Ch.8-Ch.11 MET3003 The Atmosphere an Introduction to Metereology

Given an upper-air chart, where do forecasters usually look to find favorable sites for cyclogenesis? Where do anticyclones usually form in relation to the wavy flow aloft?

low pressure regions--where cyclones are

What air mass and source region provide the greatest amount of moisture to the eastern and central United States?

mT

How does a stationary front produce precipitation when its position does not change or when it changes very slowly?

when stationary fronts start to move, they become warm fronts or cold fronts depending upon the mass of the air.

Ch.8 Summary

● Changes to the stability of an air mass can result from temperature differences between an air mass and the surface and/or vertical movements induced by cyclones, anticyclones, or topography. ● The day-to-day weather we experience depends on the temperature, stability, and moisture content of the air mass we are experiencing. Among the numerous weather phenomena associated with various air masses are lake-effect snows, nor'easters, and the Pineapple Express.

cold conveyor belt

(blue arrow) is airflow that starts at the surface ahead (north) of the warm front and flows westward toward the center of the cyclone (Figure 9-25). Flowing beneath the warm conveyor belt, this air is moist- ened by the evaporation of raindrops falling through it. (Near the Atlantic Ocean this conveyor belt has a marine origin and feeds significant moisture into the storm.) Con- vergence causes this air stream to rise as it nears the center of the cyclone. During its ascent, this air cools adiabatically, becomes saturated, and contributes to the cyclone's precip- itation. As the cold conveyor belt reaches the middle tro- posphere, some of the flow rotates cyclonically around the low to produce the distinctive "comma head" of the mature storm system (Figure 9-26). The remaining flow turns right (clockwise) and becomes incorporated into the general westerly flow. Here it parallels the flow of the warm con- veyor belt and may generate precipitation.

four locations where midlatitude cyclones that affect North America tend to form

1- east side of the rockies 2- north pacific 3- gulf of mexico 3- cape hatteras

Mechanisms that trigger the movement of warm humid air rising in an unstable environment (what causes thunderstorms)

1- unequal heating of Earth's surface, significantly contributes to the formation of air-mass thunderstorms 2- along a front or mountain slope 3- diverging winds aloft frequently contribute to the formation of these storms because they tend to draw air from lower levels upward beneath them. most occur in the tropics where moist warm air is most in US occur in eastern rockies and new mexico

List two reasons why cold-front weather is usually more severe than warm-front weather.

1- when air behind cold fronts move over the warm surface, radiation emitted from the earth can heat the air and produce shallow convection, which produce thunderstorms 2- cold fronts are steeper

Why do some thunderstorms persist for many hours?

A key factor is the existence of strong vertical wind shear—that is, changes in wind direction and/or speed at different heights. When such conditions prevail, the updrafts that provide the storm with mois- ture do not remain vertical but become tilted. Because of this, the precipitation that forms high in the cloud falls into the downdraft rather than into the updraft, as occurs in air- mass thunderstorms. This allows the updraft to maintain its strength and continue to build upward. Sometimes the updrafts are suffici- ently strong that the cloud top is able to push its way into the stable lower stratosphere, a situation called overshooting. Beneath the cumulonimbus tower, where downdrafts reach the surface, the denser cool air spreads out along the ground. The leading edge of this outflowing downdraft acts like a wedge, forcing warm, moist surface air into the thun- derstorm. In this way, the downdrafts act to maintain the updrafts, which in turn sustain the thunderstorm.

multiple-vortex tornadoes

A tornado may consist of a single vortex, but within many stronger tornadoes are smaller intense whirls called suction vortices that orbit the center of the larger tornado circulation. All tornadoes.

Ch. 9 Summary

According to the polar front theory, midlatitude cyclones form along fronts and proceed through a generally predictable life cycle. Along the polar front, where two air masses of different densities are moving parallel to the front and in opposite directions, cyclogenesis (cyclone formation) occurs, and the frontal surface takes on a wave shape that is usually several hundred kilometers long. Once a wave forms, warm air advances poleward, invading the area formerly occupied by colder air. This change in the direction of the surface flow causes a readjustment in the pressure pattern that results in somewhat circular isobars, with the low pressure centered at the apex of the wave. Usually, the cold front advances faster than the warm front and gradually closes the warm sector and lifts the warm front. This process is known as occlusion. Eventually, all of the warm sector is forced aloft, and cold air surrounds the cyclone at low levels. At this point, the cyclone has exhausted its source of energy and the once highly organized counterclockwise flow ceases to exist. Guided by the westerlies aloft, cyclones generally move eastward across the United States. Airflow aloft (divergence and convergence) plays an important role in maintaining cyclonic and anticyclonic circulation. In cyclones, divergence aloft supports the inward flow at the surface During the colder months, when temperature gradients are steepest, cyclonic storms move at their fastest rate. Furthermore, the westerly airflow aloft tends to steer these developing pressure systems in a general west-to-east direction.

