Weather map showing an occluded front
Click on image for full size
Windows to the Universe image by Lisa Gardiner
Sometimes a cold front follows right behind a warm front. A warm air mass pushes into a colder air mass (the warm front) and then another cold air mass pushes into the warm air mass (the cold front). Because cold fronts move faster, the cold front is likely to overtake the warm front. This is known as an occluded front.
At an occluded front, the cold air mass from the cold front meets the cool air that was ahead of the warm front. The warm air rises as these air masses come together. Occluded fronts usually form around areas of low atmospheric pressure.
There is often precipitation along an occluded front from cumulonimbus or nimbostratus clouds. Wind changes direction as the front passes and the temperature changes too. The temperature may warm or cool. After the front passes, the sky is usually clearer and the air is drier.
On a weather map, like the one on the left, an occluded front looks like a purple line with half triangles and half semicircles along it pointing in the direction that the front is moving. It ends at a low pressure area shown with a large ‘L’ on the map, and at the other end connects to cold and warm fronts.
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