When meteorologists forecast a winter storm one of the important predictions they make is the type, or types, of precipitation that are likely to fall. Will freezing rain cause an ice storm? Will ice pellets, called sleet, leave the ground covered in slush? Will it rain or snow?
To answer these questions, they look at the temperature of air masses in the storm. When a warm air mass is pushed above a cold air mass with freezing temperatures, various types of icy precipitation can form. This type of collision of air masses is called a warm front. If the cold air mass is thick, raindrops will freeze into ice pellets called sleet. Sleet pellets are smaller than snowflakes and do not have crystal shapes. If the cold air mass is thin, the rain will be super-cooled on its way down allowing it to freeze instantly when it gets to the ground leaving a layer of ice. This super-cooled rain is called freezing rain, and the resulting icy conditions are often called an ice storm.
If the cold air mass near the ground moves out of the way, then the rain will not freeze as it falls. If the warm air mass moves out of the way and it is cold between the storm clouds and the ground, then snowflakes will fall.
A single winter storm can produce several different types of precipitation. Rain, snow, and other icy precipitation, such as sleet, graupel, and hail, are all possible as the air masses change and move during the storm.