Siberian express

After such an air mass forms over the frozen expanses near the Arctic Circle, the winds aloft sometimes direct it toward the south and east. When an outbreak takes place, it is popularly called the "Siberian Express" by the news media, even though the air mass did not originate in Siberia.

source regions

Areas in which air masses originate

development of cyclonic flow

As a wave evolves, warm air advances poleward to form a warm front, while cold air moves equatorward to form a cold front. This change in the direction of the surface flow is accompanied by a readjustment in the pressure pattern and results in somewhat circular isobars, with the low- est pressure located at the crest of the wave. The result- ing flow is an inward-directed, counterclockwise circula- tion. Once the cyclonic circulation develops, general convergence results in forceful lifting, especially where warm air is overrunning colder air. Because the warm air is moving perpendicular to the front, we can conclude that the warm air is invading a region formerly occupied by cold air. Therefore, this must be a warm front. Similar reasoning indicates that to the left (west) of the wave front, cold air from the northwest is displacing the air of the warm sector and generating a cold front. In addition, a strong jet stream in the flow aloft frequently precedes the formation of a surface cyclone. This fact strongly suggests that upper-level flow contributes to the formation of these rotating storm systems. However, when the upper air begins to meander widely from north to south, forming high-amplitude waves of alternating troughs (lows) and ridges (highs), cyclonic activity intensifies. Moreover, when surface cyclones form, almost invariably they are centered below the jet stream core and downwind from an upper-level low

mature stage

As the downdraft leaves the base of the cloud, precipitation is released, marking the beginning of the cloud's mature stage (see Figure 10-4b). At the surface the cool downdrafts spread laterally and can be felt before the actual precipitation reaches the ground. The sharp, cool gusts at the surface are indicative of the downdrafts aloft. During the mature stage, updrafts exist side by side with downdrafts and continue to enlarge the cloud. most active stage with heavy rain and possibly small hail, in which downdrafts are found side by side with updrafts.

return stroke

As the electrons at the lower end of the conductive path move earthward, electrons positioned successively higher up the channel begin to migrate downward. Because the path of electron flow is continually being extended upward, the accompanying electrical discharge has been appropri- ately named a return stroke.

Describe the modifications that occur as a cP air mass passes across a large ice-free lake in winter.

lake effect snow

summer

Because cA air is present principally in the win- ter, only cP air has any influence on our summer weather, and this effect is considerably reduced when compared with winter.

air-mass weather

Because it may take several days for an air mass to tra- verse an area, the region under its influence will probably ex- perience generally constant weather conditions

two criteria for source regions

Because the atmosphere is heated chiefly from below and gains its moisture by evaporation from Earth's surface, the nature of the source region largely determines the initial characteristics of an air mass. An ideal source region must meet two essential criteria. First, it must be an extensive and physically uniform area. A region having highly irregular topography or one that has a surface consisting of both water and land is not satisfactory. The second criterion is that the area be characterized by a general stagnation of atmospheric circulation so that air will stay over the region long enough to come to some mea- sure of equilibrium with the surface. In general, it means regions dominated by stationary or slow-moving anticy- clones, with their extensive areas of calm or light winds

isohyets

lines connecting places having equal rainfall

mature stage of a midlatitude cyclone

During the mature stage of a mid- latitude cyclone the pressure sur- rounding the low continues to drop, causing winds to strengthen and frontal weather to develop.

mT

Eastern subtropical Pacific Warm and humid entire year Stable entire year a. In winter it brings fog, drizzle, and occasional moderate precipitation to northwestern Mexico and the southwestern United States b. In summer occasionally reaches the western United States and is a source of moisture for infrequent convectional thunderstorms

mesocyclone

For example, this could occur if the surface flow is from the south or southeast and the winds aloft increase in speed and become more west- erly with height. If a thunderstorm develops in such a wind environment, the updraft is made to rotate. It is within this column of cyclonically rotating air, called the mesocyclone, that tornadoes often form

mT (maritime tropical)

Gulf of Mexico, Caribbean Sea, western Atlantic Warm and humid entire year Unstable entire year a. In winter it usually becomes mTw, moving northward and bringing occasional widespread precipitation or advection fog b. In summer hot and humid conditions, frequent cumulus development, and showers or thunderstorms

polar front theory (norwegian cyclone model)

In 1921 the work of these scientists resulted in a publication outlining a compelling model of how midlatitude cyclones progress through stages of birth, growth, and decay. These insights, which marked a turning point in atmospheric science, became known as the polar- front theory—also referred to as the Norwegian cyclone model.According to the Norwegian model, cyclones form along fronts and proceed through a generally predictable life cycle. This cycle can last from a few days to more than a week, depending on atmospheric conditions.

How might vertical movements induced by a pressure system or topography act to modify an air mass?

In addition to modifications resulting from tempera- ture differences between an air mass and the surface below, upward and downward movements induced by cyclones and anticyclones or topography can also affect the stability of an air mass. Such modifications are often called mechanical or dynamic and are usually independent of the changes caused by surface cooling or heating. For example, significant modification can result when an air mass is drawn into a low. Here convergence and lifting dominate and the air mass is rendered more unstable. Conversely, the subsidence associated with anticyclones acts to stabilize an air mass. Similar alterations in stabil- ity occur when an air mass is lifted over highlands or de- scends the leeward side of a mountain barrier. In the first case, the air's stability is reduced; in the second case, the air becomes more stable.

