'9I3-] OF THE UNITED STATES. 265 



climatic characteristics can be derived therefrom. Indeed, the sun- 

 shine data furnished by the weather observatories is not even quanti- 

 tative in any adequate sense. 



In the face of these difficulties ecology has been forced to turn 

 entirely away from the available meteorological data. It is appar- 

 ent at once that the water-extracting power of the aerial environ- 

 ment is effective through the evaporating power of the air and the 

 intensity of sunlight. The sunlight factor appears frequently to be 

 of comparatively little importance in the climatic moisture relation, 

 though its effects in removing water from moist objects such as plants 

 can now be measured and automatically integrated with considerable 

 readiness.* The evaporating power of the air (a complex of the 

 eflfects of vapor pressure and wind movement) appears, on the other 

 hand, to be generally of prime importance. This fact has long been 

 recognized and meteorologists outside of the United States have 

 accumulated a vast amount of information upon evaporation as a 

 climatic factor. ^"^ Meeting with difficulties in the standardization of 

 atmometers, many workers have turned their attention to attempts 

 to derive a formula by which evaporation might be computed from 

 the meteorological factors usually measured. An enormous amount 

 of work has been done in this line, but the results are of little value 

 for climatological purposes. At the same time various students of 

 climatology and of plant activities have devised numerous forms of 

 atmometers, for measuring and automatically integrating the evapo- 

 rating power of the air directly. Since the latter is a very complex 

 factor, it comes about that data from different kinds of instruments 

 cannot be readily reduced to a common standard, so that there has 

 been some hesitation in making evaporation measurements a general 

 feature of climatological work. It is nevertheless true that, for our 

 present purposes at least, all that is required is that some one form 



* Livingston, B. E., " A Radio-atmometer for Measuring Light Intensi- 

 ties," Plant World, 14: 96-99, 191 1; "Light Intensity and Transpiration," 

 Bot. Gas., 52: 418-438, 1911. 



* In this connection see Livingston, Grace J., " An Annotated Bibliog- 

 raphy of Evaporation," Mo. Weather Rev., 36: 181-6, 301-6, 375-81, 1908; 

 37: 68-72, 103-9, 157-60, 193-9. 248-52, 1909. Also reprinted and repaged 

 1-121, 1909. The subject has very recently attracted much more attention 

 than formerly, especially from agriculturalists and ecologists. 



