Instructor's Manual and Test Bank to accompany Meteorology Today, 10th Edition
Jonathan D. W. Kahl
University of Wisconsin-Milwaukee
Chapter 12
Middle-Latitude Cyclones
Summary
The structure of middle latitude storm systems and some of the factors that govern their
development are covered in this chapter. The chapter begins with a review of the polar front theory of
wave cyclones, originally formulated by a group of Norwegian meteorologists after the end of World War
I. The vertical structure of middle latitude storms and the influence of upper-level wind patterns on storm
development are examined next. 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 focus section “Northeasters.”
The chapter explains how fast moving shortwaves can perturb the upper-level longwave flow
pattern and produce regions of temperature advection, or baroclinic zones in the atmosphere. The
resulting air motions can enhance storm development. Strong upper level divergence is also found at
certain positions near the jet streak, a region of strong winds in the core of the jet stream. The concept of
vorticity is introduced and the relationships between vorticity, upper level divergence, rising air motions
and surface storm development are examined in focus sections on “A Closer Look at Convergence and
Divergence” and “Jet Streaks and Storms”. The concept of vorticity is used to explain why an area of low
pressure will often form on the downwind side of a mountain range. Vorticity is further explored in the
focus section “Vorticity and Longwaves”.
Material introduced in this chapter is tied together with the so-called blizzard of ‘83 that battered
the eastern United States during February 1983. The chapter concludes with a discussion of polar lows,
strong cyclones occurring poleward of the polar front.
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Teaching Suggestions
1.
Examine Pacific and Atlantic surface analysis maps to find middle latitude storm storms
approaching and leaving the US. Examples of storms in various stages of development can often be seen.
2.
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.
Contours on the 500 mb analysis and forecast maps, showing local vorticity maxima and minima, are
helpful also.
3.
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?
4.
Using realtime weather images, compare the size, wind speed, and precipitation patterns between
a middle latitude cyclone and a tropical storm. Discuss similarities and differences.
5.
Is your area affected by a particular type of mid-latitude storm (such as northeasters or Alberta
clippers)? If so, discuss aspects of these storms as they occur. If they don’t occur, discuss archived
weather maps showing the passage of this type of storm. Map archives can be found here:
http://www.hpc.ncep.noaa.gov/html/sfc_archive.shtml
6.
During the passage of a mid-latitude cyclone through your area, examine surface and upper-air
weather charts and compare the position of fronts, troughs and ridges, and jet streams.
7.
Compare surface and upper-air hemispheric charts to relate the hemispheric pattern of upper-air
waves to surface cyclones. Some resources for hemispheric weather charts:
Surface: http://www.esrl.noaa.gov/psd/map/images/fnl/mslp_01.fnl.html
Upper-air: http://www.esrl.noaa.gov/psd/map/images/avn/250_wnd_anl.avn.html
8.
Over a period of a few weeks, determine whether your area is currently being affected by
convergence or divergence. Relate this to the current weather conditions.
9.
Examine an upper-level weather chart for winds in the different quadrants of a jet streak. Do the
wind speeds conform to the model shown in the focus section on Jet Streaks?
Student Projects
1.
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.
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2.
Use current surface weather charts to identify any large high or low pressure systems in the
United States. Do you think these systems are intensifying or decaying? Why?
3.
Locate a cold or warm front on a realtime surface weather chart. Then attempt to locate the warm
or cold air aloft using realtime upper-air weather charts.
4.
Locate a middle-latitude cyclone on a realtime surface weather chart and determine whether the
current cyclone is in the early, middle, or late stage of its life cycle.
5.
Compare and contrast a current midlatitude cyclone in both the northern and southern
hemispheres.
Answers to Questions for Review
1.
Answer not provided.
2.
There is no longer a temperature contrast at the surface.
3.
The Gulf of Mexico, the Atlantic Ocean east of the Carolinas, the eastern slopes of the Rockies
and the Sierra Nevada.
4.
Without upper-level divergence, the converging surface winds would increase the central
pressure, thus decreasing the pressure gradient and stopping the convergence.
5.
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.
6.
It will intensify. When upper-level divergence is stronger than surface convergence, surface
pressures drop and the low intensifies.
7.
A cyclone forming on the eastern side of the Rockies in Alberta, Canada, then rapidly skirting
across the northern tier states.
8.
