Published: 2008-01-08 12:27:57
Updated: 2008-01-08 12:27:57
Posted January 8, 2008
By Diane Nichols
MIKE MOSS SAYS: Diane, The difference in the two is when the precipitation exists as a liquid and when it freezes. In both cases, the precipitation usually begins as snow aloft that falls though a warmer layer below that is sufficiently deep to melt the snow into raindrops. If the air below that level remains above freezing all the way to the surface, only rain will occur at the ground.
Sometimes, however, there is another layer of subfreezing air at lower altitudes that the raindrops fall through. If that layer is deep enough, the rain may freeze into more or less spherical, mainly clear pellets of ice, in which case they are referred to as sleet. Sleet tends to make kind of a unique tapping sound as it lands, and you can usually see the pellets bounce off of hard surfaces.
If the raindrops fall into a layer of subfreezing air near the surface that is quite shallow, they may become "supercooled," with their temperature below freezing but the drops not yet frozen into solid form. If these droplets then land on ground, decks, tree limbs, power lines and so on that have a temperature below freezing, they may spread into a clear layer and then freeze, producing a coating of clear ice. This is what we call freezing rain, and it can be an especially high impact form of wintry weather, because only a small amount of this clear ice on roadways is extremely slippery. Also, more than a quarter inch or so of freezing rain can weigh down trees and tree limbs in a way that can cause them to lay over or break, impacting power supplies on a scattered basis. If the ice builds up to more than half an inch or so, some power lines begins to break on their own or strain their connections, and power outages may become widespread, in addition to significant damage to trees and large tree branches.