case studies

In summary, this example demonstrates the effect of a spring cyclone on midlatitude weather. Within three days, Fort Worth, Texas, temperatures changed from warm to unseasonably cold. Thunderstorms with hail were followed by cold clear skies. You can see how the north-south temperature gradient, which is most pronounced in the spring, generates these intense storms. Recall that it is the role of these storms to transfer heat from the tropics poleward. Because of Earth's rotation, however, this latitudinal heat exchange is complex. If Earth rotated more slowly, a more leisurely north-south flow would exist that might reduce the temperature gradient, in which case the midlatitudes would not experience stormy weather.

cP

Interior Canada and Alaska Very cold and dry in winter Stable entire year a. Cold waves in winter b. Modified to cPk in winter over Great Lakes, bringing lake-effect snow to leeward shores

lake effect snow

Lake-effect storms account for a high percentage of the snowfall in many areas adjacent to the lakes. The strips of land that are most frequently affected, called snowbelts, are shown in Figure 8-5What causes lake-effect snow? The answer is closely linked to the differential heating of water and land (Chap- ter 3) and to the concept of atmospheric instability (Chapter 4). During the summer months, bodies of water, including the Great Lakes, absorb huge quantities of energy from the Sun and from the warm air that passes over them. Although these water bodies do not reach particularly high tempera- tures, they nevertheless represent huge reservoirs of heat. The surrounding land, in contrast, cannot store heat nearly as effectively. Consequently, during autumn and winter, the temperature of the land drops quickly, whereas water bod- ies lose their heat more gradually and cool slowly.From late November through late January the con- trasts in average temperatures between water and land range from about 8°C in the southern Great Lakes to 17°C farther north. However, the temperature differences can be much greater (perhaps 25°C) when a very cold cP or cA air mass pushes southward across the lakes. When such a dramatic temperature contrast exists, the lakes in- teract with the air to produce major lake-effect stormsDuring its journey, the air acquires large quantities of heat and moisture from the relatively warm lake surface. By the time it reaches the opposite shore, this cPk air is humid and unstable, and heavy snow showers are likely.

backdoor cold fronts

Most cold fronts arrive from the west or north- west, whereas backdoor cold fronts come in from the east or northeast, hence their name.Backdoor fronts are primarily springtime events that tend to bring cold temperatures, low clouds, and drizzle, although thunderstorms occur occasionally. Backdoor cold fronts are less frequent in summer, but when they occur, the cool air can provide welcome relief from midsummer heat waves in the northeastern United States.

mP (maritime polar)

North Pacific Mild (cool) and humid entire year Unstable in winter Stable in summer a. Low clouds and showers in winter b. Heavy orographic precipitation on windward side of western mountains in winter c. Low stratus and fog along coast in summer; modified to cP inland

cT

Northern interior Mexico and southwestern U.S. (summer only) Hot and dry Unstable a. Hot, dry, and cloudless, rarely influencing areas outside source region b. Occasional drought to southern Great Plains

mP

Northwestern Atlantic Cold and humid in winter Cool and humid in summer Unstable in winter Stable in summer a. Occasional nor'easter in winter b. Occasional periods of clear, cool weather in summer

location for midlatitude cyclones

Notice that a prime site for cyclone formation is along the east side of the Rocky Mountains. Other important sites are in the North Pacific and the Gulf of Mexico. midlatitude cyclones form in areas where significant temperature contrasts occur in the lower troposphere

stationary front

Occasionally, airflow on both sides of a front is neither toward the cold air mass nor toward the warm air mass. Rather, it is almost parallel to the line of the front. Consequently, the surface position of the front does not move, or it moves sluggishly.On a weather map, a stationary front is shown with blue triangles pointing into the warm air and red semicircles pointing into the cold air.Because some overrunning usually occurs along stationary fronts, gentle to moderate precipitation is likely. Stationary fronts may remain over an area for several days, in which case flooding is possible. When stationary fronts begin to move, they become cold or warm fronts, depending on which air mass advances.

blocking pattern

Occasionally, large anticyclones persist over a region for several days or even weeks. Once in place, these stagnant anticyclones block or redirect the migration of midlatitude cyclones. Thus, they are sometimes called blocking highs. Low-pressure systems (cyclones) can also generate a blocking pattern. These lows, called cut-off low pressure systems, are literally cut off from the west-to-east flow in the jet stream. Without a connection to the prevailing flow aloft, these lows remain over the same area for days, often producing dreary weather and large quantities of precipitation

movement

Once formed, most midlatitude cyclones tend to travel in an easterly direction across North America and then follow a more northeasterly path into the North Atlantic exceptions: 1- panhandle hook: Developing in southern Colorado near the Texas and Oklahoma pan- handles, these cyclones first travel toward the southeast and then bend and travel sharply northward across Wisconsin and into Canada. 2- alberta clipper: is a cold, windy cyclonic storm that forms on the eastern side of the Canadian Rockies in the province of Alberta. dive southeastward into Montana or the Dakotas and then track across the Great Lakes, bringing dra- matically lower temperatures. dry. 3- nor'easter: From the Mid-Atlantic coast to New England, the classic storm is called a nor'easter (see Figure 9-17). These storms are called nor'easters because the winds preceding them in coastal areas are from the northeast.

