HYGROMETICAL TABLES. 



lxi 



is o°i, and above — 5°o the interval is i°. The computation has been made 

 for B = 760 mm. but on each page of the table is given a correction, 

 Ae X AB, computed for B = 660, or AP = 100 mm., and for the values 

 of t' indicated. The correction is a linear function of AB. For atmospheric 

 pressures less than 760 mm. it is to be added to the tabular values of e, 

 while for atmospheric pressures greater than 760 mm. it is to be subtracted. 

 The values of e are given to 0.001 mm. for /' less than — 5°o, and to 0.01 

 mm. for t' greater than — 5?o. 



Example: 



Given, / = 104 C; t' = 8°3 C, and B = 740 mm. With t' = 8/3 and 

 t —t' — 2° 1 as arguments, Table 77 gives for e the value 7.15 mm. 



760 - 740 



AB = — = 0.2. Ae X AB = 0.14 X 0.2 = 0.03. 



100 



Corrected value of e =7.18 mm. 



For / — /' = o a vapor pressure of 7.18 mm. corresponds to a tempera- 

 ture t' = t = 6°3 C, which is the saturation, or dew-point tempera- 

 ture for the data given. 



TABLE 78. 



Table 78. Relative humidity — Temperature Centigrade. 



This table gives the vapor pressure corresponding to air temperatures 

 from — 45 C. to + 55° C. at degree intervals (side argument) and for per- 

 centage of saturation at 10 per cent intervals (top argument). It is com- 

 puted from the same formula as Table 76, namely, 

 e = e s X relative humidity. 



Below a temperature of + 5°o the values of e are given to 0.01 mm.; 

 above 50 they are given to 0.1 mm. 



Example: 



In the dew-point example given above, the computed vapor pressure 

 is 7.18 mm. Entering Table 78 with air temperature 10.4 as side ar- 

 gument, we obtain vapor pressure 



6.6 mm. = relative humidity 70 



and 



7.18 - 6.6 = 0.58 mm. " " — = 6 



10 



therefore, vapor pressure 



7.18 mm. with / = 10.4 C. " " = 76 



TABLE 79. 



Table 79 . Rate of decrease of vapor pressure with altitude for mountain stations. 



From hygrometric observations made at various mountain stations on 

 the Himalayas, Mount Ararat, Teneriffe, and the Alps, Dr. J. Hann 

 (Lehrbuch der Meteorologie Dritte Auflage, S. 230) has deduced the fol- 

 lowing empirical formula showing the average relation between the vapor 



