Chapter 8
Air Masses, Fronts, and Middle-Latitude Cyclones
Summary
This chapter begins with an examination of 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 properties. Once upper level winds
cause an air mass to move, the air mass will carry characteristics of its source with it and may have
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.
The different types of fronts that form at the boundaries between air masses are discussed
next. When warm and cold air masses move toward each other, the warm, low-density air is forced
upward. The rising motion is most gradual in the case of a warm front, and precipitation can occur
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 a treatment of the structure of middle latitude storm systems
and some of the factors that govern their development. Following a review of the polar front
theory of wave cyclones, the vertical structure of middle latitude storms and the influence of upperlevel wind patterns on storm development are discussed. 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.
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.
2.
A line of thunderstorms forming along a strong cold front can often be seen clearly on
satellite photographs.
3.
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.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 8: Air Masses, Fronts, and Middle-Latitude Cyclones
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4.
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.
5.
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.
6.
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.
7.
Fronts, and the polar front theory are discussed in Chapter 8 (Motion) in Hands-On
Meteorology.
8.
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 surface-level convergence puts more marbles in the can?
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
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
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 8: Air Masses, Fronts, and Middle-Latitude Cyclones
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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.
Use the Forecasting section of the ThomsonNow web site m to identify any
large high or low pressure systems in the United States. Do you think these systems are
intensifying or decaying? Why?
5.
Use Forecasting activity on the ThomsonNow web site to determine what type
of air mass is affecting your area today.
6.
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.
7.
Use the Forecasting section of the ThomsonNow web site to examine the
relationship between temperature advection, fronts and isobars. Describe this
relationship.
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
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 8: Air Masses, Fronts, and Middle-Latitude Cyclones
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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.
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.
12.
a. mT b. cP c. mP d. cT e. cP f. mP g. mT
a) Warm front
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 8: Air Masses, Fronts, and Middle-Latitude Cyclones
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b) Cold Front:
c) 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 toward
the low center, wind speeds may increase, producing an increase in kinetic energy. As the open
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 8: Air Masses, Fronts, and Middle-Latitude Cyclones
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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 uper low, located behind, or to
the west of the surface low.
18.
Upper-level divergence (usually associated with a shortwave), surface convergence, strong
horizontal temperature contrast, temperature advection.
19.
Without upper-level divergence, the converging surface winds would increase the central
pressure, thus decreasing the pressure gradient and stopping the convergence.
20.
The polar front jet stream provides areas of divergence aloft.
21.
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.
5.
As a cold front moves eastward, mT air is drawn up from the Gulf of Mexico ahead of it.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 8: Air Masses, Fronts, and Middle-Latitude Cyclones
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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.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 8: Air Masses, Fronts, and Middle-Latitude Cyclones
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