leader

Once ionized, the air becomes a conductive path with a radius of roughly 10 centimeters and a length of 50 meters. This path is called a leader

cumulus stage

The development of a cumulonimbus tower requires a continuous supply of moist air. The release of latent heat al- lows each new surge of warm air to rise higher than the last, adding to the height of the cloud. This phase in the develop- ment of a thunderstorm, called the cumulus stage, is domi- nated by updrafts. Once the cloud passes beyond the freezing level, the Bergeron process begins producing precipitation. Eventually, the accumulation of precipitation in the cloud is too great for the updrafts to support. The falling precipitation causes drag on the air and initiates a downdraft. The creation of the downdraft is further aided by the influx of cool, dry air surrounding the cloud, a process termed entrainment. This process intensifies the downdraft because the air added during entrainment is cool and there- fore heavy; possibly of greater importance, it is dry. It thus causes some of the falling precipitation to evaporate (a cool- ing process), thereby cooling the air within the downdraft

thunder

The electrical discharge of lightning superheats the air im- mediately around the lightning channel. In less than a sec- ond the temperature rises by as much as 33,000°C. When air is heated this quickly, it expands explosively and produces the sound waves we hear as thunder. Because lightning and thunder occur simultaneously, it is possible to estimate the distance to the stroke. Lightning is seen instantaneously, but the relatively slow sound waves, which travel approxi- mately 330 meters (1000 feet) per second, reach us a little later. If thunder is heard 5 seconds after the lightning is seen, the lightning occurred about 1650 meters away (ap- proximately 1 mile).

dart leader

The first stroke is usually followed by additional strokes that apparently drain charges from higher areas within the cloud. Each subsequent stroke begins with a dart leader that once again ionizes the channel and carries the cloud potential toward the ground. The dart leader is continu- ous and less branched than the step leader.

what do you need to maintain/create a supercell?

The huge quantities of latent heat needed to sustain a su- percell require special conditions that keep the lower tropo- sphere warm and moisture rich. Studies suggest that the exis- tence of an inversion layer a few kilometers above the surface helps to provide this basic requirement. The inversion prevents the mixing of warm, humid air in the lower troposphere with cold, dry air above. Conse- quently, surface heating continues to increase the tempera- ture and moisture content of the layer of air trapped below the inversion.

k or w

The k or w designation gives an in- dication of the stability of an air mass and hence the weather that might be expected. An air mass that is colder than the surface is going to be warmed in its lower layers. This fact causes greater instability that favors the ascent of the heated lower air and creates the possibility of cloud formation and precipitation. Indeed, a k air mass is often characterized by cumulus clouds, and if precipitation occurs, it will be of the shower or thunder- storm variety. Also, visibility is generally good (except in rain) because of the stirring and overturning of the air. Conversely, when an air mass is warmer than the sur- face over which it is moving, its lower layers are chilled. A surface inversion that increases the stability of the air mass often develops. This condition does not favor the as- cent of air, and so it opposes cloud formation and precipita- tion. Any clouds that do form will be stratus clouds, and precipitation, if any, will be light to moderate. Moreover, because of the lack of vertical movements, smoke and dust often become concentrated in the lower layers of the air mass and cause poor visibility. During certain times of the year, fogs, especially the advection type, may also be com- mon in some regions.

classification of air masses

The lowercase letter m (for maritime) or the lowercase letter c (for continental) is used to designate the nature of the surface in the source region and hence the humidity characteristics of the air mass. Because maritime air masses form over oceans, they have a relatively high water-vapor content compared to continental air masses that originate over landmasses. ex: cA continental artic The classification of an air mass depends on the latitude of the source region and the nature of the surface in the area of origin—ocean or continent. The latitude of the source region indicates the temperature conditions within the air mass, and the nature of the surface below strongly influ- ences the moisture content of the air. Notice that the list does not include mA (maritime arctic). These air masses are not listed because they seldom, if ever, form. Although arctic air masses form over the Arc- tic Ocean, this water body is largely ice covered throughout the year. Consequently, the air masses that originate here consistently have the moisture characteristics associated with a continental source region

modern view of cyclogenisis

The modern view of cyclogenesis, called the conveyor belt model, provides a good picture of the airflow within a cyclonic system. It consists of three interacting air streams: two that originate near the surface and ascend and a third that originates in the uppermost troposphere. In summary, the conveyor belt model of a midlatitude cyclone provides a three-dimensional picture of the major circulation of these storm systems. It also accounts for the distribution of pre- cipitation and the comma-shaped cloud pattern characteristic of mature cyclonic storms.

relationship between anticyclones and cyclones

The surface air that feeds a cyclone, for example, generally originates as surface air flowing out of an anticyclone (Figure 9-15). Consequently, cyclones and anticyclones are typically adjacent to one another. Like a cyclone, an anticyclone depends on the flow aloft to maintain its circulation. In an anticyclone, divergence at the surface is balanced by convergence aloft and general subsidence of the air column (Figure 9-15).

heat lightning

The thunder that we hear as a rumble is produced along a long lightning path located at some distance from the ob- server

the warm conveyor belt

The warm conveyor belt (shown in red) carries warm, moist air from the Gulf of Mexico into the warm sector of the midlatitude cyclone. As this air stream flows northward, convergence causes it to slowly ascend. When it reaches the sloping boundary of the warm front, it rises even more rapidly over the cold air that lies beyond (north of) the front. During its ascent, the warm, humid air cools adiabatically and produces a wide band of clouds and precipitation. Depending on atmospheric conditions, drizzle, rain, freezing rain (glaze), and snow are possible. When this air stream reaches the middle troposphere, it begins to turn right (eastward) and eventually joins the gen- eral west-to-east flow aloft. The warm conveyor belt is the main precipitation-producing air stream in a midlatitude cyclone.

