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C H A P T E R 7 • A I R M A S S E S A N D W E AT H E R S Y S T E M S
Coast of the United States. When such an air mass does reach the
Washington, Oregon, and California coasts, it brings with it unusual freezing temperatures that do great damage to agriculture.
Maritime Polar Air Masses During winter months,
the oceans tend to be warmer than the land, so an mP air mass
tends to be warmer than its counterpart on land (the cP air mass).
Much mP air is originally cold, dry cP air that has moved to a
position over the ocean. There, it is modified by the warmer water and collects heat and moisture. Thus, mP air is cold (although
not as cold as cP air) and damp, with a tendency toward instability. The northern Pacific Ocean serves as the source region for
mP air masses, which, because of the general westerly circulation
of the atmosphere in the middle latitudes, affect the weather of
the northwestern United States and southwestern Canada. When
this mP air meets an uplift mechanism (such as a mass of colder,
denser air or coastal mountain ranges), the result is usually very
cloudy weather with a great deal of precipitation. An mP air mass
may still be the source of many midwestern snowstorms even after crossing the western mountain ranges.
Generally, an mP air mass that develops over the northern
Atlantic Ocean does not affect the weather of the United States because such an air mass tends to flow eastward toward Europe. However, on some occasions, there may be a reversal of the dominant
wind direction accompanying a low pressure system, and New England can be made miserable by the cool, damp winds, rain, and snow
of a weather system called a nor’easter. A nor’easter may, at times,
bring serious winter storm conditions to our New England states.
Maritime Tropical Air Masses The Gulf of Mexico and
subtropical Atlantic and Pacific Oceans serve as source regions for
mT air masses that have a great influence on the weather of the
United States and at times southeastern Canada. During winter, the
waters are warm, and the air above is warm, and moist. As the warm,
moist air moves northward up the Mississippi lowlands, it travels over
increasingly cooler land surfaces. The lower layers of air are chilled,
and dense advection fog often results. When it reaches the cP air
migrating southward from Canada, the warm mT air is forced to rise
over the colder, drier cP air, and significant precipitation can occur.
The longer days and more intense insolation of summer
months modify an mT air mass at the source region by increasing
its temperature and moisture content. However, during summer,
the land is warmer than the nearby waters, and as the mT air mass
moves onto the land, the instability of the air mass increases. This
air mass is a factor in the formation of great thunderstorms and
convective precipitation on hot, humid days, and it is also responsible for much of the hot, humid weather of the southeastern and
eastern United States.
Maritime tropical air masses also form over the Pacific Ocean
in the subtropical latitudes. These air masses tend to be slightly
cooler than those that form over the Gulf of Mexico and the
Atlantic, partly because of their passage over the cooler California
Current. A Pacific mT air mass is also more stable because of
the strong subsidence associated with the eastern portion of the
Pacific subtropical high. This air mass contributes to the dry summers of Southern California and occasionally brings moisture in
winter as it rises over the mountains of the Pacific Coast.
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Continental Tropical Air Masses A fifth type of air
mass may affect North America, but it is the least important to
the weather of the United States and Canada. This is the cT air
mass that develops over large, homogeneous land surfaces in the
subtropics. The Sahara Desert of North Africa is a prime example
of a source region for this type of air mass. The weather typical
of the cT air mass is usually very hot and dry, with clear skies and
major heating from the sun during daytime.
In North America, there is little land in the correct latitudes
to serve as a source region for a cT air mass of any significant
proportion. A small cT air mass can form over the deserts of the
southwestern United States and central Mexico in the summer. In
the source region, a cT air mass provides hot, dry, clear weather.
When it moves eastward, however, it is usually greatly modified as
it comes in contact with larger and stronger air masses of different
temperature, humidity, and density values. At times, cT air from
Mexico and Texas meets with mT air from the Gulf of Mexico.
This boundary is known as a dry line. Here, the drier air is denser
and will lift the moister air over it. This mechanism of uplift may
act as a trigger for precipitation episodes and perhaps thunderstorm activity.
