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
Temperature Cold Type Warm Type	cold-cool cold	dropping rising	colder milder
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