55061 07 ch07 p170 197 pp3 - cengage · maritime polar air masses during winter months, the oceans...

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 S174

Coast of the United States. When such an air mass does reach the Washington, Oregon, and California coasts, it brings with it un-usual 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 wa-ter and collects heat and moisture. Thus, mP air is cold (although not as cold as cP air) and damp, with a tendency toward instabil-ity. 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 af-ter 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 be-cause such an air mass tends to flow eastward toward Europe. How-ever, on some occasions, there may be a reversal of the dominant wind direction accompanying a low pressure system, and New Eng-land 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 respon-sible 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 sum-mers of Southern California and occasionally brings moisture in winter as it rises over the mountains of the Pacific Coast.

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 thunder-storm activity.

FrontsWe 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 ten-dency 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 fea-ture of an air mass is that it maintains the primary characteris-tics 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 temper-ature 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 bound-ary line separating two different air masses, a front is actually a three-dimensional surface with length, width, and height. To em-phasize 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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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.”

Cold FrontA 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 horizon-tal distance. If the warm air mass is unstable and has a high mois-ture 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 distur-

bances or sharp changes in temperature, air pressure, and wind.

Warm FrontWhen 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 up-lift 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 vio-lent 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 some-times be indicated by the series of cloud types that precede them.

Stationary and Occluded FrontsWhen two air masses have converged and formed a frontal boundary but then nei-ther 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

Warm air

Cumulonimbus

Cumulus

Cirrus anvil top

mass

Cold air

mass

Cold front surface

● FIGURE 7.2Cross 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.

Warm air mass

Warm front surface

Nimbostratus

Stratus

Altostratus

Cirrostratus

Cirrus

Cool air mass

● FIGURE 7.3Cross 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?

F R O N T S



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 fron-tal 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 CyclonesEmbedded 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 po-lar air and warm subtropical air, commonly meet at a boundary called the polar front. Though the polar front may be a continuous

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 varia-tion 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, espe-cially 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 ex-amining 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 in-tensity, 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 con-verge toward that center in an attempt to equalize pressure. If we visualize air moving in toward the center of the low pressure sys-tem, 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. Visualiz-ing 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

Cold front

Stationaryfront

Warm front

Occludedfront

● FIGURE 7.4The four major frontal symbols used on weather maps.


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diverging winds. In addition, an anticyclone tends to be a fair-weather system; the subsiding air in its center increases in tem-perature 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-

sure decreases toward the center, and in an anticyclone, pressure in-creases 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 pres-

sure much lower at the center than at the outer portions of the system, the winds will converge toward the center with considerable velocity.

The situation is easier to visualize if we imagine these pressure systems as landforms. A cyclone is shaped like a basin ( ● Fig. 7.5). If we are filling the basin with water, we know that the water will flow in faster the steeper 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 will the air blowing out of an area of very high pressure move rapidly.

On a surface weather map, cyclones and anticyclones are depicted by concentric iso-bars of increasing pressure toward the center of a high and of decreasing pressure toward the center of a low. Usually a high will cover a larger area than a low, but both pressure systems are capable of covering and affecting extensive areas. There are times when nearly the entire midwestern United States is under the influence of the same system. The average diameter of an anticyclone is about 1500 ki-lometers (900 mi); that of a cyclone is about 1000 kilometers (600 mi).

General Movement The cyclones and anticyclones of the middle latitudes are steered, or guided, along a path reflecting the config-uration 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 anticy-clones 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 storm tracks ( ● Fig. 7.6). These storm tracks vary

Cyclone Anticyclone

Low High A B

1016

1012

1008

1004

Low High

1020 mb

1024

10

28

1032

A B

● FIGURE 7.5The horizontal and vertical structure of pressure systems. Close spacing of isobars around a cyclone or anticyclone indicates a steep pressure gradient that will produce strong winds. Wide spacing of isobars indicates a weaker system.Where would be the strongest winds in this figure? Where would be the weakest winds?

