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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 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 |
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| 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 |
Warm Front |
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dry line |