what may happen to an air mass as it moves away from its source region

The day-to-day fire atmospheric condition in a given surface area depends, to a large extent, on either the grapheme of the prevailing air mass, or the interaction of 2 or more air masses.

The conditions within an air mass—whether cool or warm, humid or dry out, clear or cloudy—depends on the temperature and humidity structure of the air mass. These elements will be altered by local conditions, to be sure, merely they tend to remain overall characteristic of the air mass. As an air mass moves away from its source region, its characteristics will exist modified, but these changes, and the resulting changes in fire weather condition, are gradual from day to day.

When one air mass gives way to another in a region, fire atmospheric condition may change abruptly—sometimes with violent winds—equally the front, or leading edge of the new air mass, passes. If the frontal passage is accompanied by precipitation, the fire conditions may ease. Merely if it is dry, the fire weather may become disquisitional, if only for a curt time.

Sections

• Formation and Modification of Air Masses
• Air-Mass Weather
• Variations in Air-Mass Weather condition
• Fronts
• Summary

In chapter 5 we learned that in the primary and secondary circulations there are regions where high-force per unit area cells tend to form and stagnate. Usually, these regions take compatible surface temperature and moisture characteristics. Air within these loftier-pressure level cells, resting or moving slowly over land or bounding main areas that have uniform properties, tends to acquire respective characteristics—the coldness of polar regions, the rut of the tropics, the moisture of the oceans, or the dryness of the continents.

A body of air, usually 1,000 miles or more across, which has assumed uniform characteristics of temperature and moisture, is coiled an air mass.

A body of air, ordinarily 1,000 miles or more across, which has assumed compatible characteristics, is called an air mass. Within horizontal layers, the temperature and humidity properties of an air mass are fairly uniform. The depth of the region in which this horizontal uniformity exists may vary from a few thousand anxiety in common cold, wintertime air masses to several miles in warm, tropical air masses.

Weather within an air mass will vary locally from day to day due to heating, cooling, atmospheric precipitation, and other processes. These variations, nonetheless, usually follow a sequence that may be quite unlike the weather events in an adjacent air mass.

Where two or more air masses come together, the boundary between them may be quite distinct; information technology is chosen a front. Frontal zones, where lighter air masses are forced over denser air masses, are regions of considerable weather condition activity.

In this chapter, nosotros volition consider starting time the dissimilar types of air masses and the atmospheric condition associated with them, and then the dissimilar kinds of fronts and frontal weather.

Formation and Modification of Air Masses

The region where an air mass acquires its characteristic properties of temperature and moisture is called its source region. Ocean areas, snow- or water ice-covered land areas, and wide desert areas are mutual source regions. Those areas producing air masses which enter the fireoccurrence regions of North America are:

1. The tropical Atlantic, Caribbean, Gulf of Mexico, and the tropical Pacific, which are uniformly warm and moist.
2. The Northern Pacific and Northern Atlantic, which are uniformly absurd and moist.
3. Interior Alaska, Northern Canada, and the Chill, which are uniformly cold and dry during the winter months.
4. Northern Mexico and Southwestern United States, which are usually hot and dry during the summertime months.

The time required for a torso of air to come to gauge equilibrium with the surface over which it is resting may vary from a few days to 10 days or ii weeks, depending largely on whether the body of air is initially colder or warmer than the temperature of its source region. If the air is colder, information technology is heated from below. Convective currents are produced, which carry the heat and wet aloft and speedily change the air to a considerable height.

The oceans and the country are both important air-mass sources.

On the other manus, if the air is initially warmer than the surface, it is cooled from below. This cooling stabilizes the air and cuts off convection. Cooling of the air in a higher place the surface must take identify by conduction and radiations, and these are slow processes. Thus, a longer fourth dimension—up to ii weeks—is required for the development of common cold air masses, and even and then these air masses are only a few 1000 feet thick.

Air masses that form over a source region vary in temperature and moisture from flavor to season, as does the source region. This is especially true of continental source regions. High-latitude continental source regions are much colder and drier in the winter than in the summer, and tropical continental source regions are much hotter and drier in summer than in winter.

Air masses are classified according to their source region. Several systems of classification have been proposed, but nosotros volition consider only the simplest. Air masses originating in loftier latitudes are called polar (P), and those originating in tropical regions are called tropical (T). Air masses are further classified according to the underlying surface in the source region as maritime for h2o and continental for land. The "thousand" for maritime or "c" for continental precedes the P or T. Thus, the 4 basic types of air masses are designated as: mP, mT, cP, and cT, co-ordinate to their source region. Information technology is natural that air stagnating for some fourth dimension in a polar region will become cold, or in a tropical region will become warm. And air spending old over water becomes moist, at least in the lower layers, while air over land becomes dry.

For convenience, the iv basic air mass types are often referred to as moist cold, moist warm, dry cold, and dry warm.

