Weather Wonders: Winter Precipitation Types
Discovery Place Nature
It’s winter in the Carolinas! During this time of year, maybe more than any other, forecasts can be tricky mainly because of precipitation types and totals.
There are many variables that Elisa Raffa, a Discovery Place experience specialist and meteorologist in Charlotte, uses to try to figure out whether the future brings snow, sleet, rain or a mixture. Trying to solve this puzzle is one of the most interesting challenges of her work!
One tricky determining factor in how much snow we get is temperature. We don’t just mean the temperature around our heads at the surface; we also need to know the temperature upstairs in the atmosphere.
Let’s think of the atmosphere as a tall column of air, and sometimes we get a layer of warm air to stack on top of the cold air. Meteorologists call it a “warm nose” because it looks like a pointy nose!
If the air is warm through the whole column, any precipitation that falls will be rain. If there is a thin layer of cold air at the surface and all warm air on top of that, the precipitation will fall as rain until it reaches the ground.
With temperatures cold on the ground, the rain could freeze on contact and become freezing rain! Freezing rain looks like a glaze of ice, often freezing on tree branches and weighing them down.
Sleet occurs when the warm air is a very small layer at the top, with a much bigger layer of freezing air at the bottom. This allows precipitation to initially fall as rain but freeze in the middle of the air. Sleet looks like clear little balls of ice that bounce and pitter-patter on the ground.
Finally, when the whole column of air is cold, precipitation will fall as snow. To make matters trickier, even if you get temperatures cold enough for snow, temperature and moisture still play a role in what type of snow.
If temperatures are just flirting with freezing, the cold air will be filled with a lot of moisture. Warmer air holds more moisture.
What’s this mean? A snow day at 34 degrees will have a lot of moisture and be very wet, compact snow. It will compact onto the ground as it hits and maybe not stack up as much.
Typically, we get snow with temperatures more comfortably below freezing. In that more typical set-up, the temperature and snow ratio gives 10” of snow for every 1” of water!
Sometimes, we can get very cold Arctic air to generate snow. Since the air is so cold, it doesn’t have as much moisture, so it’s a very dry, fluffy, powdery snow.
This kind of snow doesn’t stick or compact as well, but it stacks high. This could double our snow amount on the ground for the same amount of water in the atmosphere!
Winter forecasting can give meteorologists major headaches! Temperature and moisture depend heavily on the track of the storm.
As we just learned, temperature and moisture are the prime suspects in determining precipitation type and totals. So just a small jog in a storm track can mean big changes in precipitation type and totals.
Charlotte forecasts often get busted because it’s too warm for snow. Warmer temperatures also mean more of an icy mix and less snow.
However, just a 2-degree difference can mean a difference between sleet or freezing rain. So be patient with winter forecasts and always check for updates.