With all the freezing rain and sleet occurring in the Midwest and Southern Plains, I am wondering...How does freezing rain really happen? Why doesn't it just snow? And how is freezing rain different from sleet and hail?
While often beautiful (just look at the rose image to the right) freezing rain can be deadly. (The Midwest often gets devastating storms of this nature.) The sparkle of shimmering ice coating almost any surface makes freezing rain one of the most deadly types of winter storms. With accumulations of several centimeters, roads can be dangerously slick while trees and power lines cannot withstand the heavy weight of the extra added ice.
How Does Freezing Rain Form?
Freezing Rain happens when precipitation falls in liquid form as raindrops and immediately freezes as it hits most any surface. This can occur as rain or snow falls through a deep layer of warm air aloft. This warmer layer is usually 2,000 to 3,000 feet above the sub-freezing layer. This turns the snow to rain or keeps the rain in liquid form. As the rain falls, it passes through a very cold, but shallow layer of cold air at the surface and once it hits the ground, trees, cars or other items it freezes and forms a coating or glaze over the surfaces.
It is important to note that freezing rain is in liquid form until it strikes a cold surface. The water droplets are supercooled and freeze on contact. Just like Goldilocks and the Three Bears, conditions must be just right. If the cold layer near the ground is too deep, the rain will freeze again and fall as ice pellets or sleet. If the cold layer is too shallow, it will simply rain.
How Does Freezing Rain Differ From Sleet?
Sleet forms when a raindrop or a snowflake partially melts as it falls through a layer of warm air higher in the atmosphere and turns back into ice as it falls through a deep layer of cold air at the surface. This deep layer of sub-freezing air is usually 3,000 to 4,000 feet thick. Sleet is usually tiny clear ice pellets that bounce when they hit the ground. Sleet is sometimes confused with hail.
Why Doesn't It Just Snow?
Snow begins in the atmosphere as water condenses into a tiny droplet. As more and more water vapor condenses onto its surface, the droplet grows. Cold air then freezes this water into an ice crystal. As long as the air temperature remains at or below freezing the snow will reach the ground.
The Bottom Line on Winter Precipitation
- Snow forms when the entire layer of air is sub-freezing.
- Sleet forms when the layer of sub-freezing air is fairly deep, 3,000 to 4,000 feet.
- Freezing rain forms when the sub-freezing layer is very shallow.