Air mass
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In meteorology, an air mass is a large volume of air that have characteristics of temperature and water vapor content. Air masses cover many hundreds or thousands of square miles, and slowly change in accordance with the surface below them.
[edit] Classification and Notation
Air masses are classified according to their approximate environmental origin. An air mass is classified as Arctic, Polar, Tropical, or Equatorial. It is also classified as either maritime or continental. Maritime air is a moist air mass, whereas continental air is relatively dry.
Air masses are noted on some weather charts using a particular system of notation. However, air mass terminology is somewhat subjective, that is, it is not defined by any definite, universally-accepted quantitative parameters. It is used primarily for general descriptive purposes in meteorological discussions.
Air masses of oceanic origin are denoted with a lower-case "m" ("maritime"), while air masses of continental origin are denoted with a lower-case "c" ("continental"). Air masses are also denoted as either Arctic (upper-case "A", or "AA" for Antarctic air masses), polar (upper-case "P"), tropical (upper-case "T"), or equatorial (upper-case "E"). These two sets of attributes are used in combinations depending on the air mass being described. 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".
In older charts, an upper case "S" was occasionally used to denote something called a "superior" air mass. This was regarded as an adiabatically drying and warming air mass descending from aloft. In South Asia, an upper case "M" (for "monsoon") has been occasionally used to denote an air mass within the summer monsoon regime in that region.
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). An example of this might be a polar air mass blowing over the Gulf Stream, denoted as "cPk". Occasionally, especially on older charts, 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'.
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).
[edit] Movement and fronts
Air mass terms refer to the fact that air masses acquire characteristics from a given region which they may occupy for any significant length of time. For example, Polar air masses form during the northern winter as intense nighttime radiation and loss of daylight chills the air to great depths, and maritime air masses generally form over oceans and seas where widespread evaporation occurs. When air masses move into regions with different environmental conditions, they are modified. Even without movement, air masses can vary gradually over distance, with one type gradually becoming another. The boundary between two air masses can also be comparatively sharp. Where it is, the boundary is termed a "front". Fronts are often characterized by inclement weather, and are usually associated with areas of low atmospheric pressure termed frontal systems.
