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Chapter 9
WEATHER PATTERNS
Learning Objectives
After reading, studying, and discussing this chapter, students should be able to do the following:
1.
Recognize midlatitude cyclones as the primary weather producers in the middle latitudes
and describe their size, movement, duration, circulation, and fronts commonly associated
with them.
2.
Compare and contrast warm fronts and cold fronts in terms of their structure and associated
weather.
3.
Describe stationary fronts, occluded fronts, and drylines.
4.
Distinguish between cold-type and warm-type occluded fronts.
5.
Outline and explain the stages in the life cycle of a wave cyclone as described by the Polar
Front Model.
6.
Describe the changes in wind direction, pressure tendency, cloud type and coverage,
precipitation, and temperature when either a warm, a cold, or an occluded front passes.
7.
Relate the development and movement of cyclones to upper-level flow including the roles
of speed divergence, directional divergence, and vorticity.
8.
List the areas where most cyclones that influence North America form and what types
of air masses are associated with each.
9.
Describe what blocking highs are, when and where they tend to form, and the effects
that they have on weather.
10. Comment on the intensity of cyclonic storms during the spring and explain why they
can be so strong.
11. Define the terms listed in the vocabulary review.
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Chapter Outline
I. Polar-Front Theory (Norwegian
4. Traveling cyclones
Cyclone Model)
a. Patterns of
A. Midlatitude cyclone
movement
II. Fronts
b. Airflow aloft and
A. Warm
cyclone migration
B. Cold
5. Anticyclonic weather
C. Stationary
and blocking highs
D. Occluded
IV. Case Study of a Midlatitude Cyclone
1. Cold type
A. Violent spring weather
2. Warm type
B. Weather in Peoria
E. Drylines
III. Life Cycle of a Midlatitude Cyclone
A. Formation
1.Clash of Air masses
2. Cyclogenesis
B. Development of Cyclonic
Flow
C. Occlusion
1.Beginning of the end
D. Idealized Weather of a
Midlatitude Cyclone
1.Cyclone Formation
2. Cyclonic and
anticyclonic circulation
3. Divergence and
convergence aloft
a. Speed divergence
b. Directional
divergence
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Answers to the Chapter Review
1.
Early Norwegian meteorologists visualized boundary surfaces separating air
masses of different densities as analogous to battle lines and called them fronts, like
battlefronts.
2. The frontal surface would be about 2 km above the ground.
3.
The weather associated with warm fronts is generally much more mild than that
associated with cold fronts. Warm fronts usually produce light to moderate
precipitation over a large area and for an extended period. After a warm front passes,
temperatures gradually rise. Cold front weather is usually more violent than warm
front weather with more intense precipitation over a smaller area. Cold fronts often
produce severe weather including thunderstorms and tornadoes. A marked
temperature drop and wind shift also usually accompanies the passage of cold fronts.
4.
Because cold fronts are steeper and move more rapidly than warm fronts, the
vertical displacement of air is often rapid enough such that latent heat released by
condensing water vapor adds appreciably to the air’s buoyancy. The common result is
cumulonimbus clouds with their characteristic heavy showers and occasionally severe
weather.
5.
The first line of the proverb pertains to warm fronts. It refers to the fact that these
fronts are often preceded by a succession of clouds that foretell of the coming
nimbostratus clouds that usually produce light to moderate precipitation of relatively
long duration. The second line refers to a cold front. Since cold fronts usually do not
have a series of clouds preceding them, there is little or no forewarning of the
cumulonimbus clouds and showery precipitation that often accompany a cold front.
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Precipitation associated with cold fronts is “soon past” because cold fronts usually
move relatively quickly.
6.
Gentle to moderate precipitation is likely along stationary fronts because
overrunning usually occurs along them.
7.
A cold-type occluded front forms when the air behind a cold front is colder than
the air underlying the warm front that it is overtaking. When the air behind the cold
front is warmer than the air underlying the warm front, a warm-type occluded front is
said to exist. The weather associated with cold-type occluded fronts often resembles
the weather of a cold front, whereas conditions resembling warm front weather more
commonly accompany warm-type occluded fronts.
