As the temperature drops above and below the freezing point this time of year, there are a variety of precipitation types that can occur. Why so much variety, though? It all depends on what the temperature is doing above your head, in the atmosphere, not just at the ground.
Ice Storm Power Line and Tree Damage - Source: NOAA (NWS)
Rain
Even in the summer, when rain is the main form of precipitation, if you go high enough up in the atmosphere these summertime raindrops likely began as snow. As the flakes fall to the ground, though, they encounter an above-freezing layer near the ground and melt into liquid raindrops.
Snow
On the other end of the spectrum, snow occurs when the entire depth of the atmosphere, from the cloud layer to the ground, is either below freezing, and/or its wet-bulb temperature never gets above 32°F. The wet-bulb temperature is the temperature a parcel of air would have if it were cooled to the saturation point. As a snowflake falls, if it encounters a layer of air above 32°F, it will start to melt. However, if the surrounding air is not 100% saturated, then the partially-melted snowflake will also start to evaporate. The process of evaporation causes the air immediately surrounding the snowflake to cool to the wet-bulb temperature. Thus, snow can fall at the ground, even when the actual temperature is a bit above freezing, as long as the wet-bulb temperature is 32°F or below.
Sleet
In cases where there is a warm (above freezing) layer aloft in the atmosphere, however, the other two main types of cold-weather precipitation can fall: sleet and freezing rain. For sleet to occur, there must be a relatively shallow warm layer a few thousand feet above the ground. This warm layer is just deep enough that the snowflakes falling from cloud level have just enough time to melt into raindrops within this layer, but then refreeze into ice pellets as they encounter a below-freezing layer closer to the ground.
Freezing Rain
For freezing rain to occur, the conditions are similar to those for sleet. The main differences are the depths of the warm and cold layers, with the cold layer near the surface being perhaps as little as 1,000 feet thick. The precipitation starts out as snow at cloud level, melts to rain, and then freezes again only upon contact with the ground or other surfaces. This freezing rain, or glaze, can prove very hazardous to travel. Layers of ice make road conditions very slick and hard to handle while driving. The other threat is downed tree limbs and power lines which cause power outages. As ice accumulates on tree branches and power lines, it adds weight to them. Once the weight is heavy enough, it will cause the branches and power lines to snap.
Make Your Own Winter Weather
The University of Wisconsin has a neat link where you can try your hand at “being Mother Nature” and make your own types of winter precipitation by adjusting temperature and humidity throughout a simulated atmosphere.
The different types of cold-weather precipitation, whether it is rain, snow, sleet, or freezing rain, show just how complicated weather forecasting this time of year can be. It depends not just upon the temperature that your backyard thermometer is showing, but also upon what’s going on above your head.
