WEATHER — WOOLARD 



235 



36 30 25 30 \5 10 



10 -15 -20 -25 -30 -35 -40 -45 



.7'OAir 



Figure 8. — Thermodynamic diagram for plotting the so-called tephigram of an upper 

 air sounding. The observed temperatui-e (centigrade) Is plotted as abscissa, against 

 entropy as ordinate (an equivalent logarithmic scale of potential temperature is added 

 for convenience). The curved lines show the socjuences of tliermodynaniic states fol- 

 lowed by saturated air in pseudoadiabatic processes under different initial conditions, 

 as specified by realized entropy of the air, temperature, pressure in millibars (shown 

 by sloping solid straight lines) and saturation water vapor content (sloping dashed 

 straight lines). An area on this diagram represents energy. The relations of the 

 graph of the sounding to the isentropic lines and pseudoadiabats indicate conditions of 

 stability in the atmosphere, and the amount of energy available or required for the 

 occurrence of meteorological processes. 



aerological soundings. The charts show (among other things) the 

 elevation contours of the selected isentropic surface as indicated by 

 the barometric pressure distribution over the surface ; the distribution 

 of specific humidity as indicated by the values of the pressure at 

 which condensation would begin in an adiabatic expansion; and the 

 wind velocity over this surface (fig. 9). The motion of the upper air 

 is in general so nearly adiabatic, as long as no condensation occurs, 

 that the flow must everywhere be practically along an isentropic 

 surface. The flow of individual dry and humid currents in each sur- 

 face, and the gradual lateral mixing of these currents over the surface, 

 can be followed from day to day by means of the specific humidity. 

 The extent to which the motions of the currents are upward or down- 

 ward along the slopes of the isentropic surface, together with the 

 conditions of relative humidity, condensation levels, and motion of 



