Thursday, November 20, 2008

Winter Weather Awareness Week - part 4

...Winter weather awareness in the Mid-South...

Today's topic is winter precipitation types.

Snow -- snow forms in the clouds and remains as snow all the way to the ground. It most commonly takes the form of snowflakes...which are the familiar six-sided ice crystals. It may also fall in the form of snow pellets or snow grains.

Snow flurries are most commonly seen as a few snowflakes falling...although visibilities can be reduced at times. In the Mid-South...the term snow flurries is used to indicate that no accumulation is expected.

Snow showers is a term not often used in the Mid-South. In this type of precipitation...the snow falls at varying intensities over brief periods of time. Accumulation may occur...especially during moderate to heavy snow showers. Blowing snow most commonly refers to snow that is already on the ground and is lifted into the air by the wind.

In the Mid-South...heavier snows most commonly occur when cold air is already in place over the region and a strong upper level low pressure system moves out of the southwestern United States. The low serves to pull moist air northward into the cold air. Light snow or snow flurries can also occur in the cold air that follows the passage of an Arctic cold front.

Sleet - sleet consists of pellets of ice. In fact...for people who have trouble with the difference between sleet and freezing may be easier to associate sleet with its technical name...which is ice pellets. For sleet to form...snow begins falling from the clouds but then goes through a layer of above-freezing air thousands of feet above the ground. This causes the snow to change to rain. Then...the rain goes through a layer of below-freezing air...usually at least two to three thousand feet thick...and the precipitation turns into pellets of ice.

Sleet typically occurs in a fairly narrow band. This band usually moves...but at times may remain nearly stationary...resulting in accumulations of sleet. In the Mid-South...sleet most commonly occurs in a narrow band between an area of rain to the south and an area of snow to the north.

Freezing rain -- this weather phenomenon is sometimes called glaze...because of the glaze of ice it puts on surfaces at the ground. Freezing rain most commonly occurs when precipitation falls from the clouds as snow...then goes through an above-freezing layer...which turns the precipitation to rain. Then...the rain reaches the ground where temperatures are below freezing. The rain then freezes as it hits exposed objects. In the worst cases...everything becomes coated with a layer of ice.

In the Mid-South...freezing rain commonly occurs as an Arctic high pressure system begins to move away from the state. In this situation...cold air is still lingering at the ground...but warmer southerly winds from the Gulf of Mexico begin bringing moisture back over the top of the cold air. Since the air at the ground has not warmed above freezing...the rain that falls freezes on the ground and other objects. Freezing rain...and its cousin freezing drizzle...often develop during the late night hours...creating icy conditions for morning rush hour.

Freezing fog -- while this is not precipitation falling from the is another winter weather hazard. Freezing fog typically develops on clear...calm nights when temperatures are below freezing. Fog forms and freezes...usually on bridges...overpasses...and other elevated roadways. It can create quite a surprise for motorists...due to the presence of clear skies overhead.

Frost -- frost describes the formation of thin ice crystals on the ground or other surfaces in the form of scales...needles...feathers...or fans. Frost forms when water vapor in the air turns directly to ice crystals on an object. The temperature of the object must be below freezing for frost to occur. However...frost is sometimes seen on the ground when official temperatures are reported to be above freezing. This is because the official temperature is taken about five feet above the ground...where the air can be a few degrees warmer than the temperature at ground level.

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