Winter is always a tricky time of the year for forecasters across the country. Freezing rain, sleet, and snow are all types of winter precipitation that we can experience here during the winter season which can cause quite a headache for meteorologists. One small change in the atmosphere can knock down snow totals drastically or it can exceed what was expected. These “perfect” conditions must not only exist at the surface, but also in the space all the way up to the cloud. Jody James, Warning Coordination Meteorologist at the National Weather Service in Lubbock explains.
“A lot of people think just because you get down below freezing you’ll get snow and really there’s something that we look for when we’re looking at how cold it is all the way up in the cloud. Temperatures have to be down to around 10 degrees Fahrenheit before you start to get those snowflakes to develop in the cloud. If it’s warmer than that, even though it may be very cold outside, you can get rain or drizzle, or freezing drizzle.”
This is caused by a “warm layer” in the atmosphere where the temperatures are above freezing.
“So when we as meteorologist are looking at a forecast, we’re not only looking at how cold it is outside, but we’re looking at that temperature as you come all the way down from two to three miles up, or fifteen thousand feet and what happens to that snowflake as it develops in the cloud.“
Obviously, if the atmosphere is above freezing from the cloud to the ground, you will get liquid rain. However, when it comes to wintry precipitation, the warm layer’s size can make a big impact on what will land at the surface.
Freezing rain starts as snow in the cloud and goes through a relatively large warm layer, allowing it to melt and become completely liquid. However, as it approaches the surface it goes through another layer where temperatures are below freezing. Therefore when it lands, the drop is supercooled and freezes on impact. This type of winter precipitation is one of the main ways black ice forms on the roadways and is also a culprit for many power outages across the nation during the winter months.
Sleet also starts out as snow and it also goes through a warm layer in the atmosphere, but this warm layer is much smaller. Therefore, the snow only partially melts and then it refreezes. This allows the soft texture of snow to stay relatively intact making this precipitation type a bit of a nuisance but much less dangerous than freezing rain.
And finally snow stays in its initial state when there is no warm layer in the atmosphere.