CLIMATIC CONDIIIONS OF THE UNITED STATES. 349 



mountainous region, west to about the middle of Washington, Oregon, 

 and Cahfornia, is shown as an area of low vapor-pressures (below 

 0.300 inch), this area apparently extending a little into Canada at 

 the north, but not reaching the Mexican boundary at the south. The 

 whole Pacific coast region is shown as having very low values (mainly 

 between 0.300 and 0.350 inch), about like those of the region of the 

 one hundred and third meridian, in the plains area. Southern Florida 

 has the highest values, that for Key West being 0.707 inch. 



(3) Normal Mean Aqueous Vapor-phessure fob the Year. (Table 19, Plate 64.) 



The means here used are taken directly from Bigelow's tables. They 

 are reproduced in the fifth column of table 19 and are shown graphi- 

 cally by the chart of plate 64. 



The zonation shown by this chart is so similar to that of plate 63 

 that no special discussion is here needed. Neither is it necessary to 

 derive any generahzation from these two vapor-pressure charts, since 

 the discussion given for plate 63 brings out all the points that might 

 be mentioned in a generalized statement. 



E. RELATIVE AIR HUMIDITY. 



(1) Preliminary Considerations. 



Data of relative air humidity, obtained by means of stationary or 

 whirled wet and dry bulb thermometers, have been accumulated for 

 many years at the various stations of the United States Signal Service 

 and of the United States Weather Bureau. These were brought 

 together by Stockman,^ as monthly and annual means for the period 

 1888-1901,*^ for 130 stations. Stockman's means are stated to have 

 been ''derived from observations made at 8 a. m. and 8 p. m., seventy- 

 fifth meridian time." They form the original source of our studies of 

 humidity in the United States. 



The theoretical inadequacy of data of relative air humidity in char- 

 acterizing climate is tacitly suggested by a brief but well-chosen state- 

 ment with which Stockman prefaces the table above referred to. He 

 says: 



"The relative hvimidity furnishes no direct indication of the absolute amount of mois- 

 ture in the air. For purposes of comparison consideration should be given to the fact 

 that as the temperature is lowered the capacity of the air to retain moisture is decreased. 

 Of two stations having the same average percentage of relative humidity, but different 

 mean temperatures, that station which has the higher mean temperature will have the 

 greater amount of moisture." 



We have already emphasized the fact that vapor-tension deficit 

 appears to be the chmatic feature which should be recorded in con- 

 nection with air humidity, this being the difference (in pressure units, 

 as bars or as millimeters of a mercury column) between the maximum 



1 Stockman, W. B., Temperature and relative humidity data, U. S. Dept. Agric, Weather 

 Bur. Bull. O: 25-29. 1905. 



