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SLEET AND FREEZING RAIN
Winter storms normally bring snow to much of Inland Northwest. But sometimes,
periods of freezing rain and sleet can precede or accompany the snow as well,
especially across the Columbia Basin and the Palouse. All extratropical storms
are a mixture of warm and cold air, a clash of subtropical and polar air streams.
In parts of the storm, especially ahead of a warm front, warm air is flowing
over cold air near the ground. The result is a wedge of air that's above freezing
between a layer of cold air at the surface and a layer of cold air aloft.
Precipitation usually begins
as snow at cloud level and melts into rain as it falls through the warm layer
of air below. The type of precipitation is dependent on the depth and temperature
of the cold air near the surface. In places where the warm air extends all the
way to the to the ground, the precipitation will fall as rain. If a shallow
layer of cold air at the surface is near freezing, the falling rain cools to
near freezing but does not turn into ice until it hits something - this is freezing
rain. When a layer of sub- freezing air is a little thicker, the falling rain
freezes into ice pellets - generally called sleet. In places where there is
no layer of warm air, the snow falls all of the way to the ground. Often, rain,
freezing rain, sleet and snow can fall on the same places through the duration
of a storm.
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