Frontal Systems, Continued
5. Temperatures. Temperature is relatively high before passage. After
passage, the temperature decreases very rapidly with slow-moving fronts.
Such a rapid temperature change does not accompany the passage of
fast-moving cold fronts; the real temperature change is usually seen
some distance (as far as 50 to 100 miles) behind the front.
6. Dew point. The dew-point temperature generally helps to locate
fronts. This is especially true in mountainous regions. A drop in the
dew point is observed with the passage of either type of cold front.
7. Visibility and ceiling. With the approach and passage of a
slow-moving cold front, the visibility and ceilings decrease and remain
low after the passage until well within the cold air. Fast-moving cold
fronts are preceded by regions of poor visibility and low ceilings due to
shower activity. After passage of fast-moving cold fronts, the ceiling
rapidly becomes unlimited and the visibility unrestricted.
Occluded Fronts Because the occlusion is a combination of a cold front and a warm front,
the resulting weather is a combination of conditions that exists with
both. Ahead of a cold-type occlusion, as the warm air is lifted, all
clouds associated with a warm front are found producing typical
prefrontal precipitation extensively for a distance of 250 to 300 miles.
Typical cold front weather is found throughout the narrow belt in the
vicinity of the surface front. However, the thunderstorms are less
intense than those of a typical cold front. This occurs because the
source of warm air has been cut off from the surface, and the energy
received comes only from the warm air trapped aloft. Instability
showers often follow the cold front when the cold air is unstable. The
most violent weather occurs on the upper front for a distance of 50 to
100 miles north of the northern tip of the warm sector. After the
occlusion has passed, the weather usually clears rapidly. The weather
associated with the warm occlusion is very similar to that of the cold
occlusion. With the warm occlusion, the high-level thunderstorms
associated with the upper cold front develop quite some distance ahead
of the surface front (up to 200 miles), and the weather band, in general,
is wider (up to 400 miles). The air behind the cold front, flowing up the
warm frontal surface, causes cumuliform-type clouds to form. In this
area, precipitation and severe icing may be found. The most violent
weather occurs on the upper front, 50 to 100 miles north of the northern
tip of the warm sector.