64 



CONSTRUCTION OK ISOBARIC CHARTS 



next proceed to construct upon the same set of coordinates by extrapolation, the curve 

 showing the value of f^ at each level surface of the station- vertical, for 8 A.M. This 

 curve for our example, and as drawn on the same coordinate plane, is shown in Fig. 2, 



Fig. 1. The curves of virtual temperatures at Omalia tor 

 each value of the gravity potential as calculated from kite rec- 

 ords for September 23, 1898. Ascending curve A, descending 

 curve B. 



Fig. 2. The curves of virtual temperatures at Omaha from 

 Fig. 1 with the interpolated curve C for the hour of the synoptic 

 map, or 8 a. m., 75th meridian time, September 23, 1898. 



where the 8 A. M. extrapolated {f„ T^)-curve is given as the heavy Hne (C) together 

 with the curves in dotted lines, obtained directly from the observations of the day as 

 already shown in Fig. 1. From the extrapolated (<„ r)-curve of Fig. 2 for 8 A. M. 

 may now be read off the following values for the average virtual temperatures (tr) at 

 8 A. M. of the day in question. 



