.HUMIDITY 



47 



74. Sums, means, and 

 curves. An approximate hu- 

 midity sum can be obtained 

 by adding the absolute hu- 

 midities for each of the 

 twenty-four hours, and ex- 

 pressing the results in grains 

 per cubic foot. It is possible 

 to establish a general ratio 

 between this sum and the 

 transpiration* sum of the 

 plant, but its value is not 

 great at present. Means of 

 absolute and of relative hu- 

 midity are readily determin- 

 able from the psych rograph 

 records; the latter are the 

 most useful. The mean of 

 relative humidity for the 

 twenty- four hours of a day 

 is the average of the twenty- 

 four hour htmiidities. 

 From these means the sea- 

 sonal mean is computed in 

 the same manner. A close 

 approximation, usually with- 

 in I degree, may be obtained 

 in either case by averaging 

 the maximum and minimum 

 for the period concerned. 

 Various kinds of curves are 

 of value in representing 

 variation in humidity. Ob- 

 viously, these must be 

 derived from the psychro- 

 graph, or from the psychro- 

 meter when the series is 

 sufficiently complete. The 

 level curve indicates the 

 variation in different sta- 

 tions at the same time. 



Boiling 212 



'<o 

 u 



to 

 a> 



<D 

 OJD 



Melting Ice 32 



30 

 25 

 .20 

 IS 

 10 

 5 







5 



10 

 15 

 20 



210 = 



205 - 



200 



165 - 



190 



185 - 



180 



175 



170 



J 65 



I BO 



155 



150 



145 



140 : 



135 



130 



125 - 



120 



115 



no 



105 - 

 100 



95 



90 



85 - 



80 



75 - 



70 



65 



60 



55 



SO 



45 



40 



100 



95 

 -90 



as 



-80 



75. 

 -70 



65 

 -60. 



55 

 -50 



45 

 -40 



35 

 -39 



25 

 -20 



15 

 - 10 



5 







s 



10 



IS 



20 



25 



<v 



Fig. 10. Conversion scale for temperatures. 



