AEROGRAPHER'S MATE 3 & 2 



Table 14-1. -Thermal and Mechanical Air Mass Modifications 



THE PROCESS 



WHAT TAKES PLACE 



RESULTS 



A. THERMAL 



1. Heating from below. 



2. Cooling from below. 



3. Addition of moisture. 



4. Removal of moisture. 



B. MECHANICAL 



1. Turbulent mixing. 



2. Sinking, 



3. Lifting. 



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. 



indicates the modifying process, what takes 

 place, and the resultant change in stability 

 of the air mass. It must be reiterated that 

 these processes do not occur independently, 

 but two or more processes are usually in 

 evidence at the same time. It must also be 

 stressed that the conditions indicated are only 

 average conditions and that each individual case 

 may be quite different. 



WEATHER 



Within an air mass, weather is controlled 

 primarily by the moisture content of the air, 

 the relationship between surface temperature 

 and temperature of the air mass trajectory 



and topography (upslope or downslope). Rising 

 air is cooled; descending air is warmed. 

 Condensation takes place when the air is cooled 

 to its dewpoint. A cloud warmed above the 

 dewpoint temperature evaporates and dissipates. 

 Stability tends to increase if the surface tempera- 

 ture 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 surface temperature 

 remains the same and the temperature aloft 

 is lowered. 



Smooth stratiform clouds are associated with 

 stable air, whereas turbulence, convective clouds, 

 and thunderstorms are associated with unstable 

 air. 



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