Sleet and freezing rain are other winter precipitation you need to worry about

Think of the atmosphere as a great column. If this column of air is below freezing from top to bottom, the result will be snow.

If there is hot air at the top and subzero temperatures towards the bottom of the column, you will have sleet. This is because the hot air high in the atmosphere will produce a raindrop. When the raindrop falls and enters the air below the freezing point, it will freeze before it hits the surface. Since sleet is essentially a frozen raindrop, you can actually hear the sleet as it hits the surface.

Finally, if the entire air column is above the freezing point to the surface, the raindrop will freeze on contact. This is called freezing rain and it is the most dangerous type of precipitation.

“Some of the most disastrous winter storms are mainly due to freezing rain,” says the National Weather Service.

Freezing rain freezes anything it comes in contact with, causing dangerous icing on roads, trees, power lines and your vehicle. The weight of the ice can knock down trees and power lines, causing power outages in extremely cold weather.

Ice on roads turns pavements into skating rinks and can cause major traffic accidents and interstate congestion.

It takes less than a quarter of an inch of ice to make the trip unsafe. Tree branches begin to break with a buildup of a quarter to a half inch of ice. And when more than half an inch of ice accumulates, there will be extensive tree damage and major power outages.

The importance of being right

Deciding where the rain ends and where the snow begins can make all the difference in where to use de-icing machines, aircraft de-icing, school cancellations and more. In addition, it is extremely important to know which areas will receive ice or freezing rain.

The rain-snow line is a term that is often heard when a winter storm is forecast. The term simply defines the point at which rain turns into snow. In mountainous regions, it takes on a vertical connotation and is defined by the altitude at which rain turns into snow.

If the rain-snow line is poorly forecast, it could mean that a city expects to receive all the rain from a storm and ends up receiving all the snow. It could have devastating consequences if a city does not forecast snow and has not treated roads, canceled schools, or warned its residents.

The same could also be true if a city expects to receive all the snow and ends up receiving all the rain. They would have wasted countless taxpayer dollars treating roads and shutting down businesses in the making, and then nothing happens.

Most southerners remember a “snowy day” when no snow fell. Key decisions such as school closures need to be made overnight, when winter precipitation is expected to start in the morning hours. If the temperature is slightly warmer than expected, that winter precipitation is falling as rain and you have an incorrect forecast.

For example, knowing the elevation of the rain-snow line in Colorado is critical to whether some of the mountain passes along Interstate 70 will be able to remain open. In New York, it is crucial whether the rain-snow line will reach the I-95 corridor or stay west.

Why are bridges the first to freeze?

In winter, bridges and viaducts are the first to freeze. This is because they have air on all sides, so they will cool faster than surface roads. Surface roads are naturally more isolated because the cold air is only above the surface, while the ground below keeps the pavement warmer.

Then you have what’s called a winter mix. It’s basically a bit of everything. When you are near the rain-snow line, the rain can change to sleet over time and then change to snow. Or the snow will turn to rain.

Forecasters have the important task of not only identifying the type of precipitation that will fall, but also determining when one type of winter precipitation will change to another.

Knowing what time the rain will turn into snow is essential. Will this happen before or after the evening commute?

It’s difficult, without a doubt, and not always perfect. But knowing what will fall and when is the key to staying prepared in winter.