WORLD AIR MASSESLEARNING OBJECTIVE: Describe thetrajectories and weather associated with worldair masses.NORTH AMERICAN AIR MASSES,TRAJECTORIES, AND WEATHER(WINTER)The shape and location of the North Americancontinent make it an ideal source region and also permitthe invasion of maritime air masses. You must be ableto identify these air masses and trace their trajectoriesto develop and present an in-depth weather briefing.Within an air mass, weather is controlled primarilyby the moisture content of the air, the relationshipbetween surface temperature and air mass temperature,and terrain (upslope or downslope). Rising air iscooled; descending air is warmed. Condensation takesplace when the air is cooled to its dew point. A cloudwarmed above the dew point temperature evaporatesand dissipates. Stability tends to increase if the surfacetemperature is lowered or if the temperature of the air athigher levels is increased while the surface temperatureremains the same. Stability tends to be reduced if thetemperature aloft is lowered. Smooth stratiform cloudsare associated with stable air, whereas turbulence,convective clouds, and thunderstorms are associatedwith unstable air.cPk and cAk Air in WinterThe weather conditions with cPk and cAk air overthe United States depend primarily on the trajectory ofthe air mass after it leaves its source region.Trajectories, as observed on a surface chart, areindicated as one of the trajectories (A, B, C, D, E, F, G)4-71. Heating from below.2. Cooling from below.3. Addition of moisture.4. Removal of moisture.1. Turbulent mixing.2. Sinking.3. Lifting.B. MECHANICALA. THERMODYNAMICAir mass passes from over a cold surface toa warm surface, or surface under air massis heated by sun.Air mass passes from over a warm surface toa cold surface, OR radiational cooling ofsurface under air mass takes place.By evaporation from water, ice, or snowsurfaces, or moist ground, or from rain-drops or other precipitation which fallsfrom overrunning saturated air currents.By condensation and precipitation from theair mass.Up- and down-draft.Movement down from above colder air massesor descent from high elevations to low-lands, subsidence and lateral spreading.Movement up over colder air masses or overelevations of land or to compensate forair at the same level converging.Decrease in stability.Increase in stability.Decrease in stability.Increase in stability.Tends to result in athorough mixing ofthe air through thelayer where the tur-bulence exists.Increases stability.Decreases stability.THE PROCESS HOW IT HAPPENS RESULTSAG5f0404Figure 4-4.—Air mass changes.
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