CLIMATIC CONDITIONS OF THE UNITED STATES. 289 



In other terms, the rate of evaporation is, under the assumed condi- 

 tions, proportional to the product of the maximum vapor-pressure of 

 water, for the given air-temperature, and the complement of the 

 relative humidity. 



Relative humidity is commonly measured and discussed in climato- 

 logical studies, and its complement is sometimes employed as a measure 

 of atmospheric dryness. They are both easily seen to have no definite 

 relation to the evaporating power of the air. There is here, however, 

 a general and merely qualitative relation; high relative humidity 

 usually corresponds to low atmospheric evaporating power, and the 

 reverse. We shall have to deal with relative humidity in our discus- 

 sion of the climatic conditions influencing evaporation, but this con- 

 cept is to be clearly appreciated as without logical foundation; it is 

 simply a mathematical abstraction and its value to agriculture or 

 ecology will have to be determined by direct empiricism. It may be 

 here suggested that vapor-tension deficit is the climatic dimension 

 that should be measured by ecological workers, if the analysis needs 

 to be carried so far.^ Fortunately, the evaporating power of the air 

 can be directly measured, and much more readily and usefully than 

 can this deficit, and it seems not at all necessary at present, for ecologi- 

 cal purposes, to analyze this power into its components. 



Wind. — Besides the vapor-tension deficit, atmospheric evaporating 

 power is greatly influenced by air-movement; with increasing wind, 

 ceteris paribus, the evaporation-rate is accelerated. Here again, 

 however, the relation between wind velocity and evaporation-rate is 

 not a linear one; with low velocities the effect of alteration in wind 

 velocity is great; with high velocities this effect practically vanishes, 

 and the relation of the two features for any given range of velocity 

 depends upon the kind and upon the exposure of the evaporating sur- 

 face. As has been mentioned, an enormous amount of effort has been 

 expended in attempts to find empirically a formula by which evapora- 

 tion might be calculated from measurements of other climatic condi- 

 tions, and the argument over the wind factor has been greatly pro- 

 longed. Such attempts have failed, as they always must until the 

 problem is first solved by controlled physical methods, which solution 

 has not yet been seriously attempted. When a solution is reached, 

 however, it will obviously hold only for some particular kind, size, 

 etc., of evaporating-pan or other atmometer. 



As a climatic feature that must surely influence water-loss from 

 plants, but the exact nature of whose influence is still quite beyond our 

 reach, wind velocity will be but briefly touched upon in our study. On 

 theoretical grounds this is not a promising criterion for ecological 



^ Livingston, B. E., The vapor tension deficit as an index of the moisture conditions of the air, 

 Johns Hopkins Univ. Circ, Mar. 1917, pp. 170-175. — Johnston, Earl S., The seasonal march of 

 climatic conditions as related to plant growth, Maryland Agric. Exp. Sta., in press. 