Fronts
We have seen that air masses migrate with the general circulation
of the atmosphere. Over the United States, which is influenced
primarily by the westerlies, there is a general eastward flow of the
air masses. In addition, air masses tend to diverge from areas of
high pressure and converge toward areas of low pressure. This tendency means that the tropical and polar air masses, formed within
systems of divergence, tend to flow toward areas of convergence
within the United States. As previously noted, an important feature of an air mass is that it maintains the primary characteristics first imparted to it by its source region, although some slight
modification may occur during its migration.
When air masses differ, they do so primarily in their temperature and in their moisture content, which in turn affect the air
masses’ density and atmospheric pressure. As we saw in Chapter 6,
when different air masses come together, they do not mix easily
but instead come in contact along sloping boundaries called fronts.
Although usually depicted on maps as a one-dimensional boundary line separating two different air masses, a front is actually a
three-dimensional surface with length, width, and height. To emphasize this concept, a front is sometimes referred to as a surface
of discontinuity. This surface of discontinuity is a zone that can
cover an area from 2 to 3 kilometers (1–2 mi) wide to as wide as
150 kilometers (90 mi). Hence, it is more accurate to speak of a
frontal zone rather than a frontal line.
The sloping surface of a front is created as the warmer and
lighter of the two contrasting air masses is lifted or rises above the
cooler and denser air mass. Such rising, known as frontal uplift, is a
major source of precipitation in middle-latitude countries like the
United States and Canada (as well as middle-latitude European
and Asian countries) where contrasting air masses are most likely
to converge.
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FRONTS
Cold Front
The steepness of the frontal surface is governed primarily by
the degree of difference between the two converging air masses.
When there is a sharp difference between the two air masses, as
when an mT air mass of high temperature and moisture content
meets a cP air mass with its cold, dry characteristics, the slope of the
frontal surface will be steep.With a steep slope, there will be greater
frontal uplift. Provided other conditions (for example, temperature
and moisture content) are equal, a steep slope with its greater frontal
uplift will produce heavier precipitation than will a gentler slope.
Fronts are also differentiated by determining whether the
colder air mass is moving in on the warmer one, or vice versa.
The weather that occurs along a front also depends on which air
mass is the “aggressor.”
● FIGURE
A cold front occurs when a cold air mass actively moves in on a
warmer air mass and pushes it upward. The colder air, denser and
heavier than the warm air it is displacing, stays at the surface while
forcing the warmer air to rise. As we can see in ● Figure 7.2, a cold
front usually results in a relatively steep slope in which the warm
air may rise 1 meter vertically for every 40–80 meters of horizontal distance. If the warm air mass is unstable and has a high moisture content, heavy precipitation can result, sometimes in the form
of violent thunderstorms. A squall line may result when several
storms align themselves on (or in advance of) a cold front. In any
case, cold fronts are usually associated with strong weather disturbances or sharp changes in temperature, air
pressure, and wind.
7.2
Cross section of a cold front. Cold fronts generally move rapidly, with a blunt forward edge that
drives adjacent warmer air upward. This can produce violent precipitation from the warmer air.
Cirrus
anvil
top
Warm air
Cold fr
ont s
urfac
e
mass
Cumulonimbus
Cold air
mass
Cumulus
● FIGURE
7.3
Cross section of a warm front. Warm fronts advance more slowly than cold fronts and replace
rather than displace cold air by sliding upward over it. The gentle rise of the warm air produces
stratus clouds and gentle rain.
Compare Figures 7.2 and 7.3. How are they different? How are they similar?
Warm air mass
Cirrus
Cirrostratus
Altostratus
rm
Wa
t
fron
e
fac
sur
Cool air mass
Nimbostratus
Stratus
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Warm Front
When a warmer air mass is the aggressor
and invades a region occupied by a colder
air mass, a warm front results. At a warm
front, the warmer air, as it slowly pushes
against the cold air, also rises over the colder,
denser air mass, which again stays in contact
with Earth’s surface.The slope of the surface
of discontinuity that results is usually far
gentler than that occurring in a cold front.
