If that same polar air mass moves south from Canada into the southern U. This is called a continental polar air mass cP. The motion of air mass motion is usually based upon the air flow in the upper atmosphere. As the jet stream changes intensity and position, it affects the motion and strength of air masses.
Where air masses converge, they form boundaries called "fronts". Fronts are identified by change of temperature based upon their motion.
With a cold front, a colder air mass is replacing a warmer air mass. A warm front is the opposite affect in that warm air replaces cold air. There is also a stationary front, which, as the name implies, means the boundary between two air masses does not move.
The motion of air masses also affects where a good portion of precipitation occurs. The air of cold air masses is more dense than warmer air masses. Therefore, as these cold air masses move, the dense air undercuts the warmer air masses forcing the warm air up and over the colder air causing it to rise into the atmosphere. So, fronts just don't appear at the surface of the earth, they have a vertical structure or slope to them as well. When maritime air masses travel inland, the air is forced upward over coastal mountain ranges, where it produces precipitation, hence losing its moisture content and taking on characteristics more typical of continental air masses.
Sometimes, this change is apparent in just a few short miles, depending on the proximity of the mountains to the sea. The coastal ranges essentially comprise a geographic barrier, which explains why the weather of areas nestled along the Pacific coast usually differs so greatly from the weather of areas east of the mountains. Harris holds a Bachelor of Science in Mathematics from Penn State University; she taught high school math for several years and has also worked in the field of instructional design.
The Three Types of Weather Fronts. Differences Between Mediterranean Climate and Humid Types of Cyclones. Where Are Bogs Located? What Causes Katabatic Winds? Equatorial Air Mass Characteristics. On being 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 generally clear skies. If, however, it cannot descend on the lee side of the mountains, and instead continues eastward over a dome of cold cP air, snow may occur.
East of the Rockies, mP air at the surface in winter is comparatively warm and dry, having lost much of its moisture in passing over the mountains. Skies are relatively clear. If this air mass reaches the Gulf of Mexico, it is eventually changed into an mT air mass. The outflow from the Great Basin High may give rise to strong, dry foehn winds in a number of the surrounding States. At times during the winter, 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, as well as the North Pacific, the trajectory of Atlantic mP air is limited to the northeastern seaboard.
Most of the maritime tropical air masses affecting temperate North America originate over the Gulf of Mexico or Caribbean Sea. They are warm, have a high 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 below by contact with the cooler continent and becomes stabilized in the lower levels. Fog and low stratus clouds usually occur at night and dissipate during the day as this air mass invades the Mississippi Valley and the Great 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 large cumulus clouds, heavy showers, and frequent thunderstorms result.
Maritime tropical air in winter produces nighttime cloudiness and fog in the Mississippi Valley and Great Plains and showers or snow over the Appalachians and in areas where it overruns a cooler air mass. Maritime tropical air seldom reaches as far as the Canadian border or the New England States at the surface in winter. Nevertheless, it 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 up over the denser air.
More will be said about this process in the section on fronts. The tropical Pacific is also a source region for mT air, but Pacific mT seldom enters the continent. When it does, it is usually brought in with a lowpressure system in Northern Mexico or California, where the Pacific mT air can cause heavy rainfall when rapidly forced aloft by the mountains.
Continental polar air in summer brings generally 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 up enough moisture 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 result in little surface cooling and frequently in actual heating of the air near the ground.
The air mass, therefore, may be relatively unstable in the lower layers in contrast to its extreme stability during the winter. Since the air is quite dry from the surface to high levels, the relative instability rarely produces cloudiness or precipitation. The general atmospheric circulation is weaker during the summer, and polar outbreaks move more slowly than in winter. As a result, cP air undergoes tremendous changes in passing slowly from its source region to Southern United States.
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 summer, being largely covered with crops, grass, forests, and other vegetation. Transpiration from these plants and evaporation from water bodies and moist soil increase the moisture content of cP air rather rapidly.
As the moisture content increases, cloudiness also increases. The weather associated with cP air as it passes through Canada and enters the United States is generally fair and dry. Frequent intrusions of this air give rise to much of the fire weather 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 air masses in summer originate in the same general area over the Pacific Ocean as in winter. In summer, however, the ocean is relatively cool 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 being carried to higher levels.
Aloft, this air mass remains very dry, usually even drier than summer cP, and becomes quite warm through the subsidence which takes place in the Pacific High.
As mP air approaches the Pacific coast, the cold, upwelling waters along the shore cause further cooling, increasing relative humidity, and stimulating the formation of considerable fog or low stratus clouds. Thus, along the Pacific coast, summer mP is characterized by a cool, humid marine layer from 1,, feet thick, often with fog or low stratus clouds, a strong inversion capping the marine layer, and warm, dry, subsiding air above. As mP air moves inland from the west coast, the strong 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 as mP air crosses the mountains may result 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 winter, 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.
