Air mass

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Different air masses which affect North America as well as other continents, tend to be separated by frontal boundaries Airmassesorigin.png
Different air masses which affect North America as well as other continents, tend to be separated by frontal boundaries

In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry, while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density (temperature or moisture) characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character. Classification schemes tackle an air mass's characteristics, as well as modification.

Contents

Classification and Notation

Source regions of global air masses Air masses.svg
Source regions of global air masses

The Bergeron classification is the most widely accepted form of air mass classification, though others have produced more refined versions of this scheme over different regions of the globe. [1] [2] Air mass classification involves three letters. The first letter describes its moisture properties – "c" represents continental air masses (dry), and "m" represents maritime air masses (moist). Its source region follows: "T" stands for Tropical, "P" stands for Polar, "A" stands for Arctic or Antarctic, "M" stands for monsoon, "E" stands for Equatorial, and "S" stands for adiabatically drying and warming air formed by significant downward motion in the atmosphere. For instance, an air mass originating over the desert southwest of the United States in summer may be designated "cT". An air mass originating over northern Siberia in winter may be indicated as "cA". [3]

The stability of an air mass may be shown using a third letter, either "k" (air mass colder than the surface below it) or "w" (air mass warmer than the surface below it). [3] An example of this might be a polar air mass blowing over the Gulf Stream, denoted as "cPk". Occasionally, one may also encounter the use of an apostrophe or "degree tick" denoting that a given air mass having the same notation as another it is replacing is colder than the replaced air mass (usually for polar air masses). For example, a series of fronts over the Pacific might show an air mass denoted mPk followed by another denoted mPk'. [3]

Another convention utilizing these symbols is the indication of modification or transformation of one type to another. For instance, an Arctic air mass blowing out over the Gulf of Alaska may be shown as "cA-mPk". Yet another convention indicates the layering of air masses in certain situations. For instance, the overrunning of a polar air mass by an air mass from the Gulf of Mexico over the Central United States might be shown with the notation "mT/cP" (sometimes using a horizontal line as in fraction notation). [4]

Characteristics

Tropical and equatorial air masses are hot as they develop over lower latitudes. Tropical air masses have lower pressure because hot air rises and cold air sinks. Those that develop over land (continental) are drier and hotter than those that develop over oceans, and travel poleward on the southern periphery of the subtropical ridge. [5] Maritime tropical air masses are sometimes referred to as trade air masses. Maritime tropical air masses that affect the United States originate in the Caribbean Sea, southern Gulf of Mexico, and tropical Atlantic east of Florida through the Bahamas. [6] Monsoon air masses are moist and unstable. Superior air masses are dry, and rarely reach the ground. They normally reside over maritime tropical air masses, forming a warmer and drier layer over the more moderate moist air mass below, forming what is known as a trade wind inversion over the maritime tropical air mass.

Continental Polar air masses (cP) are air masses that are cold and dry due to their continental source region. Continental polar air masses that affect North America form over interior Canada. Continental Tropical air masses (cT) are a type of tropical air produced by the subtropical ridge over large areas of land and typically originate from low-latitude deserts such as the Sahara Desert in northern Africa, which is the major source of these air masses. Other less important sources producing cT air masses are the Arabian Peninsula, the central arid/semi-arid part of Australia and deserts lying in the Southwestern United States. Continental tropical air masses are extremely hot and dry. [7] Arctic, Antarctic, and polar air masses are cold. The qualities of arctic air are developed over ice and snow-covered ground. Arctic air is deeply cold, colder than polar air masses. Arctic air can be shallow in the summer, and rapidly modify as it moves equatorward. [8] Polar air masses develop over higher latitudes over the land or ocean, are very stable, and generally shallower than arctic air. Polar air over the ocean (maritime) loses its stability as it gains moisture over warmer ocean waters. [9]

Movement and Fronts

Picture of cold front (left part of the image) moving over the Czech Republic Cold front (left) above Zdimerice in Czechia, 2017.jpg
Picture of cold front (left part of the image) moving over the Czech Republic

A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity. Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by stratiform precipitation and fog. The weather usually clears quickly after a front's passage. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift. [10]

Cold fronts and occluded fronts generally move from west to east, while warm fronts move poleward. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts. [11] When a front becomes stationary, and the density contrast across the frontal boundary vanishes, the front can degenerate into a line which separates regions of differing wind velocity, known as a shearline. [12] This is most common over the open ocean.