In fact, the warm air may rise only 1 meter
vertically for every 100 or even 200 meters
of horizontal distance. Thus, the frontal uplift that develops will not be as great as that
occurring along a cold front. The result is
that the warmer weather associated with the
passage of a warm front tends to be less violent and the changes less abrupt than those
associated with cold fronts.
If we look at ● Figure 7.3, we can see
why the advancing warm front affects the
weather of areas ahead of the actual surface
location of the frontal zone. Changes in the
weather from approaching fronts can sometimes be indicated by the series of cloud
types that precede them.
Stationary and Occluded
Fronts
When two air masses have converged and
formed a frontal boundary but then neither moves, we have a situation known as
a quasi-stationary or, as it is more commonly
called, a stationary front. Locations under
the influence of a stationary front are apt
to experience clouds, drizzle, and rain (or
possible thunderstorms) for several days. In
fact, a stationary front and its accompanying
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Cold front
Warm front
Stationary
front
● FIGURE
Occluded
front
7.4
The four major frontal symbols used on weather maps.
weather will remain until either the contrasts between the two
air masses are reduced or the circulation of the atmosphere finally
causes one of the air masses to move. If a stationary front holds a
position for a length of time, then regional flooding is likely to
occur.
An occluded front occurs when a faster-moving cold front
overtakes a warm front, pushing all of the warm air aloft.This frontal situation usually occurs in the latter stages of a middle-latitude
cyclone, which will be discussed next. Map symbols for the four
frontal types are shown in ● Figure 7.4.
Atmospheric Disturbances
Middle Latitude Cyclones
Embedded within the wind belts of the general atmospheric
circulation (see Chapter 5) are secondary circulations. These are
made up of storms and other atmospheric disturbances. We use
the term atmospheric disturbance because it is more general
than a storm and includes variations in the secondary circulation
of the atmosphere that cannot be correctly classified as storms.
Partly because our primary interest is in the weather of North
American, we concentrate on an examination of middle-latitude
cyclones, sometimes known as extratropical cyclones.
Shortly after World War I, Norwegian meteorologists Jacob
Bjerknes and Halvor Solberg put forth the polar front theory, which
provided insight into the development, movement, and dissipation
of middle-latitude storms. They recognized the middle latitudes as
an area of convergence where unlike air masses, such as cold polar air and warm subtropical air, commonly meet at a boundary
called the polar front. Though the polar front may be a continuous
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boundary circling the entire globe, it is most often fragmented
into several individual line segments. Furthermore, the polar front
tends to move north and south with the seasons and is apt to be
stronger in winter than in summer. It is along this wavy polar
front that the upper air westerlies (see again Figs. 5.16 and 5.17),
also known as the polar front jet stream, develop and flow.
Middle-latitude storms develop at the front and then travel
along it. These migrating storms, with their opposing cold, dry
polar air and warm, humid tropical air, can cause significant variation in the day-to-day weather of the locations over which they
pass. It is not unusual in some parts of the United States and
Canada for people to go to bed at the end of a beautiful warm
day in early spring and wake up to falling snow the next morning.
Such variability is common for middle-latitude weather, especially during certain times of the year when the weather changes
from a period of cold, clear, dry days to a period of snow, only to
be followed by one or two more moderate but humid days.
Cyclones and Anticyclones
Nature, Size, and Appearance on Maps We have
previously distinguished cyclones and anticyclones according to
differences in pressure and wind direction. Also, when studying
maps of world pressure distribution, we identified large areas of
semipermanent cyclonic and anticyclonic circulation in Earth’s
atmosphere (the subtropical high, for example). Now, when examining middle-latitude atmospheric disturbances, we use the
terms cyclone and anticyclone to describe the moving cells of low
and high pressure, respectively, that drift with varying regularity
in the path of the prevailing westerly winds. As systems of higher
pressure, anticyclones are usually characterized by clear skies,
gentle winds, and a general lack of precipitation. As centers for
converging, rising air, cyclones create the storms of the middle
latitudes, with associated fronts of various types.
