With this weekend’s winter storm coming up, we are forecasting all sorts of wintry precipitation. That said, when forecasting whether the precipitation type will be snow, sleet, or freezing rain, the difference is very slight.
Even when the ground temperature is at or below freezing, it is not guaranteed that you will see snow. The only difference between all types of winter precipitation is simply the air above it.
From the cloud, all precipitation starts as snow. As you approach the Earth’s surface, temperatures typically rise, but there are a couple of scenarios in which a segment of air does not follow this pattern.
When rain occurs, it starts as snow and immediately melts into water droplets, but as soon as the air just above the surface cools to 32 degrees Fahrenheit or below, it becomes freezing rain.
Freezing rain and sleet, though they are different in appearance, are very similar in how they are formed. Like freezing rain, sleet starts as snow, then melts and forms. The only difference between the two is the amount of time they spend in the warm sector of air. Sleet has a larger column of air to fall through before it hits the ground, giving it more time to freeze and turn into little ice pellets.
Lastly, you have snow; the complete opposite of rain. While rain has only warm air to fall through, snow does not see any temperatures above freezing. That said, once it falls from the cloud as snow, it simply keeps its form all the way to the ground.
Snow is a lot fluffier than sleet or rain (including freezing rain), meaning that if a forecast switches from snow to any other precipitation, its accumulation amounts will plummet. Snow has an average of a 10:1 ratio of inches of snow to inches of rain, while sleet only has about a 2:1 ratio. For example, for every inch of rain is roughly 10 inches of snow and 5 inches of sleet.
This is the main reason that snow accumulations change as often as they do, and this weekend’s system is no exception. When models were forecasting several feet of snow, it was thought that not only the air at the surface below freezing, but all air above it was too.
Now that this storm is closer to us, we are seeing that warmer air aloft is present and lowering the amount of snow accumulation, all while increasing the total amount of ice.
Take, for instance, early Sunday morning in Roanoke; temperatures at the base of the cloud, in the upper air, and at the surface are all below freezing. This would mean that snow is in the forecast for this time of day.
Later in the day, as the upper air starts to warm slightly, the temperatures at the cloud base and the surface remain below freezing. That means that as the snow falls through the cloud, it melts and then rapidly cools to the point that it either refreezes in the air or becomes freezing rain and will turn to ice on impact.
It only takes the smallest changes in the atmosphere to make the biggest differences here at the surface. That said, we recommend that you stay with your Local Weather Authority for updates on any weather maker that comes into our region!