162 Atmometric Units [360 



the index of the evaporating power of the air. From this last 

 expression comes the new term, the atmometric index of the 

 locality and time period considered. 



The atmometric index is the relative measure of the evapo- 

 rating power of the air, and it is to be expressed as a possible 

 time rate of doing work ; it is an index of a power. The unit 

 of measurement must therefore be a unit of work, but it may 

 just as well be a unit of process rate, if the same process be 

 always employed. Thus it may be stated as the amount of 

 water evaporated per unit of time. If the liquid water were 

 always at the same temperature this would actually be a 

 measure of work. That the water of evaporating surfaces 

 varies in temperature has been thus far neglected in this 

 whole line of enquiry, the errors thus arising being relatively 

 small in the present early stages of our studies. 



To determine the numerical value of the atmometric index, 

 we must also consider a factor representing some standard unit 

 of surface. It has been seen above that the power of any sur- 

 face to give off water vapor is determined by the character- 

 istics of that surface, and that the extent of the surface is not 

 by any means the only characteristic that needs to be con- 

 sidered. The shape and the direction of exposure of the sur- 

 face must be taken into account, and also the influence of the 

 non-aqueous materials that are in or behind the surface, etc. 

 It is- therefore impossible to employ a surface unit defined by 

 area alone. As soon as this is realized all attempts to ex- 

 press the atmometric index as a time rate of evaporation from 

 a square centimeter (etc.) of free water surf ace .are seen to be 

 quite useless. A free water surface is more or less nearly 

 plane and more or less nearly horizontal (depending upon the 

 wind velocity, among other things), but it may be of any 

 shape or size, and all these characteristics are important. 

 With a given set of aerial conditions two different atmometer 

 pans, for instance, can give off the same amount of water per 

 hour, per square centimeter of surface, only when they are of 

 the same size and shape. Also, the depth of the water and 