As we know from experience, no middle-latitude cyclonic
storm is ever exactly like any other. The storms vary in their intensity, their longevity, their speed, the strength of their winds,
their amount and type of cloud cover, the quantity and kind of
their precipitation, and the surface area they affect.
Because there are an endless variety of cyclones, we describe
“model cyclones” in the following discussions. Not every storm
will act in the way we describe, but certain generalizations are
helpful in understanding middle-latitude cyclones.
A cyclone has a low pressure center; thus, winds tend to converge toward that center in an attempt to equalize pressure. If we
visualize air moving in toward the center of the low pressure system, we can see that the air that is already at the center must be
displaced upward. Incoming mT air spirals upward, and the lifting
(convergence uplift also known as cyclonic uplift) that occurs in a
cyclone results in clouds and precipitation.
Anticyclones are high pressure systems in which atmospheric
pressure decreases toward the outer limits of the system.Visualizing an anticyclone, or high, we can see that air in the center of the
system must be subsiding, in turn displacing surface air outward,
away from the center of the system. Hence, an anticyclone has
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AT M O S P H E R I C D I S T U R B A N C E S
sure decreases toward the center, and in an anticyclone, pressure increases toward the center. Furthermore, the intensities of the winds
involved in these systems depend on the steepness of the pressure
gradients (the change in pressure over a horizontal distance) involved.
Thus, if there is a steep pressure gradient in a cyclone, with the pressure much lower at the center than at the outer
portions of the system, the winds will converge
● FIGURE 7.5
toward the center with considerable velocity.
The horizontal and vertical structure of pressure systems. Close spacing of isobars around a
The situation is easier to visualize if we
cyclone or anticyclone indicates a steep pressure gradient that will produce strong winds. Wide
imagine these pressure systems as landforms.
spacing of isobars indicates a weaker system.
A cyclone is shaped like a basin ( ● Fig. 7.5). If
Where would be the strongest winds in this figure? Where would be the weakest winds?
we are filling the basin with water, we know
that the water will flow in faster the steeper
Cyclone
Anticyclone
the sides and the deeper the depression. If we
visualize an anticyclone as a hill or mountain,
then we can also see that just as water flowing
down the sides of such landforms will flow
faster with increased height and steepness, so
Low
High
A
B
will the air blowing out of an area of very
high pressure move rapidly.
On a surface weather map, cyclones and
b
m
anticyclones
are depicted by concentric iso0
2
10
bars of increasing pressure toward the center
16
of a high and of decreasing pressure toward
10
the center of a low. Usually a high will cover
2
1
10
a larger area than a low, but both pressure
08
systems are capable of covering and affecting
0
1
04
extensive areas. There are times when nearly
10
Low
High
the entire midwestern United States is under
A
B
the influence of the same system.The average
diameter of an anticyclone is about 1500 kilometers (900 mi); that of a cyclone is about
1000 kilometers (600 mi).
10
32
10
28
10
24
diverging winds. In addition, an anticyclone tends to be a fairweather system; the subsiding air in its center increases in temperature and stability, reducing the opportunity for condensation.
We should note here that the pressures we are referring to in these
two systems are relative. What is important is that in a cyclone, pres-
General Movement The cyclones and
● FIGURE
7.6
Common storm tracks for the United States. Virtually all cyclonic storms move from west to
east in the prevailing westerlies and swing northeastward across the Atlantic coast. Storm
tracks originating in the Gulf of Mexico represent tropical hurricanes.
What storm tracks influence your location?
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anticyclones of the middle latitudes are steered,
or guided, along a path reflecting the configuration and speed of the upper air westerlies
(or the jet stream). The upper air flow can be
quite variable with wild oscillations. However,
a general west-to-east pattern does prevail.
