the warm front, it moves up over this colder air in the
form of an upper cold front.
The primary difference between a warm and a cold
type of occlusion is the location of the associated upper
front in relation to the surface front (fig. 4-37). In a
warm type of occlusion, the upper cold front precedes
the surface-occluded front by as much as 200 miles. In
the cold type of occlusion the upper warm front follows
the surface-occluded front by 20 to 50 miles.
Since the occluded front is a combination of a cold
front and a warm front, the resulting weather is that of
the cold fronts narrow band of violent weather and the
warm fronts widespread area of cloudiness and
precipitation occurring in combination along the
occluded front. The most violent weather occurs at the
apex or tip of the occlusion. The apex is the point on the
wave where the cold front and warm front meet to start
the occlusion process.
A cold occlusion is the occlusion that forms when a
cold front lifts the warm front and the air mass
preceding the front (fig. 4-38). The vertical and
horizontal depiction of the cold occlusion is shown in
figure 4-39. Cold occlusions are more frequent than
warm occlusions. The lifting of the warm front as it is
underrun by the cold front implies existence of an upper
warm front to the rear of the cold occlusion; actually
such a warm front aloft is rarely discernible and is
seldom delineated on a surface chart.
Most fronts approaching the Pacific coast of North
America from the west are cold occlusions. In winter
these fronts usually encounter a shallow layer of
surface air near the coastline (from about Oregon
northward) that is colder than the leading edge of cold
air to the rear of the occlusion. As the occluded front
nears this wedge of cold air, the occlusion is forced
aloft and soon is no longer discernible on a surface
chart. The usual practice in these cases is to continue to
designate the cold occlusion as though it were a surface
front because of the shallowness of the layer over which
it rides. As the occlusion crosses over the mountains, it
eventually shows up again on a surface analysis.
The passage of the cold type of occlusion over the
coastal layer of colder air presents a difficult problem of
analysis in that no surface wind shift ordinarily occurs
at the exact time of passage. However, a line of stations
UPPER WARM FRONT
UPPER COLD FRONT
A. COLD TYPE OF OCCLUSION
B. WARM TYPE OF OCCLUSION
Figure 4-37.Sketch of occlusions (in the horizontal) and associated upper fronts.
COLD TYPE OCCLUSION
Figure 4-38.Vertical cross section of a cold type of occlusion.