nor'easter

The weather associated with a wintertime invasion of mP air from the Atlantic is known locally as a nor'easter. Strong north- east winds, freezing or near-freezing temperatures, high relative humidity, and the likelihood of precipitation make this weather phenomenon an unwelcome event. A clas- sic case is presented in "Severe and Hazardous Weather,"

Tornadoes and hurricanes vs. midlatitude cyclones

Tornadoes and hurricanes are both smaller and more vi- olent than midlatitude cyclones. A midlatitude cyclone may have a diameter of 1600 kilometers (1000 miles) or more. By contrast, hurricanes average only 600 kilometers (375 miles) across, and tornadoes, with a diameter of just 0.25 kilome- ter (0.16 mile)

Weather Associated with Warm Fronts

look at table 9-1

Occlusion: The Beginning of the End

Usually, a cold front advances more rapidly than the warm front. As it moves, a cold front begins to overtake (lift) the warm front--occluded front. As occlusion begins, the storm often intensifies. How- ever, as more of the warm air is forced aloft, the surface pres- sure gradient weakens, as does the storm itself. Within one or two days the entire warm sector is forced aloft, and cold air surrounds the cyclone at the surface (Figure 9-11f). Thus, the horizontal temperature (density) difference that existed between the two contracting air masses is largely elimi- nated. At this point the cyclone has exhausted its source of energy. Friction slows the surface flow, and the once highly organized inward-directed, counterclockwise flow ceases to exist.

Cold Fronts

When cold air actively advances into a region occupied by warmer air, the zone of discontinuity is called a cold front.On a weather map, a cold front is shown by a blue line with blue triangles protruding into the area of warmer air. Air near the surface of a cold front advances more slowly than the air aloft because of friction. As a result, cold fronts steepen as they move. As a cold front approaches, generally from the west or northwest, towering clouds can often be seen in the distance. Because a cold front produces roughly the same amount of lifting as a warm front, but over a shorter distance, the precipitation is generally more intense but of shorter duration. A marked temperature drop and wind shift from the southwest to the northwest usually accompany frontal passage.The weather behind a cold front is dominated by subsiding air within a continental polar (cP) air mass. Thus, the drop in temperature is usually accompanied by clearing that begins soon after the front passes.

warm fronts

When the surface position of a front moves so that warmer air invades territory formerly occupied by cooler air. warm front is shown by a red line with red semicircles protruding into the area of cooler air. East of the Rockies, warm fronts are usually associated with maritime tropical (mT) air that enters the United States from the Gulf of Mexico and "glides" over cooler air positioned over land.Because warm fronts have relatively gentle slopes, the cloud deck that results from frontal lifting covers a large area and produces light-to-moderate precipitation for an extended duration (Figure 9-4). However, if the overriding air mass is relatively dry (low dew-point temperatures), there is minimal cloud development and no precipitation. During the hot summer months when moist conditionally unstable air is often forced aloft, towering cumulonimbus clouds and thunderstorms may occur.any precipitation that forms must fall through the cool layer below. During extended periods of light rainfall, enough of these raindrops may evaporate for saturation. to occur, resulting in the development of a stratus cloud deck. Occasionally during the winter, a relatively warm air mass is forced over a body of subfreezing air. This occurrence can create hazardous driving conditions. Raindrops become supercooled as they fall through the subfreezing air. Upon colliding with the road surface, these supercooled raindrops freeze to produce an icy layer called freezing rain or glaze.

weather of a midlatitude cyclone

With the passage of the warm front, the area behind (west of) the front, called the warm sector, is under the influ- ence of a maritime tropical air mass The warm conditions associated with the warm sector pass quickly and are replaced by gusty winds and precipitation generated along the cold front. The approach of a rapidly advancing cold front is marked by a wall of rolling black clouds (point D). Severe weather accompanied by heavy precipitation, and occasionally hail or a tornado, can be expected. The passage of the cold front is easily detected by a dramatic shift in wind direction. The warm flow from the south or southwest is replaced by cold winds from the west to northwest, resulting in a pronounced decrease in temperature. Once the front passes, the skies clear quickly as cooler, drier air invades the region A very different set of weather conditions prevails in the portion of the cyclone located north of the center of low pressure; the first hints of the approaching low-pressure center are a continual drop in air pressure and increasingly overcast conditions that bring varying amounts of precipitation. This section of the cyclone most often generates snow during the winter months Once the process of occlusion begins, the character of the storm changes. Because occluded fronts tend to move more slowly than other fronts, the entire wishbone-shaped frontal structure shown in Figure 9-11 rotates counterclock- wise. As a result, the occluded front appears to "bend over backward." This effect adds to the misery of the region influenced by the occluded front because it lingers over the area longer than the other fronts warmest: second day, coldest: third day

supercell

Worst type of storm--causes death, injury, and property damage. A supercell consists of a single, very powerful cell that at times can extend to heights of 20 kilometers (65,000 feet) and persist for many hours. These massive clouds have diameters ranging between about 20 and 50 kilometers (12 and 30 miles). Despite the single-cell structure of supercells, these storms are remarkably complex. The vertical wind profile may cause the updraft to rotate.

