NASHVILLE, Tenn. (WKRN) — Winter storms are powerful and can produce all types of precipitation. The most common is snow, but at times it also produces ice that can cause major travel problems and power outages. We’re talking about freezing rain and sleet.
The science of wintry precipitation
If the precipitation is just rain or just snow, the atmosphere is either greater than or less than 32°. When you add in a wintry mix — freezing rain or sleet — then one must evaluate the temperature at different levels of the atmosphere. Layers of warm (>32°) and cold air (<32°) help determine the type of precipitation that reaches the ground.
Snow
Snow is the coldest of winter weather precipitation. This is an accumulation of ice crystals that cling together as they fall to the ground. Snow continues to fall when the temperature remains at or below 32 degrees Fahrenheit from the base of the cloud to the ground.
Rain
As the initial precipitation falls as a solid because temperatures are below 32° at the cloud level, temperatures warm above freezing to the ground. This changes the state of the precipitation from a solid and ice to a liquid and water.
Freezing Rain
According to the National Severe Storms Laboratory (NSSL), freezing rain occurs when snowflakes fall into a warmer layer of air and melt into rain. This is the worst type of precipitation with the greatest impact on travel, property, and lives.
Then, the water hits a second layer of freezing air as it falls, it becomes cold again, but not enough to make it freeze. It freezes on contact as long as the surface the droplet hits is below 32 degrees Fahrenheit.
This freezing of water makes a glaze of ice leaving surfaces extremely slick and dangerous to walk or drive on. The water can also freeze around trees, power lines, and other surfaces. A significant accumulation of freezing rain lasting several hours is called an ice storm.
Sleet
Sleet happens when snowflakes only partly melt when they enter a warm layer of air as they fall through the atmosphere to the ground. Think of falling sleet as droplets of slush.
When the slush droplets hit the second layer of freezing air, they freeze as they fall. When they hit the ground, they are considered frozen raindrops that bounce on impact.
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