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Chapter 8
Air Masses, Fronts, and Middle Latitude Cyclones
Prepared by Jonathan D. W. Kahl
University of Wisconsin-Milwaukee
Summary
This chapter begins by examining the typical weather conditions associated with air masses and
their boundaries. Students will first see how and where air masses form, and how they are classified
according to their temperature and humidity. Once upper level winds cause an air mass to move, the air
mass will carry characteristics of its source with it and often has a strong influence on conditions in the
region it invades. Continental polar air moving down from Canada, for example, often brings clear skies
but bitterly cold temperatures to the United States in winter.
Fronts, the boundaries between adjacent air masses, are discussed next. When warm and cold air
masses move toward each other, the warm, lower-density air is forced upward. The rising motion is
gradual in the case of a warm front, where precipitation occurs over a large area ahead of the front. Air is
generally forced upward more abruptly at cold fronts, with the result that precipitation may be quite heavy
in a narrower zone near the front. Typical weather conditions that might be observed during the approach
and passage of warm, cold, and occluded fronts are summarized. Drylines and upper-air fronts are also
described
The chapter continues with an explanation of the structure of middle latitude storm systems and
some of the factors that control their development. Following a review of the polar front theory of wave
cyclones, the vertical structure of middle latitude storms and the influence of upper-level wind patterns on
storm development are presented. The important interactions between upper-air and surface features are
emphasized. Students will see, for example, that diverging motions at upper levels can create a center of
low pressure at the surface or cause an existing low to intensify. Upper level winds also determine the
direction of movement of surface cyclones and anticyclones. An illustration of cyclogenesis, the
development or strengthening of a cyclone, is given in the context of the nor'easter.
Teaching Suggestions, Demonstrations and Visual Aids
1.
When the weather at your location remains roughly constant for several days at a time, identify the
type of air mass influencing your area.
2.
Show the students a good example of the "comma-shaped" cloud pattern associated with a
mature middle latitude storm. Ask the students where they would expect the center of low pressure
and the fronts to be found.
3.
A line of thunderstorms forming along a strong cold front can often be seen clearly on satellite
photographs.
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4.
After covering the material in this chapter, students should be able to understand and enjoy
discussions of the current weather conditions depicted on surface weather charts. Show the positions and
movement of air masses and fronts. Show the upper air chart and relate this to the surface features.
5.
The Pacific and Atlantic surface analysis maps will generally show several middle latitude
storm storms approaching and leaving the US. Examples of storms in various stages of development
can often be seen.
6.
Students will sometimes be confused to find precipitation associated with a stationary front. It is
worth explaining that warm air may still override the cold air even though the cold air mass and the
frontal boundary remain stationary.
7.
Students will often have difficulty, initially, trying to locate shortwaves on an upper -level
chart. Instead of a single chart, show the students a sequence of charts covering a period of 2 or 3
days. Shortwaves will be more apparent as they move and distort different portions of the longwave
pattern.
8.
Activities and demonstrations involving fronts, and the polar front theory, are discussed in
Chapter 8 (Motion) in Hands-On Meteorology.
9.
Discuss the relationship between convergence/divergence and vertical motion using the following
analogy. Imagine a coffee can full of marbles, with several small holes drilled in the bottom of the can.
If the bottom of the can represents the surface and the top represents 500 mb, and the number of marbles
in the can represents the surface air pressure, then how can the surface pressure decrease when surfacelevel convergence puts more marbles in the can?
10.
A satellite image of the spotty pattern of cumuliform clouds or rows of clouds that are produced
when continental polar air moves out over warm ocean water can be used to complement a discussion of
the lake effect. Ask the students what type of weather conditions they would expect if the winds in such a
photograph changed direction and began to blow from the ocean toward the land.
Student Projects
1.
Provide the students with surface weather observations plotted on a map. Have the students first
locate centers of high and low pressure. Then using the weather changes summarized in the text,
have them attempt to locate warm and cold fronts on the map. The instructor can supply students
with simple examples at first and then move to more complex situations. Having students forecast
the future movement of a middle latitude storm would fit in well with material covered in the next
chapter.