Fronts

are boundary surfaces that separate air masses of different densities—one of which is usually warmer and contains more moisture than the other. However, fronts can form between any two contrasting air masses. When the vast sizes of air masses are considered, the zones (fronts) that separate them are relatively narrow and are shown as lines on weather maps. Generally, the air mass located on one side of a front moves faster than the air mass on the other side. Thus, one air mass actively advances into the region occupied by another and collides with it. It is along these zones of "conflict" that midlati- tude cyclones develop and produce much of the precipita- tion and severe weather in the belt of the westerlies.

severe thunderstorms

are capable of producing heavy down- pours and flash flooding as well as strong, gusty, straight- line winds; large hail; frequent lightning; and perhaps torna- does. the storm must have winds in excess of 93 kilometers (58 miles) per hour (50 knots) or produce hailstones larger than 1.9 centimeters (0.75 inch) in diam- eter or generate a tornado.

Midlatitude cylones

are low-pressure systems with diame- ters that often exceed 1000 kilometers (600 miles) and travel from west to east across the middle latitudes in both hemi- spheres (see Figure 9-2). Lasting from a few days to more than a week, a midlatitude cyclone in the Northern Hemi- sphere has a counterclockwise circulation pattern with air- flow directed inward toward its center. Most midlatitude cyclones have a cold front and a warm front extending from the central area of low pressure. Surface convergence and ascending air initiate cloud development that frequently produces precipitation.

tornadoes

are vio- lent windstorms that take the form of a rotating column of air, or vortex, that extends downward from a cumulonimbus cloud. Pressures within some tornadoes have been estimated to be as much as 10 percent lower than immediately outside the storm. Because of the rapid drop in pressure, air sucked into the storm expands and cools adiabatically. If the air cools below its dew point, the resulting condensation creates a pale and ominous-appearing cloud that may darken as it moves across the ground, picking up dust and debris.

air mass

as the term implies, is an immense body of air, usually 1600 kilometers (1000 miles) or more across and perhaps several kilometers thick, which is characterized by homo- geneous physical properties (in particular, temperature and moisture content) at any given altitude.

winter

cP and cA air masses are bitterly cold and very dry.

Pineapple Express

can bring extraordinary rains to southern California and other West Coast locations.

predicting weather

changes in wind direction that occur with the passage of warm and cold fronts are useful in predicting impending weather.

Continental artic

cold and dry. Continental arctic air is distinguished from cP air by its generally lower temperatures, although at times the dif- ferences may be slight. In fact, some meteorologists do not differentiate between cP and cA.

Continental polar

cold and dry. Continental polar air originates over the often snow-covered interior regions of Canada and Alaska, poleward of the 50th parallel.

Mesoscale Convective Complexes

consists of many individual thunderstorms organized into a large oval to cir- cular cluster. great plains. The transformation of afternoon air-mass thunderstorms into an MCC requires a strong low-level flow of very warm and moist air. This flow enhances instability, which in turn spurs convection and cloud development. When conditions are favorable, an MCC develops from a group of afternoon air-mass thunderstorms. though mesoscale convective complexes sometimes produce severe weather, they are also beneficial because they provide a significant portion of the growing-season rainfall to the agricultural regions of the central United States.

cyclones length

cyclones often exist for a week or longer. For this to happen, surface convergence must be offset by outflow aloft (Figure 9-15). As long as divergence (spreading out) aloft is equal to or greater than the surface inflow, the low pressure can be sustained.

drylines

develops most often over the southern Great Plains. This occurs when dry, continental tropical (cT) air originating in the Southwest meets moist, maritime tropical (mT) air from the Gulf of Mexico. Drylines are spring and summer phenomena, that most often generate a band of severe thunderstorms along a line extending from Texas to Nebraska that moves eastward across the Great Plains. A dryline is easily identified by comparing the dew-point temperatures of the cT air west of the boundary with the dew points of the mT air mass to the east (Figure 9-10).

dissipating stage

dominated by downdrafts and entrainment, causing evaporation of the structure. Once downdrafts begin, the vacating air and precipitation encourage more entrainment of the cool, dry air surrounding the cell. Eventually, the downdrafts dominate throughout the cloud and initiate the dissipating stage (see Figure 10-4c). The cooling effect of falling precip- itation and the influx of colder air aloft mark the end of the thunderstorm activity. An interesting fact is that only a modest portion—on the order of 20 percent—of the moisture that condenses in an air-mass thunderstorm actually leaves the cloud as precipitation. The remaining 80 percent evaporates back into the atmosphere.

Divergence

essential to cyclogenisis. Divergence aloft does not involve the outward flow in all directions, as in a surface anticyclone. Instead, the winds aloft generally flow from west to east along sweeping curves. One mechanism responsible for divergence aloft is a phenomenon known as speed divergence. Wind speeds can change dramatically in the vicinity of the jet stream. On entering a zone of high wind speed, air accelerates and stretches out (divergence). In contrast, when air enters a zone of slower wind speed, an air pileup (convergence) results. Other factors: direc- tional divergence and directional convergence, which result from changes in wind direction. For example, directional convergence (also called confluence) is the result of air being funneled into a restricted area.

