When winter weather arrives, it’s quite common to see a transition from rain to freezing rain, to sleet, and then snow. But how does this happen?
In the image above, you can see the differences between each one. You can see how the warm air cuts above the cold air. Since warm air is less dense, it rises, which is what creates these temperature inversions in the atmosphere, which is what gives us this mixed precipitation.
Freezing Rain
Freezing rain occurs when snow falls, melts into rain, and is able to freeze on contact with the surface. They are supercooled before reaching the ground, which why when they hit the ground, they freeze to anything that is below freezing. Freezing rain, no matter the accumulation total, is a big threat to power lines and traffic. Warnings may be issued if accumulations are expected for over an hour.
Sleet
Sleet is the less dangerous form of freezing rain. Freezing rain often transitions to sleet. This happens when the snow melts higher in the atmosphere, and is able to freeze before reaching the surface. It is often the precursor to snow. Some sleet may still have snowflakes in them if the warm pocket is small enough. Sleet can still pose a hazard if it melts and then refreezes, but it does not freeze on contact with the surface.
Snow
Snow occurs when there is no warm air to melt it. Sleet transitions to snow. It falls without any temperatures above 32 degrees Fahrenheit. This can be quite dangerous when it accumulates, and can blow around unlike sleet and freezing rain. If a system is expected to drop 5 inches of snow, a National Weather Service may consider the travel impacts, and may give out advisories, watches, or warnings for the upcoming system.
Wrap it Up
To wrap it all up, freezing rain is when snow melts on its way down to the surface, and super-cools before reaching the surface. Once it reaches the surface, it freezes on contact with any object at or below the freezing point of 32 degrees. Freezing rain can then transition to sleet, which is when the rain freezes before it makes contact with the surface. That sleet may then become snow when the warm air has been pushed out, preventing the snow from melting on its way to the surface. Each type of precipitation can present a hazard to everyday life, so make sure to stay informed when an upcoming winter weather system is announced.
Sources
https://www.weather.gov/iwx/sleetvsfreezingrain