Continuing eastward, it becomes warmer and more unstable, and picks up moisture from the earth and plants. By the time it reaches the Appalachians, it has become unstable and moist enough so that lifting can again produce showers or thunderstorms. Stratus clouds and fog along the Pacific coast 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 southward bringing cool weather 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, humid air mass.
Maritime tropical air in its source region over the Gulf of Mexico and the Caribbean in summer has properties similar to those in winter, except that it is conditionally unstable to higher levels, slightly warmer, and more moist.
In summer, mT air invades central and eastern North America very frequently, sometimes penetrating as far north as Southern Canada, bringing with it the typical heat 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. This is dissipated in the early morning by surface heating. When mT air is lifted, either by crossing mountains or by being forced to rise over cooler mP or cP air, widespread clouds, numerous showers, and intense thunderstorms are produced.
Although some of the summer thunderstorm activity in Northern Mexico and the Southwestern United States is the result of mT air from the tropical Pacific, most of it is associated with mT air from the Gulf of Mexico. This moist air is usually 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 as northern Idaho, western Montana, and Southern Canada.
Some thunderstorm activity develops as mT air spreads northwestward from the Gulf and is lifted along the eastern slopes of the Rocky Mountains. On rare occasions, mT air originating in the tropical Pacific spreads northward over Northwestern Mexico and California with thunderstorm activity. Usually this is residual mT air from a dying tropical storm. This air mass is hot, dry, and unstable, and causes droughts and heat waves when it persists for any length of time.
It is similar to the upper-level, subsiding air in the Pacific High, and may actually be produced by subsidence from aloft. In summer, cT air sometimes spreads eastward and northward to cover 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 conditions. In summer, because of the weaker general circulation, air masses move more slowly and are subject to greater modification.
In winter, when the general circulation is stronger, cold polar air masses move rapidly 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.
We must realize, however, that there are many variations in individual air masses— variations from day to night, and seasonal variations other than just in winter and summer. We will consider a few general principles to help us understand these variations. We have seen that polar air masses have time ocean origin are different from those of properties very different from those of tropical continental origin.
Because the various types of air masses, and that air masses having a man- air masses move into the middle latitudes, it is inevitable that they meet somewhere and interact. Since air masses have different densities, they tend not to mix when they come together. Instead, a discontinuity surface, or front , is found between them see page Some of the weather conditions most adverse to fire control, such as strong, gusty winds, turbulence, and lightning storms, occur in frontal zones.
Sometimes there is insufficient moisture 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 earth deflects the movement of both the cold and the warm air masses as one 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 amount of slope is dependent upon the temperature contrast between the two air masses, the difference in wind speed across the front, and the relative movements of the air masses involved; that is, whether cold air is replacing warm air at the surface or warm air is replacing cold air. On a surface weather map, only the intersection of the frontal surface with the earth is indicated.
Fronts are classified by the way they move relative to the air masses involved. At a cold front, cold air is replacing warm air. At a warm front, warm air is replacing cold air. A stationary front, as the name implies, is temporarily stalled. The central portions of air masses are usually associated with areas of high pressure, but fronts are formed in troughs of low pressure. From a position on a front, we find that the pressure rises both toward the warmer air and toward the colder air.
Because the gradient wind in the Northern Hemisphere always blows with high pressure on the right, as one faces downstream, this means that the wind blows in one direction in the cold air and a different direction in the warm air. At a given location, shown in chapter 6 , the wind shifts in a clockwise direction as a front passes—for example, from southeast to southwest or from southwest to northwest.
The wind-shift line and pressure trough line provide good clues to the weatherman for the location of fronts, but there are other indications to consider.
A temperature discontinuity exists across a front. As a rule, the greater and more abrupt the temperature contrast, the more intense the front. Weak fronts are characterized by gradual and minor changes in temperature. The moisture contrast between air masses on different sides of a front may be indicated by the dew-point temperatures.
Usually the cold air mass will be drier than the warm air mass. Other indications of front location are cloud types, pressure changes, and visibility changes.
Types of fronts are distinguished by the way they move relative to the air masses involved. If a front 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. Cold fronts are indicated on weather maps by pointed cusps, and warm fronts by semicircles, on the side toward which they are moving. A stationary front is indicated by a combination of both. See sketch. 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 cold air are slowed down. This increases the steepness of the frontal surface and causes a cold front to have a blunted appearance when viewed in cross-section. There are wide variations in the orientation and speed of cold fronts. Usually, they are oriented in a northeast-southwest direction, and they move to the east and southeast, at speeds varying from about 10 to 40 m. As a cold front approaches, the southerly winds increase in the warm air ahead of the front.
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