How did the weather in the Duluth-Superior area of Minnesota and Wisconsin compare to that in the Fort Worth area during March 23-25?

first day: strong cold winds, a lot of snow second day: more harsh winds and temperature dropped further third day: no more winds, temperature dropped further

air-mass thunderstorms

frequently occur in mT air that moves northward from the Gulf of Mexico. These warm, humid air masses contain abundant moisture in their lower levels and can be rendered unstable when heated from below or lifted along a front. Because mT air most often becomes unstable in spring and summer, when it is warmed from below by the heated land surface, it is during these seasons that air-mass thunderstorms occur most frequently. They also have a strong preference for midafternoon, when surface temperatures are highest.

why do tornadoes have such high wind speeds?

high pressure gradient

anitcyclones

high pressure systems. usually produce clear skies. However, anticyclones do not always bring desir- able weather. Large anticyclones often develop over the Arctic during the winter. These cold high-pressure centers are known to migrate as far south as the Gulf Coast, where they can impact the weather over as much as two-thirds of the United States (Figure 9-19). This dense frigid air often brings record-breaking cold temperatures.

cyclogenisis

hows six stages in the life of a typical midlatitude cyclone. The first stage in this process is called cyclogenesis, meaning cyclone formation. A midlatitude cyclone is born when two air masses of different densities (temperatures) are moving roughly parallel to a front but in opposite directions. In the clas- sic polar-front model, this would be continental polar air associated with the polar easterlies on the north side of the front and maritime tropical air driven by the westerlies on the south side of the front. The polar-front model shows that cyclogenesis (cyclone formation) occurs where a frontal surface is distorted such that it takes on the shape of an ocean wave. Several surface factors are thought to produce this wave in a frontal zone. Topographic irregularities (such as mountains), tempera- ture contrasts (as between sea and land), or ocean-current influences can disrupt the general zonal (west to east) flow sufficiently to produce a wave along a front. In addition, a strong jet stream in the flow aloft frequently precedes the formation of a surface cyclone. This fact strongly suggests that upper-level flow contributes to the formation of these rotating storm systems. Topographic irregularities (such as mountains), tempera- ture contrasts (as between sea and land), or ocean-current influences can disrupt the general zonal (west to east) flow sufficiently to produce a wave along a front

occluded front

in which a rapidly moving cold front overtakes a warm front, as shown in Figure 9-8a, b. As the cold air wedges the warm front upward, a new front forms between the advancing cold air and the air over which the warm front is gliding, a process known as occlusion (Figure 9-8b).There are two types of occlusion—cold-type occluded fronts and warm-type occluded fronts. In the occluded front shown in Figure 9-9a, the air that had been behind the cold front is colder than the cool air it is overtaking. This is the most common type of occluded front east of the Rockies and is called a cold-type occluded front. Cold-type occluded fronts frequently produce thunderstorms and thus resemble cold fronts in the type of weather generated. It is also possible for the air behind an advancing cold front to be warmer than the cold air it is overtaking. These warm-type occluded fronts (Figure 9-9b) frequently occur along the Pacific Coast, where milder maritime polar air invades more frigid polar air that had its origin over the continent. Because of the complex nature of occluded fronts, they are often drawn on weather maps as either warm or cold fronts, depending on what kind of air is the aggressor. Sometimes, however, an occluded front is drawn as a purple line with alternating purple triangles and semicircles pointing in the direction of movement.

strokes

individual components that make up each flash Moreover, each stroke consists of a downward propagating leader that is immediately followed by a luminous return stroke. Each stroke is believed to begin when the electrical field near the cloud base frees electrons in the air immedi- ately below, thereby ionizing the air

squall lines

is a relatively narrow band of thunderstorms, some of which may be severe, that develops in the warm sector of a midlatitude cyclone, usually 100 to 300 kilo- meters (60 to 180 miles) in advance of the cold front. the linear band of cumulonimbus development might stretch for 500 kilometers (300 miles) or more and consist of many individual cells in various stages of development. An aver- age squall line can last for 10 hours or more, and some have been known to remain active for more than a day. Some- times the approach of a squall line is preceded by a mamma- tus sky, consisting of dark cloud rolls that have downward pouches. The squall line forms when the divergence and result- ing lift created by the jet stream is aligned with a strong, per- sistent low-level flow of warm, humid air from the south.

dry conveyor belt

is shown as a yellow arrow in Figure 9-25. Whereas both the warm and cold conveyor belts begin at the surface, the dry air stream originates in the uppermost troposphere. As part of the upper-level westerly flow, the dry convey- or belt is relatively cold and dry. As this air stream enters the cyclone, it splits. One branch descends behind the cold front, resulting in the clear, cool conditions normally associ- ated with the passage of a cold front. In addition, this flow maintains the strong temperature contrast observed across the cold front. The other branch of the dry conveyor belt maintains its westerly flow and forms the dry slot (cloudless area) that separates the head and tail of a comma cloud pat- tern (Figure 9-26).

