The first thought most people have when they think of winter is one thing: snow. Winters are known mostly for the white, frozen precipitation that occurs when all levels of the atmosphere (and the surface) are at the freezing mark (32 Degrees FahreNheit/0 Degrees Celsius). What happens, though, when not all levels of the troposphere, the lowest region of the atmosphere where all of our weather occurs, are below freezing? Let’s explore that idea.
The troposphere is somewhat complex. It is roughly the first 10km of the atmosphere, and is often measured in terms of pressure (millibars). When precipitation occurs, it falls through various levels of the atmosphere, often travelling through different temperatures that alter the formation of whatever precipitation is falling. When the precipitation is rain that means the temperature is above the freezing mark through at least the first couple hundred millibars of the atmosphere. However, it is often not the case that all levels of the atmosphere are above freezing, especially over much of the continental U.S. In the winter, the atmosphere goes through dramatic changes in temperature; there are different air masses that can occupy different levels. When it comes to sleet, there is likely a shallow layer of warm air in between two layers of sub-freezing temperatures. As ice crystals fall into the shallow warm layer, they melt, but only enough to melt the outer layer of the ice. As they pass through the colder layer nearest to the ground, water droplets freeze on an ice nucleus, the remaining center of the ice crystal, thereby creating sleet.
Freezing rain is a different story. Freezing rain occurs when a much deeper warm, layer is wedged above a shallow cold layer near the ground. The warm layer melts ice crystals to the point where they are rain droplets. When they pass through the shallow cold layer near the ground, they freeze on contact with any sub-freezing surface on the ground. This phenomenon occurs closer to the warm front of a cyclonic low pressure system than sleet, as the warm air is able to penetrate deeper into the atmosphere.
Its freezing rain and sleet, not snow, that often cause the most hazards to the general population. Both types of precipitation are known for the slick effect they have on roadways. They can cause black ice, or icy spots on roadways or sidewalks that are hard to spot due to their clear color. This issue is even more of a problem on elevated surfaces such as bridges and overpasses where both sides are surrounded by open air, allowing surfaces to freeze more rapidly.
The Occupational Safety and Health Administration (OSHA) also advises people to be more careful when icy weather hits. Their website lists two steps people can take to greatly reduce their risk of a life-threatening fall: wear rubber over-shoes with good treads and take slower, smaller steps.
The U.S. Department of Transportation has recorded, based on a ten-year average, over 1,800 deaths annually related to snow and icy roads. Drive safely, minimize travel in the dead of winter, and be smart and we can keep that statistic from growing even more.
To learn more about other winter weather topics, click here!
© 2019 Weather Forecaster Jacob Dolinger