WORLD AIR MASSES LEARNING   OBJECTIVE:    Describe    the trajectories and weather associated with world air masses. NORTH AMERICAN AIR MASSES, TRAJECTORIES, AND WEATHER (WINTER) The  shape  and  location  of  the  North  American continent make it an ideal source region and also permit the invasion of maritime air masses. You must be able to identify these air masses and trace their trajectories to develop and present an in-depth weather briefing. Within an air mass, weather is controlled primarily by  the  moisture  content  of  the  air,  the  relationship between surface temperature and air mass temperature, and   terrain   (upslope   or   downslope).   Rising   air   is cooled; descending air is warmed. Condensation takes place when the air is cooled to its dew point. A cloud warmed  above  the  dew  point  temperature  evaporates and dissipates. Stability tends to increase if the surface temperature is lowered or if the temperature of the air at higher levels is increased while the surface temperature remains  the  same.  Stability  tends  to  be  reduced  if  the temperature aloft is lowered.  Smooth stratiform clouds are   associated   with   stable   air,   whereas   turbulence, convective  clouds,  and  thunderstorms  are  associated with unstable air. cPk and cAk Air in Winter The weather conditions with cPk and cAk air over the United States depend primarily on the trajectory of the    air    mass    after    it    leaves    its    source    region. Trajectories,   as   observed   on   a   surface   chart,   are indicated as one of  the  trajectories  (A, B, C, D, E, F, G) 4-7 1.    Heating  from  below. 2.    Cooling  from  below. 3.    Addition   of   moisture. 4.    Removal  of  moisture. 1.    Turbulent  mixing. 2.    Sinking. 3.    Lifting. B.    MECHANICAL A.    THERMODYNAMIC Air  mass  passes  from  over  a  cold  surface  to a  warm  surface,  or  surface  under  air  mass is  heated  by  sun. Air  mass  passes  from  over  a  warm  surface  to a  cold  surface,  OR  radiational  cooling  of surface  under  air  mass  takes  place. By  evaporation  from  water,  ice,  or  snow surfaces,  or  moist  ground,  or  from  rain- drops  or  other  precipitation  which  falls from  overrunning  saturated  air  currents. By  condensation  and  precipitation  from  the air  mass. Up-  and  down-draft. Movement  down  from  above  colder  air  masses or  descent  from  high  elevations  to  low- lands,  subsidence  and  lateral  spreading. Movement  up  over  colder  air  masses  or  over elevations  of  land  or  to  compensate  for air  at  the  same  level  converging. Decrease  in  stability. Increase  in  stability. Decrease  in  stability. Increase  in  stability. Tends  to  result  in  a thorough  mixing  of the  air  through  the layer  where  the  tur- bulence  exists. Increases    stability. Decreases    stability. THE  PROCESS HOW  IT  HAPPENS RESULTS AG5f0404 Figure 4-4.—Air mass changes.