When the air temperature hovers right around the freezing point during the winter, several types of precipitation may occur during the same day -- rain, sleet, snow, and freezing rain (rain that freezes upon contact with ground surfaces).
In this scenario, the initial precipitation of snow was a result of cloud and air temperature of 32 degrees F or less (freezing point). Ice crystals are formed in the clouds and eventually become large enough to fall to earth as snowflakes. The air temperature between the clouds and the ground was consistently below the freezing point, so the precipitation remained as snow as it fell. By noon, the air temperature from the ground upward had risen above the freezing point. The snow began to turn to rain as it fell through the air that was above 32 degrees F.
Later in the afternoon, the air temperature close to the ground had once again dipped below freezing, but the temperature higher in the atmosphere remained above 32 degrees. At this point, the precipitation (that started falling as rain) began to freeze as it approached the ground and became sleet. Still later, the air temperature between the ground and the clouds once again dipped below freezing, and the precipitation again became snow. By evening, the air temperature from the ground upward again rose just above the freezing point, resulting in rain. However, the temperature of certain surfaces (roadways and bridges) had fallen below the freezing point and the rain froze as it contacted these surfaces.
Hope this helps!
Edited by Chris M. on 12/29/2009 at 1:34 AM EST