234 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



possible constant, two quantities known respectively as specific hu- 

 midity and equivalent potential temperature have been found espe- 

 cially useful: The specific humidity is the mass of water vapor per 

 unit mass of humid air; the equivalent potential temperature is the 

 temperature to which air would come if subjected to a pseudoadiabatic 

 lifting (that is, a lifting which is adiabatic except that all condensa- 

 tion products drop out immediately upon formation) until all mois- 

 ture was precipitated, followed by an adiabatic return to a standard 

 pressure. 



In developing methods for representing the data from upper air 

 soundings in a form that will show the meteorological import of the 

 physical conditions and be adapted to practical needs, a number of 

 special thermodynamic diagrams have been devised and have come 

 into regular use (Refsdal, 1935; Rossby, 1932; Woolard, et al., 1926). 

 In general, these diagrams consist of networks of lines, referred to 

 appropriate coordinate systems, which show the thermodynamic 

 states of atmospheric air over a wide range of conditions and by 

 which the changes of state and the energy transformations during any 

 prescribed process may be traced out; they correspond to the "indi- 

 cator diagrams" of physics and engineering, but are much more com- 

 plex. By plotting the data from a sounding on one of these diagrams, 

 the conditions in the vertical with respect to stability, available 

 energy, etc., at different levels in the atmosphere may be determined ; 

 and many of the phenomena to be anticipated under the existing 

 meteorological circumstances may be inferred. On the so-called 

 tephigram, for example (fig. 8), the sounding is plotted on a chart 

 with absolute temperature as abscissa and entropy as ordinate, and 

 with a background of lines that show pressures, water vapor contents, 

 and irreversible adiabats of saturated air; on the "Rossby diagram," 

 which is particularly useful for bringing out significant properties of 

 the air, the sounding is plotted with the so-called mixing ratio (mass 

 of water vapor per unit mass of dry air) as abscissa and potential 

 temperature of the dry-air component of the atmosphere as ordinate, 

 and with a background of lines that show equivalent potential tem- 

 peratures, and pressure and temperature at the condensation level. 

 Other widely used charts include the adiabatic chart, the emagram, 

 and the Refsdal aerogram. 



One of the methods most recently applied in daily practice as an 

 aid in the representation and physical analysis of phenomena is the 

 construction of charts that show the meteorological conditions on se- 

 lected isentropic surfaces (surfaces of constant potential temperature) 

 in the upper air, instead of on horizontal surfaces. These isentropic 

 charts were suggested by Sir Napier Shaw some years ago, but their 

 practical construction and use requires, of course, a network of daily 