Which two air masses have the greatest influence on weather east of the Rocky Mountains? Explain your choice.

mT: During the summer, mT air masses from the Gulf, Caribbean, and adjacent Atlantic affect a much wider area of North America and are present for a greater percentage of the time than during the winter. As a result, they exert a strong and often dominating influence over the summer weather of the United States east of the Rocky Mountains. cP and cA:

middle latitude cyclone

middle latitudes refers to the area of North America roughly between southern Alaska and Florida—essentially the area of the westerlies where the primary weather producer is the mid- latitude, or middle-latitude, cyclone. has sudden and dramatic changes due to fronts.

Why do cA and cP air masses often sweep far south into the United States?

no barriers from the north where they originate to the south of the US

Where do the midlatitude cyclones that affect the Pacific Coast of the United States originate?

north pacific

categories of air masses

polar (P), arctic (A), or tropical (T)

lightning

ranks second only to floods in the number of storm-related deaths in the United States.Why? The answer relates to the wide geographic oc- currence and frequency of lightning. During the formation of a large cumulonimbus cloud, a separation of charge occurs; The objective of lightning is to equalize these electrical differences by pro- ducing a negative flow of current from the region of excess negative charge to the region with excess positive charge or vice versa. Because air is a poor conductor of electricity (good insulator), the electrical potential (charge difference) must be very high before lightning will occur. The most common type of lightning occurs between op- positely charged zones within a cloud or between clouds. About 80 percent of all lightning is of this type. It is often called sheet lightning because it produces a bright but dif- fuse illumination of the parts of the cloud in which the flash occurred. The second type of lightning, in which the electri- cal discharge occurs between the cloud and Earth's surface, is often more dramatic. This cloud-to-ground lightning repre- sents about 20 percent of lightning strokes and is the most damaging and dangerous form. As the cloud moves, the negatively charged cloud base alters the charge at the surface directly below by repelling negatively charged particles. Thus, the surface beneath the cloud acquires a net positive charge. These charge differences build to millions and even hundreds of millions of volts before a lightning stroke acts to discharge the negative re- gion of the cloud by striking the positive area of the ground below, or, more frequently, the positively charged portion of that cloud or a nearby cloud.

cA

region: Arctic basin and Greenland ice cap (winter only), Bitterly cold and very dry in winter,Stable,Cold waves in winter

dryline

squall line with severe thunderstorms can also form along a boundary called a dryline, a narrow zone along which there is an abrupt change in moisture. it forms when continental tropical (cT) air from the southwestern United States is pulled into the warm sector of a midlatitude cyclone, as shown in Figure 10-11. The denser cT air acts to lift the less dense mT air with which it is converging.* In this case, both cloud formation and storm development along the cold front are minimal because the front is advancing into dry cT air.

Describe the motion of a midlatitude cyclone in relation to the flow at the 500-millibar level.

surface is same direction as winds aloft at the 500-millibar level, but at about one-quarter to one-half the speed--"steering" effect of upper-level flow

Weather Typically Associated with a Cold Front (North America)

table 9-2

thunderstorms

the circula- tion associated with thunderstorms is characterized by strong up-and-down movements. Winds in the vicinity of a thunderstorm do not follow the inward spiral of a cyclone, but they are typically variable and gusty. thunderstorms form "on their own," away from cyclonic storms, they also form in conjunc- tion with cyclones. Hurricanes also generate widespread thunderstorm activity. Thus, thun- derstorms are related in some manner to all three types of cyclones mentioned here. dynamic thermal instability that occurs during the development of a thunderstorm. A thunder- storm is a storm that generates lightning and thunder. It frequently produces gusty winds, heavy rain, and hail. A thunderstorm may be produced by a single cumulonim- bus cloud and influence only a small area, or it may be as- sociated with clusters of cumulonimbus clouds covering a large area.

migration

the flow in the middle and upper troposphere strongly influ- ences the rate at which these pressure systems advance and the direction they follow. Generally, surface cyclones move in the same direction as winds aloft at the 500-millibar level, but at about one-quarter to one-half the speed--"steering" effect of upper-level flow.

Divergence and jetstreams

the jet stream con- tributes to the formation and inten- sification of surface low- and high- pressure systems. Changes in wind speeds and/or directions cause air either to pile up (convergence) or spread out (divergence). Upper-level convergence is favored downstream (east) from a ridge, whereas diver- gence occurs downstream from an upper-level trough. At the surface, below regions of upper-level convergence, are areas of high pressure (anticyclones), whereas upper-level divergence supports the formation and development of surface cyclonic systems (lows).

step leader

the mobile electrons in the cloud base begin to flow down this channel. This flow increases the electrical potential at the head of the leader, which causes a further extension of the conductive path through further ionization. Because this initial path extends itself earthward in short, nearly invisible bursts, it is called a step leader.

gust front

the outflow- ing cool air of the downdraft acts as a "mini cold front" as it advances into the warmer surrounding air. Frequently a roll cloud may form as warm air is lifted along the leading edge of the gust front which serves as a warning

What air mass influences the weather of the Pacific Coast more than any other?

the pacific mP

overrunning

the process of warm air gliding up and over a cold air mass.

flash

the total dis- charge—which lasts only a few tenths of a second and ap- pears as a bright streak

Why do mP air masses from the North Atlantic seldom affect the eastern United States?

they go into the Atlantic basin due to the west-to-east circulation in the middle latitudes