WEATHER — WOOLARD 233 



vancing over regions occupied by warmer air, the front between the 

 two air masses is called a cold front ; if warmer air is advancing into 

 regions occupied by colder air, the discontinuity is called a warm 

 front. Advancing warm air always tends to overrun colder heavier 

 air; while advancing cold air tends to imderrun warm air. The dis- 

 continuity formed when a cold front overtakes a warm front and dis- 

 places the warm air formerly between them to a higher level is known 

 as an occluded front. Cold fronts often give rise to the well-known 

 squall lines or wind-shift lines. Along the separating frontal surfaces, 

 the overrunning of warm air over the slopes of cold air masses, or the 

 lifting of warm air by underrunning cold air, leads to adiabatic cool- 

 ing and eventually cloud and perhaps precipitation. The interaction 

 of a cold and a warm air mass often leads to the development of a 

 cyclone, with its center on the front. In the search for the fronts, im- 

 portant evidence is provided by temperatures, winds, dewpoints, pres- 

 sure tendencies (that is, the pressure changes during the 3 hours pre- 

 ceding the observations), cloud forms and sequences, precipitation, and 

 other auxiliary information (Byers, 1937). 



A number of special procedures have been devised for putting the 

 data, especially those from the upper air, into a form that will facili- 

 tate their effective and convenient application in practice, both for the 

 identification of air masses from place to place and time to time, and 

 also for the purpose of analyzing the physical conditions and proc- 

 esses; investigations for this purpose have in recent years led to a 

 particularly noteworthy development of the thermodynamics of at- 

 mospheric phenomena, and of practical means for applying it. 

 Meteorological thermodynamics involves the investigation of the 

 energy transformations and sequence of thermodynamic properties 

 and states, during specified processes, in the atmosphere (Brunt, 1939 ; 

 Humphreys, 1929). When, for example, humid air is forced to ascend 

 in the atmosphere, it comes under less and less pressure and expands 

 and cools adiabatically ; if this process continues, condensation and 

 precipitation eventually take place. The resulting mixture of dry air 

 and widely variable amounts of water distributed in continuously 

 changing proportions between the gaseous, liquid, and solid phases, in 

 the gravitational field of the earth, forms a highly complicated sys- 

 tem; and the derivation of mathematical equations of state to show 

 the condition of the system and the sequence of phenomena through- 

 out the process is an intricate problem which involves relations far 

 more complex than the familiar PV=RT of elementary physics. 



For purposes of air mass identification, conservative properties of 

 the air are of especial importance. Some properties are more con- 

 servative than others during the modifications to which air masses are 

 usually subject; in seeking an index which will remain as nearly as 



