AEROGRAPHER'S MATE 3 & 2 



when supercooled water vapor crystallizes 

 directly into ice crystals and forms cirriform 

 clouds. 



ADIABATIC PROCESS 



This is a condition of ABSOLUTE INSTABILITY. 

 If we now remove the bowl and place the ball 

 on the flat surface (fig. 12-9 (C)), we have 

 NEUTRAL STABILITY — that is, if a force is 

 applied to the ball, it moves; but if the force 

 is removed, it will stop with no further 

 movement. 



An adiabatic process is one in which no 

 heat is added to or taken away from the mass 

 of air by exchange with the environment during 

 the process. 



DESCRIPTION 



If a given parcel of air is compressed 

 adiabatic ally, its temperature rises, since the 

 work of compression is converted into heat. 

 If a given parcel of air expands adiabatically 

 against external pressure, its temperature falls, 

 since some of the heat energy of the air parcel 

 is used in doing the work of pushing back its 

 boundaries. The heating of air during com- 

 pression causes a tire pump to be warmer when 

 inflating a tire, because work is being done on 

 the gas. The cooling of air by expansion occurs 

 when air is allowed to escape from the tire, 

 because the air is doing work as it /expands. 

 (See "Gas Laws,") 



Remember, in an adiabatic process an 

 increase in temperature is due only to 

 COMPRESSION when the air sinks or subsides. 

 A decrease in temperature is due only to 

 EXPANSION when air rises, as in convective 

 currents or going over mountains. There is no 

 addition or subtraction of heat involved. The 

 changes in temperature are due to the 

 conversion of energy from one form to another. 

 It should also be noted that the atmosphere 

 has a tendency to resist vertical motion. This 

 is known as stability. Stability can best be 

 explained by a brief description of the ball 

 and bowl analogy (fig. 12-9). 



In this analogy a bowl is set on a flat 

 surface with a ball placed inside it. The ball 

 will rest in the bottom of the bowl; but, if we 

 push the ball in any direction, it will seek out 

 the bottom of the bowl again. This is referred 

 to as ABSOLUTE STABILITY (fig. 12-9 (A)). 

 Turn the bowl upside down, position the ball 

 anywhere on the bowl's bottom surface, fig. 

 12-9 (B) and the ball will start moving on 

 its own without any other force being applied. 



Air in our atmosphere reacts in a similar 

 manner when moved up or down. If it is moved 

 up and becomes denser than the surrounding 

 air, it will return to its original position and 

 be considered STABLE, If it becomes less 

 dense than the surrounding air, it will con- 

 tinue to rise and be considered UNSTABLE. 

 When density remains the same as the 

 surrounding air after being lifted it will be 

 considered NEUTRALLY STABLE, having no 

 tendency to rise or sink further. 



LAPSE RATES 



Lapse rate is a decrease in any atmospheric 

 variable with height, but is generally applied 

 to temperature in meteorology. (See table 12-3.) 



Dry Adiabatic Lapse Rate 



If a parcel of air is lifted, its pressure 

 is decreased, since pressure decreases with 

 height, and its temperature falls due to the 

 expansion. If the air is dry and the process is 

 adiabatic, the rate of temperature fall is 1° C 

 per 100 meters of lift, or 5 1/2° F per 1,000 

 feet of lift; if the parcel descends to higher 



Table 12-3. — Lapse rates of temperature 



Lapse rate 



Per 1,000 



Per 100 





feet 



meters 



Dry adiabatic 



5 1/2° F 



1°C 



Saturation (moist) 







adiabatic 



2-3° F 



.55° C 



Average 



3.3° F 



.65° C 



Superadiabatic 



5 1/2-15° F 



1-3.42° C 



Autoconvective 



More than 



More than 





15° F 



3.42° C 



274 



