Winter storms in the United states play a game with temperatures that determines what falls from the sky.
We all know that when the temperature is below 32 degrees, we have snow, and above 32 degrees, we have rain. Well this is only in some cases, and the temperatures within the layers of the atmosphere have a great deal to do with what will become of precipitation at the ground.
All precipitation starts as snow in the cloud. As it descends through the atmosphere, it remains as snow as long as the temperature is below 32 degrees:
Temperature at the surface is 25°, and the entire layer above it is below freezing.
The second case of precipitation: there is a layer of warm air above the ground, and a large freezing layer near the ground. This will partially melt the snowflake into a near raindrop, and re-freeze it into an ice-pellet (or sleet) as it falls to the ground:
Temperature at the surface is 25°, but above it there is a layer that is above freezing.
The third case of precipitation: a deep layer of warm air exists with a shallow layer of cold air at the surface. This layer completely melts the snowflake into a raindrop. This raindrop then lands on a frozen ground, and freezes to that surface:
A deep above freezing layer above the ground and a cold one at the surface.
NOTE: These are the three main types of precipitation, and the ones that are often seen with winter weather storms. Depending on how large the warm layer is and the cold layer at the surface, we can get snow, sleet, freezing rain, ice needles, graupel, and everything inbetween! (Oh yes, hail is a summertime phenomena!)
***The most dangerous of these is freezing rain. This sticks to power lines and trees, causing power outages and damage to homes. Prolonged periods of freezing rain are most feared by meteorologists. Although the ice build up makes for some great pictures!
-Meteorologist James Gilbert
Images Thanks to http://www.srh.noaa.gov/