Midlatitude cyclonic storms that develop or intensify off the eastern seaboard of North America
then move northeastward along the coast, often bringing gale force northeasterly winds to coastal areas,
along with heavy rain, snow or sleet.
9.
Longwaves are many thousands of kilometers long; shortwaves are small disturbances or ripples.
10.
Upper-level divergence (usually associated with a shortwave), surface convergence, strong
horizontal temperature contrast, temperature advection.
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11.
Under these circumstances no significant temperature advection is occurring. Without the
necessary energy transformations associated with temperature advection, the surface system gradually
dissipates. As its winds slacken and its central pressure gradually rises, the low is said to be filling.
12.
The polar front jet stream provides areas of divergence aloft.
13.
Jet streaks along the polar front jet stream provide areas of strong convergence and divergence.
15.
Rising and sinking air in regions of convergence and divergence (conversion of potential energy
to kinetic energy), radiation to space from the tops of clouds, latent heat release.
16.
It causes rising motions, which release latent heat as water vapor condenses.
17.
Warm conveyor belt: originates at the surface in the warm sector, ahead of the cold front. As the
warm airstream moves northward, it slowly rises along the sloping warm front. Water vapor in the rising
air condenses, and clouds form. Aloft, the warm air flow gradually turns toward the northeast or east,
parallel to the upper-level flow. The rising warm air causes cloudiness to appear ahead of the surface low
and its surface warm front. From these clouds, rain or snow usually fall. Directly below the warm
conveyor belt, a cold, moist airstream, the cold conveyor, belt moves slowly westward north of the
surface low and the warm front. As the airstream moves into the vicinity of the surface low, rising air
gradually forces the cold conveyor belt upward. The rising airstream usually turns as it ascends,
producing a comma-shaped cloud. If the airstream rises high enough, it gets caught in the southwesterly
flow aloft and swings northeastward. The last conveyor belt is a dry one that forms in the upper
atmosphere. Called the dry conveyor belt, this airstream slowly descends from the northwest behind the
surface cold front, where it brings generally clearing weather. If a branch of this dry air sweeps into the
storm, it produces a clear area that, in the comma cloud, appears to pinch off the comma’s head from its
tail. This phenomenon tends to show up on satellite pictures as the storm becomes more fully developed.
18.
A vorticity maximum has on its eastern side diverging air and ascending air motions underneath.
When a vorticity maximum approaches (from the west), these features cause surface cyclones to
strengthen (deepen).
19.
Polar lows are cyclones that form poleward of the polar front. They form when frigid air from
ice-covered land or water surfaces comes in contact with warmer air that was located over ice-free waters.
The cyclone forms in a similar way to a mid-latitude cyclone, with energy received from latent heat of
condensation, rising motions, and upper-level support. Polar lows have diameters of 500-1000 km,
smaller than mid-latitude cyclones. Some have a tight spiral of convective clouds that swirls
counterclockwise about a clear area, or “eye,” which resembles the eye of a tropical hurricane. Like
hurricanes, these smaller intense storms normally have a warmer central core, strong winds (often gale
force or higher), and heavy showery precipitation that, unlike a hurricane, is in the form of snow.
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Answers to Questions for Thought
1.
The storm system began as a frontal wave along the eastern slope of the Rocky Mountains. It
developed into an open wave cyclone over the Plains, became occluded over the North Atlantic Ocean,
and eventually moved northeastward into Great Britain.
2.
The wave cyclone would dissipate. The air aloft would be converging and descending directly
above the surface cyclone.
3.
Cold advection normally occurs to the west of the upper-level trough where the air aloft is also
slowly sinking and warming.
4.
Baroclinic conditions exist only in isolated regions of the atmosphere, such as above the polar
front, where there are large pressure and temperature gradients.
5.
Baroclinic waves require sharp temperature contrasts that are usually found along frontal
boundaries. Such temperature contrasts and fronts are usually not found in the tropics.
6.
On a non-rotating earth, the earth's vorticity would be zero. An air parcel would continue to
move in a straight line in the same direction unless acted on by some external force, such as the pressure
gradient force.
7.
Pacific storms often re-develop on the eastern slopes of the Sierra Nevada Mountains because as
the air moves up and over the mountain barrier, the air's vorticity changes. This creates a cyclonic
bending of the air flow on the downwind (leeward) side of the barrier.
8.
The cut-off low would most likely appear as a closed isotherm representing a region of cold air.
9.
