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EAS 308, Meeting 3
Fronts:
A front is a boundary between two air masses. An air mass is a region of air at the
surface with nearly uniform temperature and moisture content (designated by the dewpoint temperature). Thus, there can be four air masses: warm and moist, cold and moist,
warm and dry, cold and dry. Since the oceans are the source for the moisture, the moist
air masses form over the oceans, the cold, moist air mass over the high-latitude oceans
and the warm, moist air masses over the low-latitude oceans. Further, the dry air masses
form over continental interiors, the cold, dry air masses at the high-latitudes and vice
versa for the warm, dry air masses. This leads us to Assignment 3.
Figure 29 shows the cross-section of a front. The warm, less dense air lies above the
colder, denser air. The isotherms (lines of constant temperature) illustrate which air mass
is cold and which air mass is warm. There is no information given on the moisture
content of either air mass. Further no motion is indicated so we cannot determine the
type if front. But, in Fig. 30, movement is defined and the fronts, therefore, can be typed:
warm, cold and occluded. A stationary front is a front with no movement and its symbols
will be sketched on the board.
Plan and cross-section of fronts:
Figure 32 shows a plan view of a mid-latitude cyclone and three cross-sections that
illustrate the vertical structure of the attendant clouds. Notice as you move away from
the center of the low pressure region in the warm sector that the cloud cover changes
from overcast to broken to scattered. Figure 33 shows the plan view of the distribution of
precipitation which is shown in the vertical cross-sections in Figure 32. In general, it can
be seen most of the precipitation is associated with the warm front.
Anafronts and katafronts:
If air rises up the frontal surfaces it’s an anafront and vice versa for a katafront (Fig. 34).
The anafronts are the classical fronts because they produce the precipitation. The sinking
air associated with katafronts explains why little precipitation is produced. The clouds
cannot grow tall enough to produce precipitation. The ana-warm front is illustrated in
Fig 35a and the ana-cold front is shown in Fig. 36a.
The warm and cold “conveyor belts”:
A “conveyor belt” in the atmosphere is a long and wide band which carries vast
quantities of air through the low pressure system. Figure 37 shows a plan view of both
“belts” and a 3-D view of the “warm conveyor belt”. The low pressure centers move
slowly across the surface, 15 to 30 mph, while the air flows through the systems at much
higher speeds, 30 to 60 mph. This flow of air through the system explains the “belts”.
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As illustrated in Fig. 40, the cold conveyor belts transport long “tongues” of cold polar
air from high latitudes into the low pressure region. The cold conveyor belt passes
beneath the warm conveyor belt. Where the twp “belts” cross, the most intense
precipitation occurs.
Sequence of atmospheric conditions as a frontal system passes:
The description of the clouds, the weather, the winds and the temperatures found on
pages 31 and 32 in combination with Figures 32a (warm front) and 36b (cold front)
illustrates the sequence of atmospheric conditions as the two types of front pass your
location. The only phenomena the author missed are sleet and freezing rain in the warm
front. The production of these two dangerous types of precipitation will be added to Fig
35a with a sketch on the board.
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