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Wednesday, 04 January 2006 00:00

A wintry mix

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Christmas Day brought showers interrupted by buckets full of sleet. Sometime late Christmas night or before dawn the next morning, a dusting of snow covered the lawn. Winter precipitation is often a mix in the eastern United States. Sleet and freezing rain are almost exclusively eastern phenomena.

Sleet and/or freezing rain may fall across the eastern U.S. and Canada from October through May. They occur when storm systems laden with warm moist air from the Gulf of Mexico track northeasterly and collide with cold Arctic air. According to one weather website, the typical ice storm is about 30 miles wide and 300 miles long.

These storms are triggered when the warm moist air (usually out of the southwest) overruns cold air already in place. The warm air rises creating a temperature inversion – warm air above, cold air below. As the warm air rises it cools and the moisture in it begins to condense. This condensation creates ice crystals (snowflakes) that fall.

If all the warm air has cooled sufficiently to below 32 degrees Fahrenheit, the precipitation falls as snow, but often the lower portion of the warm air mass is still above freezing. As the snow falls through this warmer air it melts and falls as rain.

Now, remember our inversion. This warm air is still above a layer of cold surface air. As the rain hits this cold air it changes once again. If the layer of cold air is thick enough and/or cold enough the rain will freeze into pellets and fall as sleet.

Under certain conditions the rain droplets will become supercooled. This means the temperature of the water in the raindrops is 32ºF or colder, but the drops are not frozen. We know these supercooled droplets of water as freezing rain.

A variety of conditions can cause supercooling. Water doesn’t instantaneously turn into ice at 32 degrees. If the cold layer of air at the surface is too thin the droplets may not have had time to freeze although the water is below 32. The size and purity of the water droplets are also factors. Larger drops take longer to freeze and droplets of pure water may reach temperatures below zero without freezing. When these supercooled raindrops strike a frozen surface like the ground, roads, power lines and/or tree branches they freeze instantly, creating the havoc we know as ice storms.

It’s pretty cut and dry on paper to isolate snow, sleet and freezing rain and define them. Nature, on the other hand, is never cut and dry. That’s why that term “wintry mix” so often creeps into our forecast this time of year. Incremental differences in air temperature and/or droplet sizes can mean the difference between snow, sleet, rain and freezing rain, and it’s common to get a mix of all of this falling simultaneously.

From a meteorologist’s perspective winter precipitation is broadly defined as:

Snow — Ice crystals in complex hexagonal forms. Snow is formed by a process known as sublimation. Water vapor turns directly into ice without passing through a liquid state.

Snowflakes — A combination of snow crystals.

Snow pellets — Ice crystals that fall through supercooled water droplets to form a frozen lumpy mass.

Sleet — Water droplets that freeze into ice as they fall.

Freezing rain — Supercooled water droplets that instantly turn into ice on contact.

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