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Occluded Front
when a cold front overtakes a warm front

A developing cyclone typically has a preceding warm front (the leading edge of a warm moist air mass) and a faster moving cold front (the leading edge of a colder drier air mass wrapping around the storm). North of the warm front is a mass of cooler air that was in place before the storm even entered the region.

As the storm intensifies, the cold front rotates around the storm and catches the warm front. This forms an occluded front, which is the boundary that separates the new cold air mass (to the west) from the older cool air mass already in place north of the warm front. Symbolically, an occluded front is represented by a solid line with alternating triangles and circles pointing the direction the front is moving. On colored weather maps, an occluded front is drawn with a solid purple line.

Changes in temperature, dew point temperature, and wind direction can occur with the passage of an occluded front. In the map below, temperatures ahead (east of) the front were reported in the low 40's while temperatures behind (west of) the front were in the 20's and 30's. The lower dew point temperatures behind the front indicate the presence of drier air.

[Image: occluded front on weather map (26K)]
Image by: WXP Purdue

A noticeable wind shift also occurred across the occluded front. East of the front, winds were reported from the east-southeast while behind the front, winds were from the west-southwest. Common characteristics associated with occluded fronts have been listed in the table below.

Before Passing While Passing After Passing
Winds southeast-south variable west to northwest
Cold Type
Warm Type



Pressure usually falling low point usually rising
Clouds in order: Ci, Cs, As, Ns Ns, sometimes Tcu and Cb Ns, As or scattered Cu
Precipitation light, moderate or heavy precipitation light, moderate or heavy continuous precipitation or showers light-to-moderate precipitation followed by general clearing
Visibility poor in precipitation poor in precipitation improving
Dew Point steady usually slight drop, especially if cold-occluded slight drop, although may rise a bit if warm-occluded
Table adapted from: Ahrens, (1994)

Warm Front
Terms for using data resources. CD-ROM available.
Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.

dry line