As an air mass leaves its source region in response to broadscale atmospheric motions, it may be colder or warmer than the surface it passes over. It is then farther classified by the addition of k for colder or w for warmer to its classification symbol. The chiliad-type air mass volition be warmed from below and will become unstable in the lower layers. A w-type air mass volition be cooled from beneath, will become stable, and will be modified slowly, and only in the lower few thousand anxiety.

Air-mass properties brainstorm irresolute as presently equally the air mass leaves its source region. The amount of modification depends upon the speed with which the air mass travels, the type of surface over which it moves, and the temperature deviation between the air mass and the underlying surface.

An air mass which moves into the source region of another airmass type, and stagnates, is transformed into that type of air mass.

Air masses are modified in several means. For the most part, these are processes which we have already considered in detail. Several of the processes unremarkably take place concurrently:

1. An air mass is heated from beneath if it passes over a warmer surface (previously warmed by the sun) or if the surface beneath a slow-moving air mass is existence currently warmed by the sun. Such modification is rapid considering of the resulting instability and convection.
2. An air mass is cooled from below if it passes over a colder surface, or if the surface is cooled by radiation. This increases the stability of the lower layers, and further modification becomes a slow process.
3. Wet may be added to an air mass by: (a) Evaporation from water surfaces, moist ground, and falling rain; (b) sublimation from ice or snowfall surfaces and falling snow or hail; and (c) transpiration from vegetation. Of these, sublimation is a relatively wearisome process by comparison.
4. Wet may be removed from an air mass by condensation and atmospheric precipitation.
5. Finally, air-mass properties may be changed by turbulent mixing, by sinking, or past lifting.

Afterwards moving a considerable distance from its source region, particularly afterward inbound a source region of another type, an air mass may lose its original distinctive characteristics entirely and acquire those of some other air-mass type. Thus, a continental polar all- mass moving out over the Gulf of Mexico takes on the characteristics of a maritime tropical air mass. Or a maritime polar air mass, after crossing the Rocky Mountains, may assume the characteristics of a continental polar air mass.

Air-Mass Weather

In that location are many differences in air masses and in the weather associated with them. Even within 1 air-mass type, at that place will be considerable variation, depending on the season, the length of time that an air mass has remained over its source region, and the path it has followed after leaving that region. We will discuss but the more distinct types of air masses and consider their most common characteristics.

Continental Polar—Winter

Continental polar air masses originate in the snow-covered interior of Canada, Alaska, and the Chill in the colder months. Lower layers of the air become quite cold, dry, and stable. Much moisture from the air is condensed onto the snowfall surface. These air masses are high-pressure areas, and there is niggling cloudiness due to the lack of moisture and to the stability of the air mass.

These are the coldest wintertime air masses, and crusade severe cold waves when moving southward through Canada and into the U.s.a.. Upon moving southward or southeastward over warmer surfaces, cP air masses change to cPk. The lower layers get unstable and turbulent. If a part of the air mass moves over the Bang-up Lakes, it picks up wet likewise as oestrus and may produce cloudiness and snow flurries or rain showers on the lee side of the Lakes, and again on the windward side of the Appalachian Mountains. In one case beyond the Appalachians, the air mass is mostly clear and slightly warmer.

Continental polar air masses in winter cause severe cold waves when they motility south through Canada and into the Primal and Eastern Usa.

If a cP air mass moves southward into the Mississippi Valley and so into the Southeast, information technology will gradually warm upward but remain dry. Modification is dull until the air mass passes beyond the snow-covered areas; then it becomes more rapid. When cP air moves out over the Gulf of Mexico, it is rapidly changed to an mT air mass. The generally clear skies and relatively depression humidities associated with cP air masses are responsible for much of the hazardous burn weather in the South and Southeast during the absurd months.

The Rocky Mountains effectively prevent virtually cP air masses from moving into the Far Westward. Just occasionally, a portion of a deep cP air mass does move southward west of the Rockies, and in then doing brings this area its coldest weather. At times the air is cold enough for snowfall to fall as far south as southern California.

Maritime Polar—Winter

The N Pacific is the common source region for maritime polar air masses. While in its source region, the air mass is common cold and has a lapse charge per unit nearly the aforementioned as the moist-adiabatic rate. If the air mass moves into the snow-covered regions of Canada, it gradually changes to a cP air mass. Maritime polar air taking that trajectory usually has had a comparatively brusque stay over the water. It is quite cold and has high relative humidity, but wet content in terms of absolute humidity is rather low. Even so, rain or snowfall showers usually result as the air is lifted over the coastal mountains.

Maritime polar air masses in winter vary co-ordinate to the length of fourth dimension they spend in the source region. Those inbound the continent farther north usually have spent but a short fourth dimension over the water and are absurd and quite dry out, but showers may occur in the mountains. Those entering the w declension farther south ore more moist and produce much rain and snow, particularly in the mountains.

Maritime polar air masses originating farther south and entering Western United states or Southwestern Canada accept had a longer overwater trajectory, are non quite so cold, and accept a higher moisture content. On existence forced over the Coast Ranges and the Rocky Mountains, an mP air mass loses much of its moisture through precipitation. As the air mass descends on the eastern slopes of the Rocky Mountains, it becomes relatively warm and dry with by and large articulate skies. If, notwithstanding, information technology cannot descend on the lee side of the mountains, and instead continues east over a dome of common cold cP air, snow may occur.