The water surface could be warmer than the air above. In this case, the water would warm the air
just above and the environmental temperature would decrease rapidly with increasing altitude (steep
environmental lapse rate). Development of cumuliform clouds under these conditions would be similar to
the Lake Effect.
10.
It means that above 500 mb is the vertical dividing line between convergence and divergence. If
divergence is occurring above the 500-mb level, then convergence is occurring below this level. If
convergence is occurring above the 500-mb level, then divergence is occurring below.
Answers to Critical Thinking Questions
Figure 12.3.
Toward the northeast. Oregon would experience warm air behind the warm front.
Figure 12.29.
Toward the northeast, following the direction of the 500-mb wind flow.
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Multiple Choice Exam Questions
1.
The polar front theory of a developing wave cyclone was conceived in
a. Norway.
b. Great Britain.
c. the United States.
d. Germany.
e. the Soviet Union.
ANSWER: A
2.
Which of the scientists below was not one of the meteorologists who helped develop the polar
front theory of a developing wave cyclone?
a. B. Bjerknes
b. C.G. Rossby
c. T. Bergeron
d. H. Solberg
e. J. Bjerknes
ANSWER: B
3.
On a surface weather map that shows an open wave cyclone, the warm sector can be observed
a. ahead of an advancing cold front.
b. behind an advancing cold front.
c. ahead of an advancing cold-occluded front.
d. behind an advancing cold-occluded front.
e. ahead of an advancing warm front.
ANSWER: A
4.
According to the model of the life cycle of a wave cyclone, the storm system is normally most
intense
a. as a frontal wave.
b. as a stable wave.
c. as an open wave.
d. as a stationary wave.
e. when the system first becomes occluded.
ANSWER: E
5.
Which below is not a name given to a large cyclonic storm system that forms in the middle
latitudes?
a. middle latitude cyclone
b. extratropical cyclone
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c. wave cyclone
d. anticyclone
ANSWER: D
6.
In the polar front theory of a developing wave cyclone, energy for the storm is usually derived
from all but one of the following:
a. rising of warm air and the sinking of cold air
b. latent heat of condensation
c. an increase in surface winds
d. heat energy stored in the ground
ANSWER: D
7.
Another term for explosive cyclogenesis used by meteorologists is
a. lee-side low development.
b. cut-off low cyclogenesis.
c. deepening shortwave.
d. baroclinic development.
e. bomb.
ANSWER: E
8.
The development or strengthening of a middle latitude storm system is called
a. convergence.
b. divergence.
c. cyclogenesis.
d. frontolysis.
ANSWER: C
9.
Cyclogenesis often occurs in regions without horizontal temperature gradients.
a. true
b. false
ANSWER: A
10.
Which region is not considered to be a region where cyclogenesis often occurs?
a. eastern slopes of the Rocky Mountains
b. Atlantic Ocean near Cape Hatteras, North Carolina
c. California
d. Great Basin of the United States
e. Gulf of Mexico
ANSWER: C
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11.
A building anticyclone means
a. the central pressure is increasing.
b. the anticyclone is moving toward the east coast.
c. separate anticyclones are merging.
d. the anticyclone is causing a middle latitude storm to form.
ANSWER: A
12.
For cyclogenesis to occur along a frontal wave, the winds aloft directly above the wave should be
a. diverging.
b. converging.
c. blowing straight from west to east.
d. increasing in speed uniformly over a broad area.
ANSWER: A
13.
The piling up of air above a region is called
a. thickening.
b. divergence.
c. cyclogenesis.
d. convergence.
ANSWER: D
14.
If the flow of air into a surface low pressure area is greater than the divergence of air aloft, the
surface pressure in the center of the low will
a. increase.
b. decrease.
c. remain the same.
d. deepen.
ANSWER: A
15.
As westerly winds blow over a north-south oriented mountain range, the airflow is deflected in
such a way that a trough forms on the ____________ side of the mountain.
a. upwind
b. downwind
c. both upwind and downwind
d. none of the above
ANSWER: B
16.
Which of the following is not associated with rising air motions?
a. overrunning
b. convergence of air at the surface
c. convergence of air aloft
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d. divergence of air aloft
ANSWER: C
17.
Cyclogenesis is the ____ of a mid-latitude cyclone.
a. development or strengthening
b. weakening or dissipation
c. term for the exact midpoint
d. none of the above
ANSWER: A
18.
