Geoscience Reference
In-Depth Information
Warm, dry
subsiding air
Stratocumulus
Stratocumulus
Weak
uplift
Light rain
Figure 7.12 Cloud structure at a kata-warm and a kata-cold front. An ana-frontal structure is shown in Figure 7.7.
rapidly than the warm front. The air of the warm sector
is raised above the ground surface as the cold front catches
up with the warm front. This is known as the stage of
occlusion , or the occluded front . The nature of the front will
now depend upon the relative temperatures of the two
cold air masses ( Figure 7.13 ). Where the air behind is
colder than that ahead, we will have a structure rather like
a cold front. If it is warmer than the air ahead, the
structure will resemble a warm front.
The detailed air movements and cloud distribution at
an occluded front are structurally complex. As fronts
represent the mixing of air of different origins, humidities,
temperatures and stabilities, it is not surprising that great
variation can occur between fronts, or even along the same
front. This may also explain why 'true' occluded fronts are
relatively rare. Frequently one or more of the frontal
components of the Bergen model shown in Figure 7.5 is
missing. Recent work suggests that fronts which seem to
have an occluded structure may have been formed by
other methods, such as growth northward from the
junction of warm and cold fronts, or 'instant' occlusions
whereby comma-shaped cloud features behind the cold
front join with open frontal waves to produce apparently
mature occluded systems over a short period of time.
Much remains to be determined about the nature of
occluded frontal systems.
At one time it was believed that it was the air rising
along the frontal surface that caused the development of
a depression. However, the role of divergence in the upper
atmosphere is now believed to be the most important
factor, the fronts being the result of the rotation of air
around the low's centre. From being a cause of the low
pressure, the front has been relegated to a consequence.
Nevertheless the weather activity associated with fronts is
still a very important aspect of the depression. Unfortu-
nately their diversity makes it difficult to generalize about
their weather properties.
WEATHER-FORMING SYSTEMS OF
THE TROPICS
Easterly waves
The weather of the trade-wind zone normally shows little
variety. It is characterized by small convectional clouds
drifting across the sky in response to the prevailing winds
( Plate 7.4 ) and is dominated by the Trade Wind Inversion
( Figure 7.14 ). Showers may develop in the afternoon, and
they are likely to be heavier and more frequent in the
summer season, but otherwise the weather remains
remarkably constant throughout the year.
-50°
-50°
Upper warm
sector air
Upper warm
sector air
-40°
-20°
-40°
-20°
Cool
Cool
Cold
Cold
Figure 7.13
Simplified cross-sections
through cold and warm
occluded fronts.
10°
10°
Cold occlusion
Warm occlusion
 
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