CLIMATIC CONDITIONS OF THE UNITED STATES. 351 



ditions for any day be as well derived from observations made, say, at 

 5 a. m. and 5 p. m. as from those made at, say, 8 a. m. and 8 p. m.? 

 When vapor-pressure deficit attracts the attention it deserves the same 

 problem will arise in connection with it, and then the local times of 

 observation will have to be seriously considered before the data may 

 be regarded as quite suitable for studies involving plant transpira- 

 tion and other ecological or agricultural features.^ 



On the whole, it needs to be borne in mind simply that the moisture 

 conditions of the air deserve as much attention as do its temperature 

 conditions, if agriculture and ecology are to employ cHmatological 

 results. Our aim in the above paragraphs has been, not to point out 

 what might or might not have been done in the past, which can not 

 now be changed, but rather to emphasize what seem to us to be the 

 needs of ecological and agricultural climatology for the future. 



We shall deal here with three kinds of indices of relative humidity: 

 (1) the normals for the period of the average frostless season, (2) for 

 the year, and (3) the means for the three summer months of 1908. 



(2) Percentages Representing Normal Mean Relative Air Humidity for the Period 

 OF THE Average Frostless Season. (Table 19, Plate 65, and Fig. 17.) 



The data here employed were derived from the monthly values 

 given by Stockman (1905), by the same general method as we have 

 heretofore resorted to in obtaining indices for the period of the average 

 frostless season from monthly normals. Our values are given in the 

 third colunm of table 19, and they are shown graphically on the chart 

 of plate 65. 



The chart of plate 65 shows that the relative humidity values are 

 high (above 75 per cent) for the Pacific coast region and also for the 

 Northeast, East, and Southeast. The lowest values (below 40 per 

 cent) are found in the arid southwest. As a whole, this chart resem- 

 bles the charts of precipitation-evaporation ratios (plates 57 to 62), 

 and certain lines are here shown as broader than the others, to bring 

 out the division of the country into climatic provinces, as was done on 

 those charts. The arid region may be characterized as having relative- 

 humidity percentage values below 50, the semiarid region shows values 

 between 50 and 65, the semihumid regions show values between 65 

 and 75, and values of over 75 characterize the humid regions. In this 

 case the northwestern humid area is extended southward, along the 

 Pacific coast, to middle California, and the adjoining semihumid area 

 extends to the Mexican boundary. The semiarid region extends east- 

 ward to about the hundredth meridian of longitude, somewhat farther 

 at the south and not as far at the north, thus agreeing, in general, with 



1 The hours of temperature observation in the United States have been very thoroughly 

 studied, in relation to the daily means derived therefrom, and several important considerations 

 bearing on the readings of the dry and wet bulb thermometers have been taken up. In these 

 connections see Bigelow, 1909. Also see O. L. Fassig, Report on the climate and weather of Bal- 

 timore, Maryland Weather Service 2: 29-312, 1907; especially pp. 152-158. 



