KOTA Territory Weather School
Precipitation type is very important when it comes to winter weather forecasting. There are multiple types of it and it can create many different, yet similar hazards.
You have rain, freezing rain, sleet, snow and graupel. Rain happens when temperatures are greater than 32° from the surface, well aloft into the atmosphere.
Freezing rain happens when you have a large warm column of air aloft to melt the precipitation and fall as rain. Near the surface, there is a shallow layer of air below freezing, which is too shallow to cause the rain to re-freeze, therefore the rain drops freeze instantly on surfaces that are below freezing.
Next comes sleet. Most of the air column is below freezing, except for a shallow part, where that allows the precipitation to melt into liquid. The cold air column is different than the freezing rain scenario in that it allows the rain drops to freeze once again and fall as ice pellets.
Snow happens when the entire column of air is at or below freezing.
Graupel is another form of wintry precipitation, but not many people are familiar with it. This forms when snowflakes fall from the clouds and pass through a shallow layer of warm air. This is not enough to melt the snowflake entirely, so it start to melt, and collects water molecules, then moves back into a cold column of air and falls to the ground as tiny snow balls. Graupel looks a lot like Dip n' Dots ice cream to give you a reference of what to look for.
Let's talk about a winter hazard that can catch you off guard. Black ice. This happens when snow piles melt during the day, or when some light freezing drizzle or freezing fog takes place. During the day the surfaces are mostly wet, but once the sun sets, it allows roadways to freeze. Sometimes the road looks to be just wet, but it is a very thin layer of ice and can be dangerously slippery. It is smart to take it easy during these conditions and when you see temperatures near or below freezing with wet roadways, since they can actually be icy.