Have students draw isotherms on the surface weather chart. A southward bulge of cold air will often
be visible to the west of a strong surface low pressure center. The cold front should correspond to the
front edge of this cold air mass. Similarly, the warm front will be found at the advancing edge of a warm
air mass east of the low.
2.
Have students record and plot daily average weather data (maximum and minimum
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temperature, average dew point and pressure, precipitation amounts) and weather observations (cloud
cover, cloud types, winds) for a few days before and following the passage of a strong front. The
change in weather conditions can sometimes be quite dramatic. Also, students will often be
surprised to see how closely the sequence of events described in the text corresponds to events in the
real world. Students could repeat this exercise for other locations.
3.
Have students describe and document an unusual weather event that occurs during the semester,
such as an outbreak of polar air, a squall line with severe thunderstorms in the southeastern US, a
strong storm with gale force winds reaching the northwestern US, or a strong storm along the East
Coast of the US. The study should be confined to air mass weather or a middle latitude storm
system. The student's report should include a surface weather map and an upper level map. In each
case students should attempt to find one or more reasons for these extreme weather conditions. Was the
central pressure in a surface low, for example, lower than normal? Was the temperature gradient
across a cold front unusually large? Was the upper level wind flow pattern atypical?
Students might also document an unusual weather event that they or someone from their family
remembers.
4.
Students can examine current weather charts to identify any large high or low pressure systems in
the United States. Do they think these systems are intensifying or decaying? Why?
5.
Have students track the development and movement of middle latitude storms as they approach
and move across the United States. Have students identify associated features on the upper-air charts.
Answers to Questions for Review
1.
a. An extremely large body of air whose properties of temperature and humidity are fairly similar
in any horizontal direction at any given altitude. b. In order for a huge mass of air to develop uniform
characteristics, its source region should be generally flat and of uniform composition with light surface
winds.
2.
Polar air originating over land will be classified cP on a surface weather map. In winter, an
extremely cold air mass that forms over the arctic is designated as cA, continental arctic. Sometimes,
however, it is difficult to distinguish between arctic and polar air masses, especially when the arctic air
mass has traveled over warmer terrain.
3.
cP air is responsible for extremely cold temperatures during winter, and refreshing cool
temperatures in summer.
4.
Lake-effect snows are snowstorms that form on the downwind side of a large lake. Cold, dry air
crossing a lake gains moisture and warmth from the water. The more buoyant air rises, forming clouds
that deposit snow on the lake's lee shore.
5.
The middle latitudes, where the central US is located and where surface temperatures and
moisture characteristics vary considerably, are not good source regions. Instead, this region is a transition
zone where air masses with different physical properties move in, clash, and produce an exciting array of
weather activity.
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6.
cP: cold, dry, stable. mP: cool, moist, unstable. cT: hot, dry stable air aloft, unstable surface air.
mT: warm, moist, usually unstable.
7.
Continental tropical (cT).
8.
Lake-effect snows are snowstorms that form on the downwind side of a large lake. Cold, dry air
crossing a lake gains moisture and warmth from the water. The more buoyant air rises, forming clouds
that deposit snow on the lake's lee shore.
9.
After the air mass spends some time over its source region, it usually begins to move in response
to the winds aloft. As it moves away from its source region, it encounters surfaces that may be warmer or
colder than itself. When the air mass is colder than the underlying surface, it is warmed from below,
which results in a steeper lapse rate and instability at low levels. In this case, increased convection and
turbulent mixing near the surface usually produce good visibility, cumuliform clouds, and showers of rain
or snow. On the other hand, when the air mass is warmer than the surface below, the lower layers are
chilled by contact with the cold earth. Warm air above cooler air produces a stable lapse rate with little
vertical mixing. This causes the accumulation of dust, smoke, and pollutants, which restricts surface
visibilities. In moist air, stratiform clouds accompanied by drizzle or fog may form.
10.
The contrast in temperature between land and water is strongest in the winter. Since land and
water are source areas for air masses, the stronger contrast creates more distinct boundaries in winter as
compared to summer.
11.
a. mT b. cP c. mP d. cT e. cP f. mP g. mT
12.
Cold front:
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Warm front:
Occluded front:
14.
