As we head into the winter season, we’ll surely have a mixed bag of precipitation at some point. So what determines the precipitation type?
Obviously, the most crucial item in forecasting the precipitation type is temperature, more specifically freezing temperatures. We also need to look at the atmosphere from the ground up. We’ll break it down into precipitation types and what conditions are needed for each.
This should be pretty obvious. Temperatures need to be above freezing from the ground up at least 1000ft. Why 1000ft? Well, that’s about how far a snowflake can fall before completely melting.
Snow is a little more complicated. Of course, if the entire atmosphere is below freezing, then the precipitation form will be snow. It can snow when surface temperatures are above freezing. The important part here is how far up is the freezing mark? Again, a snowflake can survive approximately 1000ft before melting. Usually a shallow layer of temperatures above freezing above the ground is permissible.
This is one of the more difficult forms of winter precipitation to get, especially over a wide area. Surface temperatures must be below freezing in this case. We usually like a shallow layer of sub-freezing temperatures. If the layer is too thick sleet ends up being the precipitation type. Temperatures above that shallow layer must be above freezing.
Sleet or ice pellets occur when rain falls into a sub-freezing layer above the ground, which then freezes the rain into little pellets or sleet. The surface temperatures can be either above or below freezing during this precipitation type.