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Weather and Occlusions
Name: Chris
Status: student
Age: 14
Location: N/A
Country: N/A
Date: 5/10/2004
Question:
What type of weather do you have with an occlusion?
Replies:
Chris,
As you may know, an occlusion occurs when three separate air masses come together in the same place. Normally two cold air masses lie under a warm air mass, producing stable atmospheric conditions. Occlusions normally form between the center of low pressure and where a cold front and warm front meet. The boundary between the air masses is usually shown as an occluded "front" (a line without barbs), although the horizontal extent of the area affected by the occluded "front" is generally much greater than the width of the line drawn on the weather map.
There are two kinds of occlusions, a warm occlusion and a cold occlusion.
In the cold variety the coldest air mass pushes under the cool air mass, with a warm air mass above both. This is very similar to a cold front, except in this case it is capped by warm air. Therefore, you can expect to see the same kind of stratus clouds and precipitation associated with a cold front, but you are less likely to see embedded thunderstorms develop because of the capping warm air.
In the cool variety the cooler air mass is riding up and over the colder air mass, but again with the warm air mass above both. This is more like a weak warm front situation, so you are likely to see altocumulus as well as stratus clouds, at greater heights than occur in a cold front situation. Thunderstorm formation may be more likely.
Unlike stationary fronts, occluded fronts normally move, although relatively slowly when compared to a cold front.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012
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