Modification

Lake-effect snow bands near the Korean Peninsula Snow Clouds in Korea.jpg
Lake-effect snow bands near the Korean Peninsula

Air masses can be modified in a variety of ways. Surface flux from underlying vegetation, such as forest, acts to moisten the overlying air mass. [13] Heat from underlying warmer waters can significantly modify an air mass over distances as short as 35 kilometres (22 mi) to 40 kilometres (25 mi). [14] For example, southwest of extratropical cyclones, curved cyclonic flow bringing cold air across the relatively warm water bodies can lead to narrow lake-effect snow bands. Those bands bring strong localized precipitation since large water bodies such as lakes efficiently store heat that results in significant temperature differences (larger than 13 °C or 23 °F) between the water surface and the air above. [15] Because of this temperature difference, warmth and moisture are transported upward, condensing into vertically oriented clouds (see satellite picture) which produce snow showers. The temperature decrease with height and cloud depth are directly affected by both the water temperature and the large-scale environment. The stronger the temperature decrease with height, the deeper the clouds get, and the greater the precipitation rate becomes. [16]

See also

Related Research Articles

<span class="mw-page-title-main">Cyclone</span> Large scale air mass that rotates around a strong center of low pressure

In meteorology, a cyclone is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above. Cyclones are characterized by inward-spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes, and dust devils lie within the smaller mesoscale.

<span class="mw-page-title-main">Surface weather analysis</span> Type of weather map

Surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations.

<span class="mw-page-title-main">Anticyclone</span> Weather phenomenon of high pressure, as opposed to a cyclone

An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from above. Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure.

<span class="mw-page-title-main">Precipitation</span> Product of the condensation of atmospheric water vapor that falls under gravity

In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation but colloids, because the water vapor does not condense sufficiently to precipitate. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called showers.

<span class="mw-page-title-main">Nor'easter</span> Type of cyclone occurring in the northern Atlantic coast of North America

A nor'easter, is a large-scale extratropical cyclone in the western North Atlantic Ocean. The name derives from the direction of the winds that blow from the northeast. The term is commonly used in the winter in New England and Atlantic Canada.

<span class="mw-page-title-main">High-pressure area</span> Region with higher atmospheric pressure

A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interplays between the relatively larger-scale dynamics of an entire planet's atmospheric circulation.

<span class="mw-page-title-main">Low-pressure area</span> Area with air pressures lower than adjacent areas

In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather, while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places:

<span class="mw-page-title-main">Synoptic scale meteorology</span> 1000-km-order method of measuring weather systems

In meteorology, the synoptic scale is a horizontal length scale of the order of 1,000 km (620 mi) or more. This corresponds to a horizontal scale typical of mid-latitude depressions. Most high- and low-pressure areas seen on weather maps are synoptic-scale systems, driven by the location of Rossby waves in their respective hemisphere. Low-pressure areas and their related frontal zones occur on the leading edge of a trough within the Rossby wave pattern, while high-pressure areas form on the back edge of the trough. Most precipitation areas occur near frontal zones. The word synoptic is derived from the Ancient Greek word συνοπτικός (sunoptikós), meaning "seen together".

<span class="mw-page-title-main">Trade winds</span> Equatorial east-to-west prevailing winds

The trade winds or easterlies are permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, strengthening during the winter and when the Arctic oscillation is in its warm phase. Trade winds have been used by captains of sailing ships to cross the world's oceans for centuries. They enabled European colonization of the Americas, and trade routes to become established across the Atlantic Ocean and the Pacific Ocean.

A stationary front is a weather front or transition zone between two air masses when each air mass is advancing into the other at speeds exceeding 5 knots at the ground surface. On weather maps, it is illustrated as a solid line of alternating blue spikes pointing to the warmer air mass and red domes facing the colder air mass.

<span class="mw-page-title-main">Pressure system</span> Relative peak or lull in the sea level pressure distribution

A pressure system is a peak or lull in the sea level pressure distribution. The surface pressure at sea level varies minimally, with the lowest value measured 87 kilopascals (26 inHg) and the highest recorded 108.57 kilopascals (32.06 inHg). High- and low-pressure systems evolve due to interactions of temperature differentials in the atmosphere, temperature differences between the atmosphere and water within oceans and lakes, the influence of upper-level disturbances, as well as the amount of solar heating or radiationized cooling an area receives. Pressure systems cause weather to be experienced locally. Low-pressure systems are associated with clouds and precipitation that minimize temperature changes throughout the day, whereas high-pressure systems normally associate with dry weather and mostly clear skies with larger diurnal temperature changes due to greater radiation at night and greater sunshine during the day. Pressure systems are analyzed by those in the field of meteorology within surface weather maps.

<span class="mw-page-title-main">Weather front</span> Boundary separating two masses of air of different densities

A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For instance, cold fronts can bring bands of thunderstorms and cumulonimbus precipitation or be preceded by squall lines, while warm fronts are usually preceded by stratiform precipitation and fog. In summer, subtler humidity gradients known as dry lines can trigger severe weather. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.

<span class="mw-page-title-main">Mesoscale convective system</span> Complex of thunderstorms organized on a larger scale

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

<span class="mw-page-title-main">Climate classification</span> Systems that categorize the worlds climates

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<span class="mw-page-title-main">Precipitation types</span> Characters, formations, and phases of water condensed in the atmosphere

In meteorology, the different types of precipitation often include the character, formation, or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation. Orographic precipitation occurs when moist air is forced upwards over rising terrain and condenses on the slope, such as a mountain.