● FIGURE 7.6Common 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?

AT M O S P H E R I C D I S T U R B A N C E S



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.

Cyclones Now let’s look more closely at cyclones—their ori-gin, 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 cloudi-ness 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 forma-tion 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.

As the contrasting air masses jockey for position, the clouds and precipitation that exist along the fronts are greatly intensi-fied, 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 pre-cipitation 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 tempera-tures 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 precipita-tion 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 previ-ously 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.

● FIGURE 7.7Environmental Systems: Middle-Latitude Cyclonic Systems Stages in the development of a middle-latitude 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 airCold air

Cool air

Cold front

Cold fr

ont

Cool air

Cool air

Cold air Cold air

Cool airL L

LL

Front

Front

Cold front

Cold front

Warm front

Warm

front Warm

front

Warm front

Warm air

Warm airWarm air

Warm air

Warm air

Warm air

(a) (b) (c)

(d) (e) (f)


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Cyclones and Local Weather Different parts of a 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 loca-tion. 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 origi-nated 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 spe-cific time in its journey, is presented in ● Figure 7.8a. Figure 7.8bshows 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 kilo-meters 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 ele-ment, the variation in weather that will occur in Pittsburgh, with

reference, where appropriate, to the differences that occur in De-troit 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 dis-cussed 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 accentu-ated 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 atmo-spheric 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.

● FIGURE 7.8Environmental 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.

(a)

(b)

(c)

Cold sector(cP)

A

Des Moines

Cool sector(cP)

D

C

B

C

Cold front

Precipitation

Cold air Warm air

Chicago

Detroit

D

B

PittsburghColumbusIndianapolisSt. Louis

Cold airWarm air

Discontinuitysurface


Cold airDes Moines


Pittsburgh

Detroit

Chicago L

ColumbusIndianapolisSt. Louis

Warm sector(mT)

Warm front

A




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 in-vades the area. At this point, Indianapolis has already experienced the passage of the warm front. After the cold front passes, the pres-sure will rise rapidly and the temperature will drop. In this late-spring 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 there-fore 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 associ-ated with a cyclonic storm are stronger in winter when the pres-sure (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 south-southwest 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 experi-enced 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 indi-cates 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 un-stable, 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

cold front. In our example, the cold front and the band of precipi-tation 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, loca-tions 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 cy-clone will pass relative to our location, we can make a fairly accu-rate 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 sur-face 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 com-pensated 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

C

B

A

Tropic of Cancer

Polarair

movesSouth

Tropicalair

movesNorth

● FIGURE 7.9Waves 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?


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downward. This pattern will inhibit the formation of a middle-latitude storm or cause an existing storm to weaken or even dissipate. On the other hand, the region between a trough and the next down-wind ridge (B–C in Fig. 7.9) is an area of upper-level divergence, which in turn is compensated for by surface convergence. This is an area where air is drawn upward and cyclonic circulation is encour-aged. Convergence at the surface will certainly enhance the prospects of storm development or strengthen an already existing storm.

In addition to storm development or dissipation, upper air flow will have an impact on temperatures as well. If we assume that our “average” upper air flow is from west to east, then any deviation from that pattern will cause either colder air from the north or warmer air from the south to be ad-vected 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 po-lar 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 theo-retical and the real waves in the polar front jet stream.

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

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 precipita-tion 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 pres-sures 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 Arc-tic 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 freez-ing 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.

HurricanesThough their overall diameter may be less than that of a middle latitude cyclone with its extended fronts, hurricanes are essentially the largest storms on Earth. Hurricanes are severe tropical cyclones that receive a great deal of attention from scientists and laypeople alike, primarily because of their tremendous destructive powers ( ● Fig. 7.11).

● FIGURE 7.10Polar 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?

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● FIGURE 7.11Damage incurred by Hurricane Andrew in 1992. Until the hurricanes of 2004 and 2005, the damage by Hurricane Andrew, as shown in this photo, was the costliest in U.S. history.

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