Understanding What Forces Different Types of Wintry Precipitation (credit: National Weather Service)
DISCUSSION: As we are heading deeper into the 2017-2018 Winter season, it is always important to bear in mind what factors come into play when it comes to forecasting for winter weather events. One of the first questions which should always be asked when it comes to anticipating any variety of winter weather event is to what extent (i.e., both with respect to vertical and horizontal difference) there is cold air in place. In addition, the other major issue pertains to if there happens to be warmer air (i.e., above-freezing air) at the surface and/or above the surface. If there is warmer air above the surface, this can often lead to either icing events, mixed precipitation events, or even sleet events. The main difference is determined by the exact presence as well as depth of the relatively warmer air.
If there is warmer air present through the depth of the precipitation all the way down to the surface, this leads to the occurrence of an all-rain event. However, when there ends up being a shallow layer of colder air very close to the surface, this quite often leads to the occurrence of a freezing rain event due to the fact that rain falling through a somewhat warmer layer above the surface reaches a below-freezing layer very close to and/or right above the surface of the Earth which leads to the rainfall freezing upon contact with the ground. Thus, this leads to what are most commonly recognized as destructive freezing rain events. The other relatively common possibility is for the "frozen layer" of the lower part of the atmosphere to be substantially deeper than that observed with the freezing rain scenarios which leads to sleet events where rain freezes well above the surface. This leads to ice pellets which can often have quite a high impact since upon reaching a frozen surface, they can adhere to the surface and create a dangerous situation as well. The message to be taken from all of this is that all-out snowstorms are not the only issues to be concerned about when it comes to winter weather events.
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©2017 Meteorologist Jordan Rabinowitz