The depth of this frontal zone associated with a cP air mass typically extends to altitudes as high as five kilometers, so there can be fronts in middle troposphere (and on occasion, in the upper troposphere). All along the upward slant of the cold wedge, cold air abuts with warmer air, creating an upward-slanting boundary characterized by large temperature contrasts. Then the upward slant relaxes into a much more gentle slope (e.g. The cold front is steepest in the lowest several hundred meters of the atmosphere, with a slope of about 1/100, meaning that elevation increases about 1 kilometer for every 100 kilometers of horizontal distance from the surface front. To see what I mean, focus your attention on the cross-sectional profile of an advancing continental-Polar air mass below. Therefore, the narrow frontal zone that separates the two contrasting air masses must extend upward from the surface. They are three-dimensional blobs of air, so when cold air advances at the surface, cold air at higher altitudes also advances on warm air. However, if cold air advances at a speed less than 5 knots (about 5 miles an hour), forecasters classify the front as "stationary" by convention.īut, air masses aren't just two-dimensional. As long as the surface wind on the cold side of a front is blowing at least somewhat toward the front, cold air advances and the forecasters classify the front as cold. But, now it's time to look more closely at cold fronts so that we can better understand their other weather impacts.įor starters, what determines whether cold air advances, retreats or just holds its ground? To answer this question, weather forecasters always look at the winds on the cold side of a front. As a result, temperatures and dew points often decrease after a cold front passes (as colder, drier air arrives at your location). Cold fronts, marked by a chain of blue triangles pointing in the direction of movement (toward the warmer air), often mark the boundary between a maritime-Tropical (mT) and an advancing continental-Polar (cP) air mass or perhaps the boundary between a cP air mass and an advancing continental-Arctic (cA) air mass (the coldest of the cold). You already studied the basics of cold fronts in a previous lesson, primarily the idea that a cold front is the leading edge of an advancing cold air mass. As you just learned, cold fronts form as a natural consequence of the circulation of mid-latitude cyclones the circulation causes a cold air mass to advance on the west (and eventually south) side of the low (in the Northern Hemisphere).
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