SPECIFIC VOLUME AND DENSITY OF ATMOSPHERIC AIR AND SEA-WATER. 29 



24. Virtual Temperature as a Function of the Height. In some cases the 

 height will appear instead of the pressure as one of the observed quantities. It will 

 then be convenient to be able to calculate the virtual-temperature correction as a 

 function not of pressure but of height. For this we must first know the average 

 pressure for the heights to be used as argument in the table. Using the inter- 

 national kite and balloon ascents performed in Europe for the years 1901 to 1903, 

 we have found the values of the average pressure in given dynamic heights above 

 the 1000 m-bar surface (table A). 



Table A. Average Pressures in Given Dynamic Heights above the IOOO m-bar 

 Surface, Calculated from the International Kite and Balloon Ascents in 

 Europe for the Period 1901-1903. 



Table A would give the average pressures in the corresponding heights above 

 sea-level, if the pressure at sea-level was 1000 m-bars. This pressure being about 

 760 mm. of mercury, or 1013 m-bars, all pressures in table A would have to be 

 increased by about 1.3 per cent in order to give the average pressures in the 

 corresponding heights above sea-level. But this difference is quite insignificant 

 for our present purpose. 



By means of these values of the average pressure in the standard heights, table 

 8 m has been calculated from table 7 m. The virtual temperature varying very slowly 

 with the pressure, even a great deviation of the actual pressure from the supposed 

 average value will have no influence on the correctness of the results obtained from 

 table 7 m. 



The use of table 8 m is perfectly analogous to that of table 7 M. An example of 

 a virtual-temperature diagram, drawn with the height as the independent variable 

 is also given in Chapter VI. The heights are given originally in common meters. 

 Before using table 8 m these are first to be transformed, by tables 3 m and 4 m, to 

 dynamic meters. 



The height being given originally in feet and the temperature in Fahrenheit 

 degrees, tables 2 a and 3 a of the Appendix are first used to transform the heights 

 from feet to dynamic meters, and afterwards table 13 a of the Appendix to draw the 

 virtual-temperature diagram in Fahrenheit degrees. 



25. Specific Volume and Density of the Air. The value of the virtual tem- 

 perature being found, it is easy to calculate the specific volume or the density by 

 the equation of state, which gives 