<span class="mw-page-title-main">Climate of the United States</span> Varies due to changes in latitude, and a range of geographic features

The climate of the United States varies due to changes in latitude, and a range of geographic features, including mountains and deserts. Generally, on the mainland, the climate of the U.S. becomes warmer the further south one travels, and drier the further west, until one reaches the West Coast.

<span class="mw-page-title-main">Atmospheric instability</span> Condition where the Earths atmosphere is generally considered to be unstable

Atmospheric instability is a condition where the Earth's atmosphere is considered to be unstable and as a result local weather is highly variable through distance and time. Atmospheric stability is a measure of the atmosphere's tendency to discourage vertical motion, and vertical motion is directly correlated to different types of weather systems and their severity. In unstable conditions, a lifted thing, such as a parcel of air will be warmer than the surrounding air. Because it is warmer, it is less dense and is prone to further ascent.

<span class="mw-page-title-main">Rain</span> Precipitation in the form of water droplets

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<span class="mw-page-title-main">Cold front</span> Leading edge of a cooler mass of air

A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone, at the leading edge of its cold air advection pattern—known as the cyclone's dry "conveyor belt" flow. Temperature differences across the boundary can exceed 30 °C (54 °F) from one side to the other. When enough moisture is present, rain can occur along the boundary. If there is significant instability along the boundary, a narrow line of thunderstorms can form along the frontal zone. If instability is weak, a broad shield of rain can move in behind the front, and evaporative cooling of the rain can increase the temperature difference across the front. Cold fronts are stronger in the fall and spring transition seasons and are weakest during the summer.

<span class="mw-page-title-main">Glossary of meteorology</span> List of definitions of terms and concepts commonly used in meteorology

This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.

References

  1. ↑ Please note that the latitudes in the picture are incorrect on the northern hemisphere. 60 should read 70 and 30 should read 40.
  2. ↑ H. C. Willett (June 1933). "American Air Mass Properties" (PDF). Papers in Physical Oceanography and Meteorology. Massachusetts Institute of Technology. 2 (2). Retrieved 2009-10-28.
  3. 1 2 3 Glossary of Meteorology (June 2000). "Airmass Classification". American Meteorological Society. Archived from the original on 11 June 2008. Retrieved 2008-05-22.
  4. ↑ United States Weather Bureau (1950-02-01). "Daily Weather Maps: February 1, 1950". United States Department of Commerce . Retrieved 2009-10-28.
  5. ↑ Glossary of Meteorology (June 2000). "Tropical air". American Meteorological Society. Archived from the original on 2011-06-06. Retrieved 2009-10-28.
  6. ↑ Glossary of Meteorology (June 2000). "Trade air". American Meteorological Society. Archived from the original on 2011-06-06. Retrieved 2009-10-28.
  7. ↑ Glossary of Meteorology (June 2000). "Superior air". American Meteorological Society. Archived from the original on 2011-06-06. Retrieved 2009-10-28.
  8. ↑ Glossary of Meteorology (June 2000). "Arctic air". American Meteorological Society. Archived from the original on 2012-03-15. Retrieved 2009-10-28.
  9. ↑ Glossary of Meteorology (June 2000). "Polar air". American Meteorological Society. Archived from the original on 2012-10-02. Retrieved 2009-10-28.
  10. ↑ Climate Change Research Center (2000-11-10). "Lesson 7: Clouds and Precipitation". University of New Hampshire. Archived from the original on January 11, 2005. Retrieved 2007-04-29.
  11. ↑ David Roth (2006-12-14). "Unified Surface Analysis Manual" (PDF). Hydrometeorological Prediction Center. Archived (PDF) from the original on 29 September 2006. Retrieved 2006-10-22.
  12. ↑ Glossary of Meteorology (June 2000). "Shear Line". American Meteorological Society. Archived from the original on 2007-03-14. Retrieved 2006-10-22.
  13. ↑ Jeffrey M. Freedman; David R. Fitzjarrald (August 2001). "Postfrontal Airmass Modification" (PDF). Journal of Hydrometeorology. American Meteorological Society. 2 (4): 419–437. Bibcode:2001JHyMe...2..419F. doi:10.1175/1525-7541(2001)002<0419:PAM>2.0.CO;2. Archived from the original (PDF) on 2005-11-13. Retrieved 2009-08-22.
  14. ↑ Jun Inoue; Masayuki Kawashima; Yasushi Fujiyoshi; Masaaki Wakatsuchi (October 2005). "Aircraft Observations of Air-mass Modification Over the Sea of Okhotsk during Sea-ice Growth". Boundary-Layer Meteorology. 117 (1): 111–129. Bibcode:2005BoLMe.117..111I. doi:10.1007/s10546-004-3407-y. S2CID   121768400.
  15. ↑ B. Geerts (1998). "Lake Effect Snow". University of Wyoming . Retrieved 2008-12-24.
  16. ↑ Greg Byrd (1998-06-03). "Lake Effect Snows". University Corporation for Atmospheric Research. Archived from the original on 17 June 2009. Retrieved 2009-07-12.
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