East of the Rockies, mP air at the surface in winter is comparatively warm and dry out, having lost much of its wet in passing over the mountains. Skies are relatively clear. If this air mass reaches the Gulf of Mexico, it is somewhen inverse into an mT air mass.

Maritime polar air sometimes stagnates in the Swell Bowl region of the Western United States in clan with a Swell Basin Loftier. The outflow from the Corking Bowl High may give ascent to strong, dry foehn winds in a number of the surrounding States.

At times during the wintertime, mP air is trapped in Pacific coast valleys and may persist for a week or more. Low stratus clouds and fog are produced, making these valleys some of the foggiest places on the continent during the winter.

Although mP air forms over the North Atlantic Ocean, besides as the North Pacific, the trajectory of Atlantic mP air is limited to the northeastern seaboard.

Maritime Tropical - Wintertime

Well-nigh of the maritime tropical air masses affecting temperate North America originate over the Gulf of Mexico or Caribbean area Sea. They are warm, have a loftier moisture content, and a conditionally unstable lapse rate. Maritime tropical air is brought into the southeastern and central portions of the country by the circulation around the western end of the Bermuda High. In moving inland during the winter, mT air is cooled from beneath by contact with the libation continent and becomes stabilized in the lower levels. Fog and low stratus clouds usually occur at dark and dissipate during the twenty-four hours every bit this air mass invades the Mississippi Valley and the Groovy Plains. If mT air is lifted over a cP air mass, or if it moves northeastward and is lifted on the western slopes of the Appalachians, the conditional instability is released and big cumulus clouds, heavy showers, and frequent thunderstorms effect.

Maritime tropical air in winter produces night cloudiness and fog in the Mississippi Valley and Great Plains and showers or snowfall over the Appalachians and in areas where it overruns a libation air mass.

Maritime tropical air seldom reaches as far as the Canadian border or the New England States at the surface in wintertime. Nevertheless, information technology occasionally causes heavy rain or snow in these areas, when mT air encounters a colder cP or mP air mass and is forced to rise upwards over the denser air. More will be said about this process in the section on fronts.

The tropical Pacific is likewise a source region for mT air, just Pacific mT seldom enters the continent. When information technology does, information technology is usually brought in with a lowpressure system in Northern Mexico or California, where the Pacific mT air can cause heavy rainfall when quickly forced aloft past the mountains.

Continental Polar-Summertime

Continental polar air in summer brings by and large fair and dry weather to the central and eastern portions of the continent. The air mass warms rather rapidly and becomes unstable as it moves southward. It may pick upwardly enough wet to produce some clouds.

In summer, even though the source region for cP air masses is farther north than in winter—over Northern Canada and the polar regions—the warmer surface temperatures event in petty surface cooling and ofttimes in actual heating of the air almost the ground. The air mass, therefore, may be relatively unstable in the lower layers in dissimilarity to its extreme stability during the winter. Since the air is quite dry out from the surface to high levels, the relative instability rarely produces cloudiness or precipitation.

The full general atmospheric circulation is weaker during the summer, and polar outbreaks motion more slowly than in winter. Every bit a result, cP air undergoes tremendous changes in passing slowly from its source region to Southern U.s.a.. During its southward and southeastward travel, cP air is warmed from below and becomes more unstable.

Continental areas, over which cP air travels, are relatively moist in summertime, being largely covered with crops, grass, forests, and other vegetation. Transpiration from these plants and evaporation from water bodies and moist soil increment the wet content of cP air rather rapidly. As the moisture content increases, cloudiness besides increases.

The weather associated with cP air every bit it passes through Canada and enters the United States is generally fair and dry. Frequent intrusions of this air requite rising to much of the fire atmospheric condition in the north-central and northeastern regions from spring, through summer, and into fall.

Occasionally, cP air stagnates in the Southeastern United States and accumulates sufficient moisture to produce showers and isolated thunderstorms, particularly over mountainous areas.

Maritime Polar—Summertime

Maritime polar air masses in summer originate in the aforementioned general expanse over the Pacific Ocean as in wintertime. In summertime, nonetheless, the ocean is relatively absurd compared to the land surfaces. Summer mP air is cooled from below in its source region and becomes stable. Stability in the lower layers prevents moisture from existence carried to higher levels. Aloft, this air mass remains very dry out, usually fifty-fifty drier than summertime cP, and becomes quite warm through the subsidence which takes place in the Pacific Loftier.

As mP air approaches the Pacific coast, the cold, upwelling waters along the shore crusade further cooling, increasing relative humidity, and stimulating the germination of considerable fog or low stratus clouds. Thus, along the Pacific coast, summer mP is characterized past a cool, humid marine layer from ane,000-2,000 feet thick, often with fog or depression stratus clouds, a potent inversion capping the marine layer, and warm, dry, subsiding air to a higher place.

