82 DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



Example 4. Observed time, pressure {inches of mercury) , tcmperattirc ( F. ) , and humidity 

 ( per cent) . (Table O). From the observed pressures and temperatures (columns 2 and 3) the 

 curve of true temperature Fahrenheit is drawn (the curve to the left in fig. 9). Then the curve of 

 virtual temperature for saturated air is drawn (curve to the right) by means of table 12 a of the 

 Appendix. Finally, the curve of virtual temperature is drawn between the two others in accordance 

 with the percentages of humidity (column 4). Then the horizontal lines representing the standard 

 surfaces are drawn in accordance with the following table giving the value of the standard pressures 

 in inches of mercury : 



m-bars 1000 900 Soo 700 600 500 



Inches mercury 29-53 26.58 23.62 20.67 17-72 14.77 



400 

 11. Si 



300 

 8.86 



200 

 5-91 



100 

 2 -95 



Pressure 

 indiesof 

 mercury^ . 30 



8 



-20 -<10 



Temperature E ., 



10 20 30 



w so 







SO 



Pressure, 

 rrbbars 



-w 



500 



wo 



soo 



600 



TOO 



800 



900 



1000 



-30 



-20 -10 O 



Temperature C 



10 



2o"a 



Fig. 9. Virtual-temperature diagram; pressure in inches of mercury, temperature in degrees Fahrenheit. 



The standard isobaric sheets being thus marked in the diagram, their average virtual temperatures 

 are determined in the usual way by drawing the vertical segments of line. The diagram gives these 

 temperatures in degrees Fahrenheit (column 6a), but by table 9 a of the Appendix they are changed 

 into degrees centigrade. Afterwards the thickness of the standard sheets (column 7), and the height 

 of the standard surfaces (column 8), are determined as in the preceding examples. From the dia- 

 gram also the temperature Fahrenheit at the standard surfaces can be read off, and from this tempera- 

 ture, changed into centigrade (column 9b), we'fmd the specific volume of the air at the standard sur- 

 faces (column 10). 



