KNOXVILLE, Tenn. (WATE) — Rain, sleet, ice, or snow? How much and which will East Tennessee see this winter?

That depends entirely on the temperature of the air through which the precipitation falls. That temperature can vary greatly at each level of the atmosphere. When predicting what type of winter precipitation an area will receive, a meteorologist must look at air temperatures at every level of the atmosphere.

Rain: Precipitation first falls from the clouds in the form of snow. Those flakes fall through several levels in the atmosphere, each of which can be different temperatures. If the final surface layer is deep and above 32 degrees the precipitation lands as rain.

Freezing rain:  Precipitation falls first from within a freezing layer, creating snow. Those flakes then pass through a warmer layer in the atmosphere in which the flakes melt and fall as rain. If this warmer layer is shallow, that rain will again encounter cold temperatures and refreeze when it makes contact with the ground.  This is what causes icy roads, for example.  

Figure 3. Vertical temperature profile supportive of freezing rain.
Image courtesy of the National Weather Service

Sleet: Sleet is similar to freezing rain, except it encounters a cold atmospheric level close to the earth that causes the precipitation to lands as ice pellets. Sleet is the most common precipitation in winter. 

Figure 2. Vertical temperature profile supportive of sleet.
Image courtesy of the National Weather Service

Snow: Precipitation that never encounters a warm layer in the earth’s atmosphere falls to the earth as snow.

Figure 1. Vertical temperature profile supportive of snow.
Image courtesy of the National Weather Service