As mP air moves inland from the westward declension, the potent daytime heating in interior California, Oregon, Washington, and portions of British Columbia warms the surface layers and lowers the relative humidity. The intense heating and the lifting equally mP air crosses the mountains may issue in cumulus cloud formation and occasional scattered showers and thunderstorms at high elevations. In descending the eastern slopes of the Rockies, summer mP is heated adiabatically as in wintertime, and the relative humidity may become quite low at times. When it arrives in the Plains and the Mississippi Valley, it is hardly distinguishable from cP air in the area and results in clear, dry weather condition. Continuing east, it becomes warmer and more than unstable, and picks up moisture from the earth and plants. Past the time it reaches the Appalachians, it has become unstable and moist enough so that lifting tin again produce showers or thunderstorms.

Stratus clouds and fog along the Pacific declension are characteristic of mP air in summer. Heating and lifting of the air are likely to produce clouds in the Sierras and showers or thunderstorms in the Rockies if sufficient moisture is present.

Maritime polar air formed over the colder waters of the North Atlantic in summer occasionally moves due south bringing cool weather condition and cloudiness to the Atlantic coastal areas.

Maritime tropical air moving onto the continent is conditionally unstable. Daytime heating and orographic lifting produce showers and thunderstorms in this warm, boiling air mass.

Maritime Tropical—Summer

Maritime tropical air in its source region over the Gulf of Mexico and the Caribbean in summertime has properties like to those in winter, except that it is conditionally unstable to college levels, slightly warmer, and more moist. In summertime, mT air invades key and eastern Northward America very frequently, sometimes penetrating as far north every bit Southern Canada, bringing with it the typical oestrus and oppressive humidity of those tropical source regions.

Daytime heating of the air as it moves inland produces widespread showers and thunderstorms, particularly, during the afternoon and evening. At night, there may be sufficient cooling of the world's surface to bring the temperature of the air well-nigh the basis to the dew point and produce fog or stratus clouds. This is dissipated in the early morning by surface heating.

When mT air is lifted, either past crossing mountains or by being forced to ascension over cooler mP or cP air, widespread clouds, numerous showers, and intense thunderstorms are produced.

Although some of the summertime thunderstorm activity in Northern Mexico and the Southwestern United States is the issue of mT air from the tropical Pacific, most of it is associated with mT air from the Gulf of Mexico. This moist air is normally brought in at intermediate levels by easterly and southeasterly flow. Heating and lifting by mountains set off thunderstorms as the air spreads northward along the Sierra-Cascade range, occasionally extending as far every bit northern Idaho, western Montana, and Southern Canada. Some thunderstorm activity develops as mT air spreads northwestward from the Gulf and is lifted forth the eastern slopes of the Rocky Mountains.

On rare occasions, mT air originating in the tropical Pacific spreads north over Northwestern United mexican states and California with thunderstorm action. Usually this is residual mT air from a dying tropical tempest.

Continental Tropical—Summertime

The only source regions for continental tropical air in North America are Mexico and the Southwestern U.s.a.. This air mass is hot, dry, and unstable, and causes droughts and estrus waves when it persists for any length of time. Information technology is similar to the upper-level, subsiding air in the Pacific High, and may really be produced by subsidence from aloft.

In summer, cT air sometimes spreads eastward and northward to embrace portions of the Central or Western United States. Because of its heat and dryness, it has a desiccating effect on wildland fuels, setting the stage for serious fireweather atmospheric condition.

Characteristics of winter and summer air masses are summarized in the following tables.

Table: Characteristics of Winter Air Masses

Air Mass	Lapse rate	Temperature	Surface RH	Visibility	Clouds	Atmospheric precipitation
cP at source region	Stable	Cold	High	First-class	None	None
cP over midcontinent, Southeastern Canada and Eastern United States	Variable	do.	Low	Expert, except in industrial areas and in snow flurries	Stratocumulus in hilly regions, stratocumulus or cumulus forth lee shores of Great Lakes	Snowfall flurries in hilly areas and forth lee shores of Great Lakes
mP at source region	Unstable	Moderately cool	Loftier	Expert	Cumulus	Showers
mP over west coast	practise.	Cool	practice.	exercise.	practice.	do.
mP over Rockies	do.	do.	exercise.	Good, except in mountains and during precipitation	exercise.	Showers or snow
mP over midcontinent, Southeastern Canada and Eastern United States	Stable	mild	Low	Good, except near industrial areas	None, except in mountains	None
mT at source region	Unstable	Warm	High	Good	Cumulus	Showers
mT over Southern United States	Stable in lower layers	do.	do.	Far in afternoon, poor with fog in early morning	Stratus and stratocumulus	Rain or drizzle