Consequently, people in most of the eastern
United States look at the weather occurring
to the west to see what they might expect in
the next few days. Most storms that develop in
the Great Plains or Far West move across the
United States during a period of a few days at
an average speed of about 36 kilometers per
hour (23 mph) and then travel on into the
North Atlantic before occluding.
Although neither cyclones nor anticyclones develop in exactly the same places
at the same times each year, they do tend
to develop in certain areas or regions more
frequently than in others. They also follow
the same general paths, known as stor m
tracks ( ● Fig. 7.6). These storm tracks vary
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with the seasons. In addition, because the temperature variations
between the air masses are stronger during the winter months,
the atmospheric disturbances that develop in the middle latitudes
during those months are greater in number and intensity.
As the contrasting air masses jockey for position, the clouds
and precipitation that exist along the fronts are greatly intensified, and the area affected by the storm is much greater. Along the
warm front, precipitation will be more widespread but less intense
than along the cold front. One factor that can vary the kind of precipitation occurring at the warm front is the stability of the warm
air mass. If it is stable, then its uplifting over the cold air mass may
cause only a fine drizzle or a light, powdery snow if the temperatures are cold enough. On the other hand, if the warm air mass is
moist and unstable, the uplifting may set off heavier precipitation.
As you can see by referring again to Figure 7.3, the precipitation
that falls at the warm front may appear to be coming from the
colder air. Though weather may feel cold and damp, the precipitation is actually coming from the overriding warmer air mass above,
then falling through the colder air mass to reach Earth’s surface.
Because a cold front usually moves faster, it will eventually
overtake the warm front. This produces the situation we previously identified as an occluded front. Because additional warm,
moist air will not be lifted after occlusion, condensation and the
release of latent heat energy will diminish, and the system will
soon die. Occlusions are usually accompanied by rain and are the
major process by which middle-latitude cyclones dissipate.
Cyclones Now let’s look more closely at cyclones—their origin, development, and characteristics. Warm and cold air masses
meet at the polar front where most cyclones develop. These two
contrasting air masses do not merge but may move in opposite
directions along the frontal zone. Although there may be some
slight uplift of the warmer air along the edge of the denser, colder
air, the uplift will not be significant. There may be some cloudiness and precipitation along such a frontal zone, though not of
storm caliber.
For reasons not completely understood but certainly related
to the wind flow in the upper troposphere, a wavelike kink may
develop along the polar front. This is the initial step in the formation of a fully grown middle-latitude cyclone ( ● Fig. 7.7). At this
bend in the polar front, we now have warm air pushing poleward
(a warm front) and cold air pushing equatorward (a cold front),
with a center of low pressure at the location where the two fronts
are joined.
● FIGURE
7.7
Environmental Systems: Middle-Latitude Cyclonic Systems Stages in the development of a middlelatitude cyclone. Each view represents the development somewhat eastward of the preceding view as the
cyclone travels along its storm track. Note the occlusion in (e).
In (c), where would you expect rain to develop? Why?
Cold air
Cold air
Cold air
L
ld
Co
Cool air
L
t
ron
Wa
rm
f
fron
t
Front
W
ar
nt
old
fro
C
m
fro
nt
Warm air
Warm air
(a)
Warm air
(b)
(c)
Cool air
Cool air
L
Cold air
Cold air
L
Cool air
ar
W
m
fro
nt
nt
nt
ro
df
l
Co
ld
Co
Warm air
(d)
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fro
W
ar
Front
m
fro
nt
Warm air
Warm air
(e)
(f)
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AT M O S P H E R I C D I S T U R B A N C E S
Cyclones and Local Weather Different parts of a
reference, where appropriate, to the differences that occur in Detroit as the cyclonic system moves east.
