Feb 192013

Winter Precipitation Depends on Temperatures Above

When I use the phrase “winter mix”, it refers to some combination of snow, sleet or freezing rain (ice).

The term is used whenever any of these are possible over a certain time frame.

Both sleet and freezing rain require a layer of above freezing air aloft.  The difference is in the depth of the sub-freezing layer touching the ground (lower blue areas on diagram)

While the “general” layout is for snow to occur closest to the colder air with sleet, freezing rain and rain occurring in bands toward the warmer air (usually in a southerly direction), there are exceptions.

One is terrain. An increase in elevation means colder temperatures which can change the precipitation type.  Or the opposite, occasionally cold air becomes trapped in valleys and alters precipitation type.

Also, convective (rain/thunder) bands tend to cool a column of air near the ground and turning what might be a freezing rain situation into sleet.

Here are the definitions:


This is liquid (rain) aloft which falls into subfreezing air near the surface deep enough to freeze the drops into ice balls.  Sleet “tings” when it strikes windows.  Sleet accumulates like snow but because it is essentially little ice pellets, it doesn’t accumulate nearly as fast. It actually offers some increased traction on roads when compared to snow.

Freezing Rain (Ice)

Nearly the same as sleet except that the drops stay liquid falling into a much shallower subfreezing layer at the ground. These “super-cooled” rain drops will then freeze on contact with surface objects, especially those off the ground such as trees and wires.  Freezing rain producing ice accumulations over a half inch can cause branches and wires to snap from the weight. An ice storm is born!


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