Table: Characteristics of Summer Air Masses

Air Mass	Lapse charge per unit	Temperature	Surface RH	Visibility	Clouds	Precipitation
cP at source region	Unstable	Absurd	Low	Proficient	None or few cumulus	None
cP over midcontinent, Southeastern Canada and Eastern The states	practice.	Moderately absurd	practice.	Excellent	Variable cumulus	None
mP at source region	Stable	Cool	High	Fair	Stratus, if any	None
mP over west declension	do.	practise.	do.	Adept, except poor in areas of fog	Fog or stratus	None
mP over Rockies	Unstable	Moderately cool	Moderate	Good	Cumulus	Showers at high elevations
mP over midcontinent, Southeastern Canada and Eastern U.s.	do.	Warm	Depression	practice.	Few cumulus	Showers windward side of Appalachians
mT at source region	practise.	Warm	High	do.	Cumulus, if any	Showers
mT central and eastern continent	do.	Hot	Moderate	Skillful during day except in showers; poor with fog in early morning	Fog in morning, cumulus or cumulonimbus in afternoon	Showers or thunderstorms
cT	Unstable	Hot	Low	Good except in grit storms	None	None

Variations in Air-Mass Conditions

In summer, because of the weaker general circulation, air masses move more slowly and are subject to greater modification. In wintertime, when the full general apportionment is stronger, cold polar air masses move apace away from their source region and penetrate far southward with little modification.

We have considered the usual characteristics of the principal air masses in winter and in summer. Nosotros must realize, however, that there are many variations in individual air masses— variations from day to night, and seasonal variations other than just in wintertime and summer. Nosotros will consider a few general principles to assistance us understand these variations.

1. If the surface over which an air mass is located is warmer than the air mass, the lower layers will exist heated. This results in increased instability, convective mixing and turbulence, and a lowering of surface relative humidity. If sufficient moisture is nowadays, cumulus clouds and possible showers may exist formed. The increased mixing generally results in good visibility.
2. If the surface is colder than the air mass, the lower layers are gradually cooled. This increases the stability and retards convective mixing and turbulence. Water vapor and atmospheric impurities tend to be concentrated in the lower layers, and visibility is decreased. With sufficient moisture, fog and low stratus clouds will course.
3. As a rule, air masses over state and away from their source region tend to exist cooler than the surface during the day, and warmer than the surface at night. Thus, the weather characteristics change accordingly from 24-hour interval to night.
4. In the jump, land surfaces away from source regions warm faster than the h2o or snowcovered surfaces at source regions. This leads to increased instability in the lower layers as air masses leave their source region, and causes considerable thunderstorm activity, hail, and, sometimes, tornadoes.
5. During the summertime, there is the least temperature difference betwixt polar and tropical regions. The general apportionment is weaker and then that air masses move more slowly, and spending more time in transit, are thus more subject to modification. The belt of westerlies is farther north than in wintertime. Every bit a result, tropical air masses penetrate far to the north, but polar air masses are blocked at high latitudes and exercise non penetrate far southward.
6. As the earth's surface begins to cool in the fall, air masses tend to exist more stable in the lower layers, and thunderstorm activeness is reduced. As fall progresses and winter approaches, stable common cold air near the surface becomes deeper and more persistent, encouraging the formation of fog or low stratus clouds.
7. During the wintertime, cold polar air masses move at a faster rate and penetrate far southward. The temperature contrast between polar and tropical regions increases, as does the speed of the full general circulation.

Fronts

We take seen that polar air masses accept time ocean origin are different from those of properties very dissimilar from those of tropical continental origin. Because the various types of air masses, and that air masses having a man- air masses movement into the heart latitudes, information technology is inevitable that they meet somewhere and collaborate. Since air masses have unlike densities, they tend non to mix when they come together. Instead, a aperture surface, or front, is found between them (see page 129).

Some of the weather conditions about adverse to fire control, such equally strong, gusty winds, turbulence, and lightning storms, occur in frontal zones. Sometimes there is insufficient wet in the warm air mass, or inadequate lifting of this mass, so that no precipitation occurs with the front. Strong, gusty, and shifting winds are typical of a dry frontal zone, adding greatly to the difficulty of fire control.

In a frontal zone, the warmer air mass, being lighter, will be forced over the colder air mass. The rotation of the globe deflects the movement of both the cold and the warm air masses as 1 tries to overrun or underride the other, and prevents the formation of a horizontal discontinuity surface. Instead, the frontal surface slopes up over the colder air. The slope varies from virtually 1/50 to 1/300, A 1/50 slope means that for every 50 miles horizontally, the front is i mile higher in the vertical. The amount of slope is dependent upon the temperature dissimilarity between the ii air masses, the difference in wind speed beyond the forepart, and the relative movements of the air masses involved; that is, whether common cold air is replacing warm air at the surface or warm air is replacing common cold air. On a surface weather map, but the intersection of the frontal surface with the earth is indicated. The dissimilarity between the air masses is strongest near the globe's surface, and decreases upward in the atmosphere.

Fronts are classified past the mode they motion relative to the air masses involved. At a common cold front end, cold air is replacing warm air. At a warm forepart, warm air is replacing cold air. A stationary front, every bit the name implies, is temporarily stalled.

