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A warm occlusion is the occlusion that forms when the overtaking cold front is lifted by overrunning the colder retreating air asso-ciated with the warm front. This is shown in figure 4-5-4.

The warm occlusion usually develops in the Northern Hemisphere when conditions north and ahead of the warm front are such that low temperatures exist north of the warm front. This usually occurs along the west coasts of continents when a relatively cool maritime cold front overtakes a warm front associated with a very cold continental air mass of high pressure situated over the western portion of the continent. The cold front then continues as an upper cold front above the warm front surface. The occlusion is represented as a continuation of the warm front. The cold front aloft is usually represented on all surface charts.

Figure 4-5-4.—Vertical cross section of a warm type of occlusion.

Figure 4-5-5 depicts atypical warm type of oc-clusion in both the vertical and horizontal.

Surface Characteristics

The warm type of occlusion has the same type of pressure pattern as the cold type of occlusion. The most reliable identifying characteristics of the upper front are as follows:

1. A line of marked cold frontal precipitation and clouds ahead of the occluded front

2. A slight but distinct pressure trough

3. A line of pressure-tendency discontinuities

NOTE: The pressure tendency shows a steady fall ahead of the upper cold front and, with passage, a leveling off for a short period of time. Another slight fall is evident with the approach of the surface position of the occlusion. After passage the pressure shows a steady rise.

The pressure trough is often more distinct with the upper front than with the surface front.


The weather associated with warm front oc-clusions has the characteristics of both warm and cold fronts. The sequence of clouds ahead of the occlusion is similar to the sequence of clouds ahead of a warm front; the cold front weather occurs near the upper cold front. If either the warm or cool air that is lifted is moist and unstable, showers and sometimes thunderstorms may develop. The intensity of the weather along the upper front decreases with distance from the apex. Weather conditions change rapidly in occlusions and are usually most severe during the initial stages. However, when the warm air is lifted to higher and higher altitudes, the weather activity diminishes. When showers and thunderstorms oc-cur, they are found just ahead and with the up-per cold front. Normally, there is clearing weather after passage of the upper front, but this is not always the case.

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