A lee-side low forms
a. over the central Pacific Ocean.
b. near the equator.
c. on the upwind side of a mountain.
d. on the downwind side of a mountain.
ANSWER: D
19.
In the Southern Hemisphere, convergence and divergence don’t play a role in the development of
middle latitude cyclones.
a. true
b. false
ANSWER: B
20.
Northeasters (or nor'easters) are midlatitude storms commonly found
a. along the Pacific coast of North America.
b. along the Atlantic coast of North America.
c. along the Gulf coast of North America.
d. along both the Pacific and Atlantic coasts of North America.
ANSWER: B
21.
the
A surface low pressure area with a deep upper-level trough to the west will tend to move toward
a.
b.
c.
d.
northwest.
northeast.
southwest.
southeast.
ANSWER: B
22.
When a deep upper-level trough is located to the east of a surface anticyclone, the surface
anticyclone will tend to move toward the
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a.
b.
c.
d.
northwest.
northeast.
southwest.
southeast.
ANSWER: D
23.
When an upper-level low lies directly above a surface low,
a. the surface low will probably weaken.
b. thunderstorms will develop.
c. a wave cyclone will begin to form.
d. the pressure of the surface low will decrease.
e. cyclogenesis will occur.
ANSWER: A
24.
An upper-level pool of cold air that has broken away from the main flow is called
a. a cut-off low.
b. a shortwave.
c. a wave cyclone.
d. a lee-side low.
ANSWER: A
25.
Jet streaks often occur away from jet streams.
a. true
b. false
ANSWER: B
26.
For a surface storm system to intensify, the upper-level low (or trough) should be located to the
____ of the surface low.
a. north
b. south
c. east
d. west
ANSWER: D
27.
Developing low pressure areas generally have ____ air near the surface and ____ air aloft.
a. converging, diverging
b. diverging, converging
c. converging, converging
d. diverging, diverging
ANSWER: A
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28.
Strong storms that develop over water, poleward of the polar front, are called
a. nor'easters.
b. upslope lows.
c. lee-side lows.
d. polar lows.
e. none of the above
ANSWER: D
29.
Like hurricanes, polar lows have a clear area in their center.
a. true
b. false
ANSWER: A
30.
When upper-level divergence of air above a surface low pressure area is stronger than the
convergence of surface air, the surface pressure will ____ and the storm itself will ____.
a. increase, intensify
b. increase, dissipate
c. decrease, intensify
d. decrease, dissipate
ANSWER: C
31.
The entrance region occurs ____________ of a jet streak, while the exit region occurs _________
of the jet streak.
a. downwind, upwind
b. upwind, downwind
c. ahead, above
d. downwind, above
ANSWER: B
32.
If the outflow of air around a surface high pressure area is greater than the convergence of air
aloft, you would observe
a. in increase in pressure in the center of the high.
b. movement of the high toward the northeast.
c. a decrease in the central pressure.
d. strengthening in the high.
ANSWER: C
33.
Longwaves in the middle and upper troposphere usually have lengths on the order of
a. tens of kilometers.
b. hundreds of kilometers.
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c. thousands of kilometers.
d. millions of kilometers.
ANSWER: C
34.
An upper-level trough that shows retrograde motion would probably be moving toward the
a. west.
b. east.
c. north.
d. south.
ANSWER: A
35.
Rossby waves are also known as
a. stable waves.
b. shortwaves.
c. tidal waves.
d. longwaves.
ANSWER: D
36.
Atmospheric shortwaves usually move ____ at a speed that is ____ than longwaves.
a. east to west, faster
b. west to east, faster
c. east to west, slower
d. west to east, slower
ANSWER: B
37.
The warm sector is located
a. ahead of the warm front and behind the cold front.
b. ahead of the cold front and behind the warm front.
c. below the occluded front and above the cold front.
ANSWER: B
38.
Atmospheric shortwaves usually move ____ than longwaves, and ____ when they move through
a longwave ridge.
a. faster, weaken
b. faster, strengthen
c. slower, weaken
d. slower, strengthen
ANSWER: A
39.
A small, moving disturbance imbedded in a longwave is called
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a.
b.
c.
d.
a lee-side low.
a wave cyclone.
a shortwave.
a frontal wave.
ANSWER: C
40.
