Download Occluded Front: when a cold front overtakes a warm front

Occluded Front: when a cold front overtakes a warm front
12/12/14 12:42 PM
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.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/frnts/ofdef.rxml
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Occluded Front: when a cold front overtakes a warm front
12/12/14 12:42 PM
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 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.
Winds
Temperature
Cold Type
Warm Type
Pressure
Clouds
Precipitation
Visibility
Dew Point
Before
Passing
southeastsouth
cold-cool
cold
usually falling
in order: Ci,
Cs, As, Ns
light,
moderate or
heavy
precipitation
poor in
precipitation
steady
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/frnts/ofdef.rxml
While Passing
After Passing
variable
west to northwest
dropping
rising
low point
Ns, sometimes Tcu and
Cb
colder
milder
usually rising
Ns, As or scattered Cu
light, moderate or heavy
continuous precipitation
or showers
light-to-moderate
precipitation followed
by general clearing
poor in precipitation
improving
usually slight drop,
especially if cold-
slight drop, although
may rise a bit if warmPage 2 of 3
Occluded Front: when a cold front overtakes a warm front
12/12/14 12:42 PM
occluded
occluded
Table adapted from: Ahrens, (1994)
Terms for using data resources. CD-ROM available.
Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
Warm Front
the University of Illinois at Urbana-Champaign.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/frnts/ofdef.rxml
dry line
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