The cardinal portions of air masses are commonly associated with areas of loftier force per unit area, but fronts are formed in troughs of low pressure. From a position on a front end, we find that the pressure rises both toward the warmer air and toward the colder air. Because the gradient current of air in the Northern Hemisphere e'er blows with high force per unit area on the right, as ane faces downstream, this means that the wind blows in 1 direction in the common cold air and a different direction in the warm air. At a given location, shown in affiliate vi, the air current shifts in a clockwise direction every bit a front passes—for instance, from southeast to southwest or from southwest to northwest.

The air current-shift line and pressure trough line provide expert clues to the weatherman for the location of fronts, but at that place are other indications to consider. A temperature discontinuity exists beyond a forepart. As a rule, the greater and more than abrupt the temperature dissimilarity, the more intense the front. Weak fronts are characterized by gradual and pocket-sized changes in temperature. The wet contrast between air masses on different sides of a front may exist indicated by the dew-signal temperatures. Usually the common cold air mass will be drier than the warm air mass. Other indications of front location are deject types, pressure changes, and visibility changes.

Types of fronts are distinguished by the style they move relative to the air masses involved. If a front end is moving so that cold air is replacing warm air, it is a cold front. If the warm air is advancing and replacing cold air ahead, the front is a warm front. If a front is not moving, it is a stationary front end. Cold fronts are indicated on weather maps by pointed cusps, and warm fronts past semicircles, on the side toward which they are moving. A stationary front is indicated by a combination of both. (Run into sketch.)

Common cold Fronts

The leading edge of an advancing cold air mass is a cold front. It forms a wedge which pushes under a warm air mass forcing the warm air to rise. Because of surface friction, the lowest layers of the common cold air are slowed down. This increases the steepness of the frontal surface and causes a common cold front to take a blunted appearance when viewed in cross-section. The slopes of cold fronts ordinarily vary betwixt 1/l to 1/150.

At that place are wide variations in the orientation and speed of common cold fronts. Normally, they are oriented in a northeast-southwest management, and they move to the east and southeast, at speeds varying from about 10 to 40 m.p.h. and faster in the winter.

As a cold front end approaches, the southerly winds increase in the warm air ahead of the front. Clouds appear in the management from which the forepart is approaching. The barometric pressure usually falls, reaches its lowest bespeak every bit the front passes, then rises sharply. Winds go strong and gusty and shift sharply to westerly or northwesterly equally the common cold front passes. Temperature and dew point are lower later the cold forepart passes. In frontal zones with precipitation, the heaviest precipitation usually occurs with the passage of the front. Then it may end quickly and be followed past clearing weather.

There are many exceptions to the foregoing general pattern of common cold-front passages. The severity of the atmospheric condition associated with common cold fronts depends upon the moisture and stability of the warm air, the steepness of the front, and the speed of the forepart. Since cold fronts are unremarkably steeper and motion faster than warm fronts, the accompanying ring of weather is narrower, more severe, and commonly of shorter duration than with warm fronts.

With slow-moving cold fronts and stable warm air, rain clouds of the stratus blazon grade in a wide ring over the frontal surface and extend for some distance behind the front. If the warm air is moist and conditionally unstable, thunderstorms may form, with the heaviest rainfall near the frontal zone and immediately following. If the warm air is fairly dry out and the temperature dissimilarity across the front end is small, there may be little or no precipitation and few or no clouds.

With quickly moving cold fronts, the weather is more severe and occupies a narrower ring. The disturbance is also of shorter duration than that caused by a slow-moving front. If the warm air is relatively stable, clouded skies and precipitation may occur for some distance alee of the front end, and the heaviest atmospheric precipitation may occur ahead of the surface cold forepart. If the warm air is moist and conditionally unstable, scattered showers and thunderstorms form merely ahead of the cold forepart. The conditions usually clears rapidly backside a fastmoving cold front, with colder temperatures and gusty, turbulent surface winds following the frontal passage.

Clouds and precipitation encompass a wide bail and extend some distance backside wearisome-moving common cold fronts. If the warm air is moist and stable, stratus-type clouds and steady rain occur, If the warm air is conditionally unstable, showers and thunderstorms are probable.

Nether some conditions, a line of showers and thunderstorms is formed from 50 to 300 miles ahead of, and roughly parallel to, a cold front. This is chosen a squall line. The weather associated with squall lines is often more severe than that associated with the subsequent cold front. After the passage of the squall line, the temperature, air current, and pressure normally revert to conditions similar to those present before the squall line approached. Occasionally, the showers and thunderstorms are scattered along the squall line then that some areas feel strong, gusty winds without any precipitation.

With rapidly moving cold fronts, the weather is mare severe and occupies a narrower band. If the warm air is moist and conditionally unstable, equally in this case, scattered showers and thunderstorms form just alee of the common cold front.

Dry cold fronts ofttimes cause very severe burn down weather in many sections. Dry out cold-front passages may occur in whatsoever region, just they are a major problem in the Southeast. Common cold fronts tend to be drier further away from the low-pressure heart with which they are associated. Thus, a cold forepart associated with a Low passing eastward across Southern Canada or the Northern States may exist very dry out as it passes through the Southeast. In addition, the polar air mass post-obit the cold front end may become quite unstable because of surface heating by the time information technology reaches the Southeast.