As we have previously stated, atmospheric temperature and
pressure are closely related. As temperature increases, air expands
and pressure decreases. Therefore, these two elements are discussed together. Because a cyclonic storm is composed of two
dissimilar air masses, there are usually significant temperature
contrasts. The sector of warm, humid mT air between the two
fronts of the cyclone is usually considerably warmer than the
cold cP air surrounding it. The temperature contrast is accentuated in the winter when the source region for cP air is the cold
cell of high pressure normally found in Canada at that time of
year. During the summer, the contrast between these air masses is
greatly reduced.
As a consequence of the temperature difference, the atmospheric pressure in the warm sector is considerably lower than
the atmospheric pressure in the cold air behind the cold front. Far
in advance of the warm front, the pressure is also high, but as the
warm front (see again Figure 7.3) approaches, increasingly more
cold air is replaced by uplifted warm air, thus steadily reducing
the surface pressure.
middle-latitude cyclone exhibit different weather. Therefore, the
weather that a location experiences at a particular time depends
on which portion of the middle-latitude cyclone is over the location. Also, because the entire cyclonic system tends to travel from
west to east, a specific sequence of weather can be expected at a
given location as the cyclone passes over that location.
Let’s assume that it is late spring. A cyclonic storm has originated in the southeast corner of Nebraska and is following a track
(see again Figure 7.6) across northern Illinois, northern Indiana,
northern Ohio, through Pennsylvania, and finally out over the
Atlantic Ocean. A view of this storm on a weather map, at a specific time in its journey, is presented in ● Figure 7.8a. Figure 7.8b
shows a cross-sectional view north of the center of the cyclone,
and Figure 7.8c shows a cross-sectional view south of the center
of the cyclone. As the storm continues eastward, at 33–50 kilometers per hour (20–30 mph), the sequence of weather will be
different for Detroit, where the warm and cold fronts will pass
just to the south, than for Pittsburgh, where both fronts will pass
overhead. To illustrate this point, let’s examine, element by element, the variation in weather that will occur in Pittsburgh, with
● FIGURE
7.8
Environmental Systems: Middle-Latitude Cyclonic Systems This diagram models a middle-latitude
cyclone positioned over the Midwest as the system moves eastward: (a) a map view of the weather system;
(b) a cross section along line AB north of the center of low pressure; (c) a cross section along line CD south of
the center of low pressure.
Cold sector
(cP)
Detroit
Cool sector
(cP)
A
Des Moines
B
Chicago L
D
C
St. Louis
Indianapolis
Cold front
(a)
Discontinuity
surface
Warm front
Warm air
Cold air
Des Moines
(b)
Discontinuity
surface
Columbus
Warm sector
(mT)
Precipitation
A
Pittsburgh
Cold air
Chicago
B
Detroit
Discontinuity
surface
Warm air
Cold air
C
(c)
St. Louis
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Indianapolis
Columbus
D
Pittsburgh
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Therefore, as the warm front of this late-spring cyclonic storm
approaches Pittsburgh, the pressure will decrease. After the warm
front passes through Pittsburgh where the temperature may have
been 8°C (46°F) or more, the pressure will stop falling, and the
temperature may rise up to 18°–20°C (64°F–69°F) as mT air invades the area. At this point, Indianapolis has already experienced
the passage of the warm front. After the cold front passes, the pressure will rise rapidly and the temperature will drop. In this latespring storm, the cP air temperature behind the cold front might be
2°C–5°C (35°F–40°F). Detroit, which is to the north of the center
of the cyclone, will miss the warm air sector entirely and therefore will experience a slight increase in pressure and a temperature
change from cool to cold as the cyclone moves to the east.
Changes in wind direction are one signal of the approach and
passing of a cyclonic storm. Because a cyclone is a center of low
pressure, winds flow counterclockwise toward its center. Also, winds
are caused by differences in pressure. Therefore, the winds associated with a cyclonic storm are stronger in winter when the pressure (and temperature) differences between air masses are greatest.
