Snow and sleet are still possible when surface temperatures are above freezing
GREENSBORO, N.C. -- During the snow storm that hit the Triad on February 12-13, 2014 everything from snow, sleet and freezing rain fell during the event. A WFMY News 2 and 2 Wants To Know viewer asked the question, "...How is this possible when temperatures remained below freezing?"
To understand how this is possible you must first understand how each of these three types of precipitation form. Snow only forms in conditions that are below freezing. In order for us to get snow at the surface that snowflake must remain in a sub-freezing atmosphere for most of it's decent to the ground. Once the snowflake enters a section of the atmosphere that is above freezing that snowflake will begin to melt. If the snowflake is in that layer of warm air too long the flake will turn into a drop of water.
It is at this point when the other two types of winter precipitation enter the picture. It is possible for a warm layer in the atmosphere to be elevated enough to where that melted snowflake (now rain) can fall into another sub-freezing layer of the atmosphere and re-freeze. When this water droplet refreezes it does not turn back into a snowflake, but rather into an ice pellet (sleet).
Now, if that sub-freezing layer of the atmosphere is not thick enough or cold enough to re-freeze that rain drop into an ice pellet it will fall all the way through the atmosphere as a liquid and hit the surface as rain. Once this rain hits the surface, the sub-freezing temperatures will freeze the water after it comes in contact with the ground (or trees, street signs, etc...). This is called freezing rain. The continual accumulation of water and then subsequent freezing on contact ultimately leads to a coating of ice on all surfaces that are below freezing.
Chief Meteorologist Grant Gilmore gives a visual demonstration in the attached video of how these types of precipitation form in our atmosphere. If you have a weather question for Grant you can send him an email at firstname.lastname@example.org