On an upper-level chart where the isotherms cross the isobars (or contours) and temperature
advection occurs, the atmosphere is called
a. barotropic.
b. geostrophic.
c. hydrostatic.
d. baroclinic.
ANSWER: D
41.
Temperature advection does not occur when the winds do not cross the isotherms.
a. true
b. false
ANSWER: A
42.
During baroclinic instability,
a. wave cyclones can intensify into large storm systems.
b. strong wind speed shear exists from the surface up to at least the 500 mb level.
c. rising and descending air motions exist.
d. temperature advection is occurring.
e. all of these
ANSWER: E
43.
a.
b.
c.
d.
Which of the following statements is not correct about vorticity?
The earth's vorticity in the Northern Hemisphere is positive.
The earth's vorticity is zero at the poles.
Air that spins cyclonically possesses positive vorticity.
Absolute vorticity is the sum of the earth's vorticity and the relative vorticity.
ANSWER: B
44.
If we assume that the absolute vorticity of flowing air is conserved, air moving northeastward will
bend ____ to compensate for the ____ in the earth's vorticity.
a. anticyclonically, decrease
b. anticyclonically, increase
c. cyclonically, increase
d. cyclonically, decrease
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ANSWER: B
45.
Lee-side lows are
a. storms (extratropical cyclones) that form on the eastern (lee) side of a mountain range.
b. thermal lows that form due to surface heating over deserts.
c. storms (tropical cyclones) that form near the Leeward Islands.
d. low pressure areas that form over oceans, then move onshore of the lee side of the
coastline.
ANSWER: A
46.
Vorticity is always present in the atmosphere, even during fair weather.
a. true
b. false
ANSWER: A
47.
The planetary vorticity of an air parcel moving from low toward high latitude in the Northern
Hemisphere will
a. increase.
b. decrease.
c. remain constant.
d. change from positive to negative.
ANSWER: A
48.
Vorticity refers to
a. the rising and sinking of air along weather fronts.
b. the formation of clouds.
c. the spin of air parcels.
d. the changing of the seasons.
e. the development of a wave cyclone.
ANSWER: C
49.
Which of the following is true about vorticity?
a. Vorticity can be observed by satellites, even during cloudy conditions.
b. Planetary vorticity is always present.
c. Relative vorticity can be either positive or negative.
d. all of the above
e. none of the above
ANSWER: D
50.
The type of weather system known as a “mid-latitude cyclone” cannot form over the tropical
ocean because:
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a.
b.
c.
d.
e.
surface temperature contrasts are not large.
the ocean surface has a lot of waves.
the Coriolis force is weak in the tropics.
both surface temperature contrasts are not large and the ocean surface has a lot of waves.
both surface temperature contrasts are not large and the Coriolis force is weak in the
tropics.
ANSWER: E
Essay Exam Questions
1.
Describe or illustrate the various phases in the life cycle of a middle latitude storm according to
the polar front theory.
2.
Explain how cyclogenesis works.
3.
List two regions in the United States where cyclogenesis is common. Why does cyclogenesis
preferentially occur in these areas?
4.
Draw a sketch of a 500 mb chart that clearly shows a trough and a ridge. Where would you
expect to find the maximum and minimum values of absolute vorticity? Where would you expect
to find converging and diverging wind motions?
5.
Why does middle latitude storm development occur beneath troughs rather than ridges on a 500
mb chart?
6.
With the aid of a diagram, show why an intensifying surface low pressure center which is located
just east of a deep upper-level trough will often move in a northeasterly direction.
7.
Why is it important locate and follow the movements of atmospheric shortwaves?
8.
How can shortwaves be identified and followed on 500 mb charts?
9.
How do shortwaves differ from a longwaves in the development of middle latitude storms?
10.
Describe, in words or with a sketch, a wind flow pattern that will result in upper-level divergence.
11.
Describe some of the ways in which the upper-level wind flow pattern can influence the
development and movement of a middle latitude storm system.
12.
List some of the reasons why the polar front theory doesn’t apply to storms in the tropics.
13.
When making a weather forecast, which kind of chart is more important: a surface chart or a 500
mb chart?
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14.
Suppose you observe a middle-latitude cyclone on the surface weather chart. What kind(s) of
weather chart(s) would you want to examine to determine whether the cyclone is likely to
strengthen or decay over the next 24 hours? What information would you hope to gain from each
chart type?
15.
Describe how horizontal changes in both wind direction and wind speed can cause convergence
and divergence.