The cyclone begins as a stationary front along the polar front. It represents a trough of lower
pressure with higher pressure on both sides. Cold air to the north and warm air to the south flow parallel
to the front, but in opposite directions. This type of flow sets up a cyclonic wind shear. Under the right
conditions a wavelike kink forms on the front. The wave that forms is known as a frontal wave. As the
cold air displaces the warm air upward along the cold front, and as overrunning occurs ahead of the warm
front, a narrow band of precipitation forms. Steered by the winds aloft, the system typically moves east
or northeastward and gradually becomes a fully developed open wave in 12 to 24 hours. The central
pressure is now much lower. These more tightly packed isobars create a stronger cyclonic flow, as the
winds swirl counterclockwise and inward toward the low’s center. Precipitation forms in a wide band
ahead of the warm front and along a narrow band of the cold front. The region of warm air between the
cold and warm fronts is known as the warm sector. Here, the weather tends to be partly cloudy, although
scattered showers may develop if the air is conditionally unstable. As the air masses try to attain
equilibrium, warm air rises and cold air sinks, transforming potential energy into kinetic energy.
Condensation supplies energy to the system in the form of latent heat. And, as the surface air converges
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toward the low center, wind speeds may increase, producing an increase in kinetic energy. As the open
wave moves eastward, central pressures continue to decrease, and the winds blow more vigorously. The
faster-moving cold front constantly inches closer to the warm front, squeezing the warm sector into a
smaller area, and the wave quickly develops into a mature cyclone. The cold front eventually overtakes
the warm front and the system becomes occluded. At this point, the storm is usually most intense, with
clouds and precipitation covering a large area. Without the supply of energy provided by the rising warm,
moist air, the old storm system dies out and gradually disappears.
15.
The polar front is a semi-continuous global boundary separating cold polar air from warm
subtropical air, thus providing ideal conditions for the formation of large mid-latitude cyclones.
16.
The Gulf of Mexico, the Atlantic Ocean east of the Carolinas, the eastern slopes of the Rockies
and the Sierra Nevada.
17.
Behind the cold front there is cold air both at the surface and aloft. This cold, dense air is helping
to maintain the surface anticyclone. But aloft, in a region of cold air, constant pressure surfaces are
squeezed closer together because in the dense air the atmospheric pressure decreases rapidly with height.
Consequently in the cold air aloft we find the upper low, located behind, or to the west of the surface low.
18.
It will intensify, because more air will be removed above the storm than is brought in at the
surface. This causes the surface air pressure to decrease, intensifying the storm.
19.
Upper-level divergence (usually associated with a shortwave), surface convergence, strong
horizontal temperature contrast, temperature advection.
20.
Without upper-level divergence, the converging surface winds would increase the central
pressure, thus decreasing the pressure gradient and stopping the convergence.
21.
The polar front jet stream provides areas of divergence aloft.
22.
Because the upper-level wind flow, ahead of the trough, is generally southwesterly.
Answers to Questions for Thought and Exploration
1.
Very unlikely, since the lake wouldn’t be expected to thaw in February.
2.
On the eastern side of the anticyclone the winds are northerly. Cold winds and cP air can bring
record low temperatures. On the western side of the anticyclone the winds are southerly. As the
anticyclone drifts eastward, the southerly winds carry warm mT air into the region.
3.
In winter, cold fronts are well developed. When warm, humid mT air is drawn northward ahead
of the front the warm air is lifted, often producing stormy weather. In winter, along a warm front the air
is usually stable as warm air lies above cold air. In summer, along a warm front, warm, humid, unstable
air rides up and over only slightly cooler surface air. Often the rising unstable air is able to produce
towering clouds, showers, and even thunderstorms.
4.
A warm front.
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5.
As a cold front moves eastward, mT air is drawn up from the Gulf of Mexico ahead of it.
6.
The equator-to-pole temperature gradient is stronger in winter, so the temperature contrast
between adjacent air masses will also be stronger in winter.
8.
Because midlatitude cyclones form and move along the polar front in a wavelike manner.
9.
Both pressure systems are 'steered' by a wave in the 500-mb flow.
10.
The wave cyclone would dissipate. The air aloft would be converging and descending directly
above the surface cyclone.
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