Chapter 14 — AIR MASSES AND FRONTS 



During the life span of an air mass, some 

 of the physical properties, such as the surface 

 air temperature and humidity, are apt to change 

 frequently and rapidly due to lifting, conduction, 

 radiation, evaporation, or some local topo- 

 graphical feature. Some properties of the air 

 mass remain almost constant for a period of 

 time. An element of an air mass that has 

 little change from day to day is considered 

 as being conservative. A nonconservative 

 property is one that changes frequently and 

 rapidly. 



Although a strictly conservative property 

 probably does not exist in the atmosphere, 

 there are certain properties that for a short 

 period of time, and under certain conditions, 

 are so nearly constant that they may be con- 

 sidered as conservative. 



The most important physical properties in 

 air masses are those that concern temperature 

 and moisture. The various expressions of 

 temperature and moisture are summarized in 



table 14-2. The Aerographer's Mate should 

 learn the significance of the various properties 

 listed in this table. 



Since many nonadiabatic processes occur 

 near the surface of the earth, it becomes 

 necessary to study the adiabatic conditions of 

 the upper atmosphere in classifying an air 

 mass. Some authorities differ as to the pro- 

 perties to use in air-mass identification. 

 However, from table 14-2 it can be seen that 

 the equivalent potential temperature and the 

 potential wet-bulb temperature are conservative 

 relative to both dry adiabatic and moist 

 adiabatic temperature changes, while other 

 elements are conservative in regard to only 

 one type or nonconservative in regard to both 

 types of temperature change. 



In order to understand table 14-2 better, 

 it is imperative that the Aerographer's Mate 

 learn the definitions of the various properties 

 listed in that table. Temperature, relative 

 humidity, dewpoint, and mixing ratio were 



Table 14-2. — Conservative characteristics of the physical properties of air masses with respect to 



adiabatic temperature changes 





Conservative with respect to 



Property 



Dry adiabatic 



temperature 



changes 



Moist adiabatic 



temperature 



changes 



Temperature 



No 



No 



Relative humidity 



No 



Yes 



Dewpoint temperature* 



Quasi-conservative 



No 



Wet-bulb temperature 



No 



No 



Mixing ratio* 



Yes 



No 



Potential temperature 



Yes 



No 



Equivalent temperature 



No 



No 



Equivalent potential temperature 



Yes 



Yes 



Potential wet-bulb temperature. 



Yes 



Yes 



♦These two properties are conservative with respect to nonadiabatic temperature changes; 

 therefore it is advantageous to use these elements in the analysis of surface weather charts. 



323 



