather. Earlier in the day there had been a intense but short burst of snow that left a half-inch everywhere. Then it started sleeting, and when I left my apartment it was raining. I thought innocently that the liquid water was good, as it would melt the snow. But as I continued I realized the rain was a silent evil--it was freezing rain, icing anything it hit. Running my hand over a car window showed the rain was freezing as soon as it hit the glass. It's beautiful in addition to being evil as the trees began glazing over--even fallen, rotting leaves were gorgeous, covered in a glossy encapsulation. A flat glass covering a light became scalloped with ice so well that it appeared as if the glass was installed that way, and it wasn't obvious until I saw the flat glass on another pane.
The WGN Weather blog has a description of the processes and temperature profiles that define what precipitation you get in a winter storm such as Saturday's. In nearly all precipitating clouds the precipitation starts out as ice crystals. Depending on how the temperature increases towards the ground you can get snow, rain, or some bizarro type. The storm started cold--it was cold from the top to the bottom, and so the precipitation was all snow. As the storm progressed warm air from the south increased in the midlevel altitudes, melting the snow, but the air close to the ground, where the wind velocity is not so high, was still cold, and the rain droplets (ex-snow crystals) refroze to fall as sleet. Then, as the warm layer got thicker, there wasn't enough cold air to refreeze the rain, leaving it liquid and falling to the ground (which still was below freezing) as freezing rain. Finally later in the evening the cold air on the ground was whisked away and we had just a cold, but liquid, rain.