The combination of strong, gusty winds and dry, unstable air creates serious fire weather. The second of 2 common cold fronts passing through the Southeast in rapid succession also tends to exist dry. The warm air mass ahead of the first cold forepart may exist moist and produce atmospheric precipitation, but the air mass betwixt the first and second fronts commonly will not have had time to acquire much moisture. Therefore, the 2d cold-front passage may be dry out and will be the more serious from the fire-control standpoint.

The dry out, trailing ends of cold fronts crusade serious fire atmospheric condition wherever they occur. Along the Pacific coast, the winds behind such common cold fronts are, at times, from a northeasterly direction. This offshore direction means that the air flows from high elevations to depression elevations and has foehn characteristics. The potent, shifting, gusty winds of the cold-front end passage combine with the dry foehn wind to the rear of the front to produce a shortlived but extremely disquisitional fire-weather condition.

Warm Fronts

The leading edge of an advancing warm air mass is called a warm front. The warm air is overtaking and replacing the common cold air, but at the same fourth dimension sliding up over the wedge of common cold air. Warm fronts are flatter than common cold fronts, having slopes ranging from i/100 to 1/300. Because of this flatness, cloudiness and precipitation extend over a wide area alee of the front, providing, of course, that at that place is sufficient wet in the warm air.

If the warm air above a warm forepart is moist and stable, clouds are of the stratus type. The sequence of cloud types is cirrus, cirrostratus, altostratus, and nimbostratus. Atmospheric precipitation is steady and increases gradually with the arroyo of a front end.

Warm fronts are less distinct than cold fronts and more difficult to locate on weather maps. This is particularly true in rough terrain where highelevation areas may extend upwardly into the warm air before the warm front has been felt at lower elevation stations.

The commencement indication of the approach of warm, moist air in the upper levels ahead of the surface warm front may be very loftier, thin, cirrostratus clouds which requite the sky a milky appearance. These are followed by middle-level clouds which darken and thicken as precipitation begins. This sequence may be interrupted past short clearing periods, just the advent of successively lower deject types indicates the steady approach of the warm front. Rains may precede the inflow of the surface warm front by as much as 300 miles. Pelting falling through the cold air raises the humidity to the saturation level and causes the formation of low stratus clouds.

If the warm air in a higher place the warm front is moist and stable, the clouds which grade are of the stratus type. The sequence is cirrus, cirrostratus, altostratus, and nimbostratus. Atmospheric precipitation is a steady type and increases gradually with the arroyo of the surface front.

If the warm air above a warm forepart is moist and conditionally unstable, altocumulus and cumulonimbus clouds form. Often, thunderstorms volition be embedded in the cloud masses.

If the warm air is moist and conditionally unstable, altocumulus and cumulonimbus clouds grade, and, often, thunderstorms volition be embedded in the cloud masses that normally accompany a warm front.

The charge per unit of motion of warm fronts is about one-half that of cold fronts. Winds are usually non as stiff or gusty with the arroyo of warm fronts as with cold fronts. The shift in air current is generally from an easterly to a southerly management as a warm front passes. After it passes, temperatures rise, precipitation commonly stops, and clouds diminish or vanish completely.

From the standpoint of fire atmospheric condition, warm fronts associated with moist air are a real do good. The accompanying precipitation is widespread and long-lasting, and ordinarily is sufficient to thoroughly moisten forest fuels, reducing the fire danger.

Stationary Fronts

Surface winds on either side of a stationary front tend to blow parallel to the front, but in opposite directions. Stationary fronts are indicated on weather maps by alternate sharp cusps and semicircles on apposite sides of the front end.

When the forces acting on two adjacent air masses are such that the frontal zone shows little movement, the front end is chosen a stationary front end. Surface winds on either side of the front tend to blow parallel to the front, but in opposite directions. Weather conditions occurring with a stationary front are variable; usually they are like to those constitute with a warm front, though less intense. If the air is dry out, there may be little cloudiness or precipitation. If the air is moist, there may be continuous precipitation with stable, warm air, or showers and thunderstorms with conditionally unstable, warm air. The precipitation area is likely to exist broader than that associated with a cold front, but not as extensive as with a warm front.

Stationary fronts may apace change back to moving fronts as a slight imbalance of forces interim on the air masses develops. A stationary front may oscillate back and along, causing changing winds and weather conditions at a given location. It may become a common cold or warm forepart, or a frontal wave may develop, equally nosotros will see in the next department.

Frontal Waves and Occlusions

A frontal surface is similar to a water surface. A disturbance such as air current can crusade the formation of waves on the h2o. If the wave moves toward the shoreline, it grows until it becomes topheavy and breaks. Similarly, along frontal surfaces in the atmosphere a disturbance may grade a wave. This disturbance may be a topographic irregularity, the influence of an upperlevel trough, or a change in the air current field cause by local convection. Waves ordinarily course on stationary fronts or deadening-moving cold fronts, where winds on the ii sides of the forepart are blowing parallel to the front with a strong shearing motion.