In our example, Pittsburgh is located to the south and east
of the center of low pressure and ahead of the warm front, and it
is experiencing winds from the southeast. As the entire cyclonic
system moves east, the winds in Pittsburgh will shift to the southsouthwest after the warm front passes. Indianapolis is currently in
this position. After the cold front passes, the winds in Pittsburgh
will be out of the north-northwest. St. Louis has already experienced the passage of the cold front and currently has winds from
the northwest. The changing direction of wind, clockwise around
the compass from east to southeast to south to southwest to west
and northwest, is called a veering wind shift and indicates that
your position is south of the center of a low. On the other hand,
Detroit, which is also experiencing winds from the southeast,
will undergo a completely different sequence of directional wind
changes as the cyclonic storm moves eastward. Detroit’s winds
will shift to the northeast as the center of the storm passes to
the south. Chicago has just undergone this shift. Finally, after the
storm has passed, the winds will blow from the northwest. Des
Moines, to the west of the storm, currently has northwest winds.
Such a change of wind direction, from east to northeast to north
to northwest, is called a backing wind shift, as the wind “backs”
counterclockwise around the compass. A backing wind shift indicates that you are north of the cyclone’s center.
The type and intensity of precipitation and cloud cover
also vary as a cyclonic disturbance moves through a location. In
Pittsburgh, the first sign of the approaching warm front will be
high cirrus clouds. As the warm front continues to approach, the
clouds will thicken and lower. When the warm front is within
150–300 kilometers (90–180 mi) of Pittsburgh, light rain and
drizzle may begin, and stratus clouds will blanket the sky.
After the warm front has passed, precipitation will stop and
the skies will clear. However, if the warm, moist mT air is unstable, convective showers may result, especially during the spring
and summer months with their high afternoon sun angles.
As the cold front passes, warm air in its path will be forced to
move aloft rapidly. This may mean that there will be a cold, hard
rain, but the band of precipitation normally will not be very wide
because of the steep angle of the surface of discontinuity along a
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cold front. In our example, the cold front and the band of precipitation have just passed St. Louis.
Thus, Pittsburgh can expect three zones of precipitation as
the cyclonic system passes over its location: (1) a broad area of
cold showers and drizzle in advance of the warm front; (2) a zone
within the moist, subtropical air from the south where scattered
convectional showers can occur; and (3) a narrow band of hard
rainfall associated with the cold front (Fig. 7.8c). However, locations to the north of the center of the cyclonic storm, such as
Detroit, will usually experience a single, broad band of light rains
resulting from the lifting of warm air above cold air from the
north (Fig. 7.8b). In winter, the precipitation is likely to be snow,
especially in locations just to the northwest of the center of the
storm, where the humid mT air overlies extremely cold cP air.
As you can see, different portions of a middle-latitude cyclone
are accompanied by different weather. If we know where the cyclone will pass relative to our location, we can make a fairly accurate forecast of what our weather will be like as the storm moves
east along its track (see Map Interpretation: Weather Maps).
Cyclones and the Upper Air Flow The upper air
wind flow greatly influences our surface weather. We have already
discussed the role of these upper air winds in the steering of surface storm systems. Another less obvious influence of the upper air
flow is related to the undulating, wavelike flow so often exhibited
by the upper air. As the air moves its way through these waves, it
undergoes divergence or convergence because of the atmospheric
dynamics associated with curved flow. This upper air convergence
and divergence greatly influence the surface storms below.
The region between a ridge and the next downwind trough
(A–B in ● Fig. 7.9) is an area of upper-level convergence. In our
atmosphere, an action taken in one part of the atmosphere is compensated for by an opposite reaction somewhere else. In this case, the
upper air convergence is compensated for by divergence at the surface.
In this area, anticyclonic circulation is promoted as the air is pushed
● FIGURE
7.9
Waves in the polar front jet stream. The upper air wind pattern, such as
that depicted here, can have a significant influence on temperatures and
precipitation on Earth’s surface.
Where would you expect storms to develop?