8.
Cyclones form along fronts. Cyclonic formation (cyclogenesis) occurs when two
air masses of different densities (temperatures) are moving roughly parallel to the
front, but in opposite directions.
9.
If conditions are suitable for cyclogenesis, the frontal surface that separates two
contrasting air masses will take on a wave shape that is usually several kilometers
long. Midlatitude cyclones that intensify develop waves that change in shape over
time, much like a gentle ocean swell does as it moves into shallow water and becomes
a tall breaking wave.
10.
During occlusion, the warm sector is forced aloft. When this process is complete,
the cyclone has lost its primary source of energy, the sinking of cold air and the rising
of warm. Hence, the storm comes to an end.
11.
When a midlatitude cyclone passes to the north of an observer, a warm front
passes first followed by a cold front. Different types of weather are associated with
the passage of each front type.
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a. As the warm front approaches, winds generally blow from the east or southeast,
and as the warm front passes, winds shift to the south or southwest. When the cold
front passes, a wind shift from the southwest to the northwest occurs.
b. Pressure drops steadily until the warm front passes. Then, pressure tendency
steadies. Pressure rises again after the cold front passes.
c. As a warm front nears, cirrus clouds would be sighted first, followed by
cirrostratus, altostratus, and finally nimbostratus. Skies clear after the warm front
passes. Once the cold front arrives, cumulonimbus clouds usually appear. Skies
again clear after the cold front passes.
d. As a warm front moves in, cloud cover would get progressively greater, from a few
tenths coverage with cirrus to completely overcast with the coming of the
nimbostratus. Skies clear after the front passes. Later, with the approach of the
cold front, cumulonimbus clouds fill much of the sky until after the front passes.
e. As a warm front moves in, gentle precipitation would begin as the nimbostratus
clouds moved overhead. Precipitation would cease after the front passed. When
the cold front arrives, heavy precipitation and the possibility of hail and tornado
activity would occur. Precipitation would end again after the front passed.
f. As the warm front passed, temperatures would rise. Later, the passage of the cold
front would be accompanied by a drop in temperature.
12.
As the low approaches, cool temperatures are the rule, and winds are easterly
because the warm sector of the cyclone is to the south. The pressure drops and the
sky becomes increasingly overcast. Further, precipitation is to be expected, and if it
is winter or early spring, possibly snow, sleet, or glaze. As the occluded front slowly
passes, winds shift from the north or northeast to the northwest. The sky begins to
clear and the barometric tendency rises. Temperatures, however, remain cool or cold.
13.
When winds shift in a clockwise manner, as from east to south, it is termed a
veering wind shift. Such wind shifts occur when the center of a wave cyclone passes
to the north of an observer. When winds shift in a counterclockwise direction, it is
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called a backing wind shift. Such a shift is to be expected when the center of a low
passes to the south of an observer.
14.
In order for cyclones to be maintained at the surface, surface convergence must be
offset by outflow aloft. As long as divergence aloft is equal to or greater than the
surface inflow, the cyclone can be sustained.
15.
Speed divergence occurs when air enters a zone of high wind speed, accelerates,
and stretches out. Speed convergence occurs when air enters a zone of slow wind
speed and air pileup results.
16.
Upper-level divergence supports cyclogenesis. This usually occurs downstream
of an upper-level trough. Anticyclones usually form downstream of upper-level
ridges because this is where upper-level convergence is favored.
17.
Blocking highs can block the eastward migration of cyclones, keeping one region
dry and keeping another region continually under the influence of cyclonic storms.
Another influence of blocking highs is their contribution to air pollution episodes.
This can happen since subsidence within an anticyclone can produce a temperature
inversion and light winds associated with the center of an anticyclone do little to
disperse polluted air.
18.
Intense cyclonic storms traveling across the United States can affect large areas
while producing blizzards, numerous severe thunderstorms, hailstorms, and
tornadoes.
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