When a section of a front is disturbed, the warm air begins to menses up over and displace some of the common cold air. Cold air to the rear of the disturbance displaces some of the warm air. Thus, one department of the front begins to deed like a warm front, and the side by side section like a common cold front. This deformation is called a frontal moving ridge.

The pressure at the meridian of the frontal wave falls, and a low-pressure center with a counterclockwise (cyclonic) circulation is formed. If the pressure continues to fall, the moving ridge may develop into a major cyclonic system. The Low and its frontal wave by and large movement in the direction of the wind menses in the warm air, which is usually toward the east or northeast.

The life bicycle of a frontal moving ridge includes the post-obit steps: A. A disturbed section of a front. B. Cold air begins to displace warm air to the rear of the disturbance, and warm air alee tends to override the cold air. The front ahead of the disturbance becomes a warm front, and the portion to the rear becomes a cold front. C. A cyclonic circulation is established and pressure falls at the crest of the wave. D. After the cold forepart overtakes the worm front, an occlusion is formed and the organisation enters its dying phases.

As the organisation moves, the cold front moves faster than the warm front and eventually overtakes the warm front. The warm air is forced aloft between the cold air behind the cold forepart and the retreating cold air ahead of the warm front. The resulting combined front end is called an occlusion or occluded front. This is the time of maximum intensity of the wave cyclone. The pressure becomes quite low in the occluded organisation with strong winds around the Low Usually the system is accompanied by widespread cloudiness and precipitation. The heaviest atmospheric precipitation occurs to the north of the low-force per unit area heart.

Equally the occlusion continues to grow in length, the cyclonic circulation diminishes in intensity, the low-pressure center begins to fill, and the frontal motility slows downwards.

There are 2 types of occluded fronts— a warm-front type and a cold-forepart blazon—depending on whether the surface air ahead of the occlusion is warmer or colder than the air to the rear.

The cold-front type is predominant over most of the continent, especially the key and eastern regions. The conditions and winds with the passage of a common cold-front end occlusion are similar to those with a cold front. Alee of the occlusion, the weather and deject sequence is much like that associated with warm fronts.

Most warm-front occlusions are found along the west declension. The air mass to the rear is warmer than the air mass ahead. Therefore, when the common cold front overtakes the warm forepart, it rides up the warm-front surface and becomes an upper cold forepart.

A cross department through a cold-front occlusion shows the warm air having been lifted in a higher place the two colder air masses. At the surface, cold air is displacing cool air. The conditions and winds associated with the frontal passage are similar to those with a cold front.

The weather associated with a warm-forepart occlusion has characteristics of both warm-forepart and common cold-front atmospheric condition. The sequence of clouds and atmospheric condition ahead of the occlusion is similar to that of a warm forepart. Cold-front weather occurs nigh the upper common cold front. With moist and conditionally unstable air, thunderstorms may occur. At the surface, the passage of a warm-front occlusion is much similar that of a warm forepart. The rainy season in the Pacific Northwest, British Columbia, and southeastern Alaska is dominated past a succession of warm-front occlusions that movement in from the Pacific.

Common cold fronts crossing the Rocky Mountains from the westward are forced to ascension over the mountains. Quite frequently in wintertime, a very common cold air moss is located due east of the mountains. The common cold front then does non return to the surface, but rides aloft ever the common cold air equally an upper cold front. The frontal activity takes identify above the cold air.

Another type of upper cold front end should be mentioned. Common cold fronts approaching the Rocky Mountains from the west are forced to rise and cross over the mountains. Quite frequently in wintertime, a very common cold air mass is located e of the mountains. And so, the cold front does not return to the surface, just rides aloft over the cold air as an upper common cold front often accompanied past thundershowers. When such a front meets an mT air mass, and underrides it, a very unstable condition is produced that will result in numerous thunderstorms and, occasionally, tornadoes.

Summary

When air stagnates in a region where surface characteristics are compatible, it acquires those characteristics and becomes an air mass. Warm, moist air masses are formed over tropical waters; cold, moist air masses over the northern oceans; cold, dry air masses over the northern continent; and warm, dry air masses over arid regions.

Air masses accept feature weather in their source regions. Simply, as air masses leave their source regions, they are modified co-ordinate to the surface over which they travel, and the air-mass weather changes.

In frontal zones, where differing air masses meet, considerable weather is concentrated. Cloudiness, precipitation, and strong and shifting winds are feature of frontal passages; but, occasionally, frontal passages are dry out and adversely touch on burn down behavior.

In discussing many of the topics and so far, it has been necessary to mention different types of clouds from time to fourth dimension. Different deject types are associated with stability and instability, and certain cloud sequences are feature of different frontal systems. In the post-obit affiliate, nosotros will discuss types of clouds more fully and examine the precipitation processes that develop in clouds.

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Source: https://www.nwcg.gov/publications/pms425-1/air-masses-and-fronts

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