A
Polar
air
moves
South
C
Tropical
air
moves
North
B
Tropic of Cancer
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181
Dennis Laws/U.S. Navy/Fleet Numerical Meteorology & Oceanography Center
AT M O S P H E R I C D I S T U R B A N C E S
● FIGURE
7.10
Polar front jet stream analysis. This wind analysis map was produced at an altitude of
300 mb (approximately 10,000 m or 33,000 ft above sea level). At this height the
long waves of the jet stream can more easily be seen. In this true-winds depiction,
the troughs and ridges are not as smooth and regular as they are in theory. Knots
(kts), or nautical miles per hour, are a little faster than statute miles per hour.
Which country does most of this pattern occupy?
center of an anticyclone encourages stability as the air is
warmed adiabatically while sinking toward the surface.
Consequently, the air can hold additional moisture as
its capacity increases with increasing temperatures. The
weather resulting from the influence of an anticyclone is
often clear, with no rainfall. There are, however, certain
conditions under which there can be some precipitation within a high pressure system. When such a system
passes near or crosses a large body of water, the resulting
evaporation can cause variations in humidity significant
enough to result in some precipitation.
There are two sources for the relatively high pressures that are associated with anticyclones in the middle
latitudes of North America. Some anticyclones move
into the middle latitudes form northern Canada and the
Arctic Ocean in what are called outbreaks of cold Arctic air. These outbreaks can be quite extensive, covering
much of the midwestern and eastern United States. The
temperatures in an anticyclone that has developed in a
cA air mass can be markedly lower than those expected
for any given time of year. They may be far below freezing in the winter. Other anticyclones are generated in
zones of high pressure in the subtropics.When they move
across the United States toward the north and northeast,
they bring waves of hot, clear weather in summer and
unseasonably warm days in the winter months.
Anticyclones Just as cyclones are centers of low
pressure that are typified by the convergence and uplift
of air, so anticyclones are cells of high pressure in which
air descends and diverges. The subsidence of air in the
55061_07_Ch07_p170_197 pp3.indd 181
NOAA 7.15: CIMSS at UW-Madison, produced by the Tropical Cyclones team
downward. This pattern will inhibit the formation of a middleHurricanes
latitude storm or cause an existing storm to weaken or even dissipate.
On the other hand, the region between a trough and the next downThough their overall diameter may be less than that of a middle latitude
wind ridge (B–C in Fig. 7.9) is an area of upper-level divergence,
cyclone with its extended fronts, hurricanes are essentially the largest
which in turn is compensated for by surface convergence. This is an
storms on Earth. Hurricanes are severe tropical cyclones that receive a
area where air is drawn upward and cyclonic circulation is encourgreat deal of attention from scientists and laypeople alike, primarily
aged. Convergence at the surface will certainly enhance the prospects
because of their tremendous destructive powers ( ● Fig. 7.11).
of storm development or strengthen an already existing storm.
In addition to storm development or dissipation, upper
● FIGURE 7.11
air flow will have an impact on temperatures as well. If we
Damage incurred by Hurricane Andrew in 1992. Until the hurricanes of 2004
assume that our “average” upper air flow is from west to east,
and 2005, the damage by Hurricane Andrew, as shown in this photo, was the
then any deviation from that pattern will cause either colder
costliest in U.S. history.
air from the north or warmer air from the south to be advected into an area. Thus, after the atmosphere has been in a
wavelike pattern for a few days, the areas in the vicinity of a
trough (area B in Fig. 7.9) will be colder than normal as polar air from higher latitudes is brought into that area. Just the
opposite occurs at locations near a ridge (area C in Fig. 7.9).
In this case, warmer air from more southerly latitudes than
would be the case with west-to-east flow is advected into
the area near the ridge. ● Figure 7.10 shows that in reality,
the jet stream curves with less regularity. Comparing Figures
7.9 and 7.10 you can see the difference between the theoretical and the real waves in the polar front jet stream.
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