CLIMATIC CONDITIONS OF THE UNITED STATES. 291 



carried out by the United States Weather Bureau, consists in the 

 determination of the number of hours of sunshine occurring each day 

 at the various stations. It is to be emphasized that the sunshine 

 recorders now generally in use give but httle information as to the 

 intensity of the sunshine itself; they record the duration aspect of that 

 range of intensities which is called direct sunshine, but the limits of 

 this range have never attracted attention and are not established, so 

 that the whole mass of data so derived are anything but precise. Never- 

 theless, some of the sunshine data of the United States Weather Bureau 

 will be considered below, since they furnish the only available measure- 

 ments having any bearing at all upon the matter before us. 



B. ATMOSPHERIC EVAPORATING POWER IN THE UNITED STATES. 

 (1) Very Limited Nature of Available Data. 



To obtain data bearing on atmospheric evaporating power it is only 

 necessary to operate a number of atmometers of the same form in the 

 various climatic regions dealt with, being sure that all have similar 

 local exposures. The importance of this condition to plant and animal 

 life and the relative ease with which it may be measured makes it 

 appear surprising that practically no organized study of evaporation 

 throughout the United States has yet been undertaken. Had evapora- 

 tion been recorded as thoroughly as precipitation has been, we should now 

 be able to construct relatively satisfactory atmometric charts, but the 

 almost utter lack of data makes this practically impossible at present. 

 To render our position in this connection still less satisfying, it is to be 

 remembered that observations of any climatic condition, extending 

 through a single year, are of but little value; if evaporation measure- 

 ments were to be systematically begun in the present and were to be 

 systematically continued, it would require many years of records to 

 render these measurements as valuable climatologically as are those of 

 temperature and precipitation at the present time. It seems now, 

 however, that students of climate will hardly be able to persist much 

 longer in their too common attitude of ignoring the evaporating power 

 of the air. As we have emphasized, this climatic feature is probably 

 as important from the standpoint of agriculture and etiological plant 

 geography as either temperature or precipitation, and its investigation 

 seems likely to be carried forward first by agriculturists and ecologists. 



While it is possible to collect from the literature numerous instances 

 in which evaporation has been measured at a single station for a longer 

 or shorter period of years, such measurements can not usually be 

 correlated with those for other stations, either because the same years 

 are not involved or because different kinds or sizes of atmometers have 

 been employed. Aside from such cases,^ which are all valuable — ^at 



^ Most of these cases are mentioned in: Livingston, Grace J., An annotated bibliography o 

 evaporation. Monthly Weather Rev. 36: 181-186,301-306, 375-381, 1908; 37: 68-72, 103-109 

 157-160, 193-199, 248-253, 1909. 



