6th to 12th Grade
What causes the different types of frozen precipitation?
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 there is cold air in place. In addition, the other major issue pertains to whether there happens to be warmer air (i.e., above-freezing air) at the surface and/or within some given layer(s) 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 which can also lead to widespread life-threatening conditions both on the ground and in the air. The main difference is determined by the exact presence as well as depth of the relatively warmer air, if there is any warmer air present near and/or above the surface.
If there is warmer air present through the depth of the low/mid-levels of the atmosphere 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 (i.e., generally within the lowest 2,500 to 3,000 feet above the surface) of colder air very close to the surface, this can lead to the occurrence of a freezing rain event. This is because rain falling through a somewhat warmer layer above the surface (i.e., often within the last 1,000 to 2,000 feet) before reaching a sub-freezing layer very close to and/or right above the surface of the Earth facilitates a sudden freezing of the falling rain droplets and thus, icing events.
Thus, such situations often lead to what are most commonly recognized as destructive freezing rain events. The other possibility is for the "frozen layer" of the lower part of the atmosphere (i.e., the lowest 4,000 to 5,000 feet) 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. Moreover, to always pay attention to forecasts from your local National Weather Service office for your area both days prior to and in the hours leading up to the event itself. In addition, be sure to always stay tuned to situational updates from our team at the Global Weather and Climate Center for the latest updates.
Below are some sample soundings that demonstrate how different temperatures can lead to different precipitation types.
Source: NWS Central Region Headquarters