AIR MASS CLASSIFICATION LEARNING  OBJECTIVE: Define air mass classification and describe how the classification will change when characteristics modify. Air masses are classified according to geographic source  region,  moisture  content,  and  thermodynamic process. Geographic Origin The   geographical   classification   of   air   masses, which  refers  to  the  source  region  of  the  air  mass, divides air masses into four basic categories: arctic or antarctic (A), polar (P), tropical (T), and equatorial (E). An    additional    geographical    classification    is    the superior (5) air mass. The superior air mass is generally found aloft over the southwestern United States, but is sometimes located at or near the surface. Moisture Content The    arctic    (A),    polar    (P),    and    tropical    (T) classifications  are  further  broken  down  by  moisture content. An air mass is considered to be maritime (m) if its source of origin is over an oceanic surface. If the air mass  originates  over  a  land  surface,  it  is  considered continental (c). Thus, a moist, maritime arctic air mass is designated m; and a drier, continental arctic air mass is designated c.  Equatorial (E) air is found exclusively over the ocean surface in the vicinity of the equator and is designated neither c nor m but simply E. Thermodynamic Process The  thermodynamic  classification  applies  to  the relative warmth or coldness of the air mass. A warm air mass (w) is warmer than the underlying surface; a cold air mass (k) is colder than the underlying surface. For example,  a  continental  polar  cold  air  mass  over  a warmer    surface    is    classified    as    cPk.    An    mTw classification indicates that the air mass is a maritime tropical warm air mass and overlays a cooler surface. Air masses can usually be identified by the type of clouds  within  them.  Cold  air  masses  usually  show cumuliform clouds, whereas warm air masses contain stratiform   clouds.   Sometimes,   and   with   some   air masses, the thermodynamic classification may change from  night  to  day.  A  particular  air  mass  may  show  k characteristics during the day and w characteristics at night and vice versa.  The designators and descriptions for the classifications of air masses are listed in table 4-1. 4-4 Designator Description cAk Continental arctic air that is colder than the surface over which it lies. cAw Continental arctic air that is warmer than the surface over which it lies. mAk Maritime arctic air that is colder than the surface over which it lies. cPw Continental polar air that is warmer than the surface over which it is moving. cPk Continental polar air that is colder than the surface over which it is moving. mPw Maritime polar air that is warmer than the surface over which it is moving. mPk Maritime polar air that is colder than the surface over which it is moving. mTw Maritime tropical air that is warmer than the surface over which it is moving. mTk Maritime tropical air that is colder than the surface over which it is moving. cTw Continental tropical air that is warmer than the surface over which it is moving. cTk Continental tropical air that is colder than the surface over which it is moving. Ek Maritime equatorial air that is colder than the surface over which it is moving. Ew Maritime equatorial air that is warmer than the surface over which it is moving. S Superior air, found generally aloft over the southwestern United States, and occassionally at or near the surface. Table 4-1.—Classification of Air Masses