168 Atmometric Units [366 



With open pans it is easier to measure depth than volume, for 

 rough approximations. It is true that volume or weight read- 

 ings from other types of instrument than those employing an 

 open pan, may also be multiplied by a constant throughout the 

 series, and this constant might be the area of the surface em- 

 ployed, or any other number that may be chosen. But it can- 

 not be too strongly emphasized that such treatment is to be 

 applied only to series of readings that are already comparable, 

 and that no constants can be found by which readings from 

 different types of instruments may be rendered comparable. 



The use of depth units in comparing water losses from open 

 pans has introduced and supported a fallacy that is extremely 

 hard to combat in the minds of those who have not given the 

 subject of atmometry serious attention. This fallacy is based 

 upon the mistaken idea that the area of the evaporating sur- 

 face is the only surface characteristic that can influence the 

 rate of evaporation. If different sizes of pans are employed 

 the readings are incomparable, and they remain incompar- 

 able even after each one has been divided by the area of its 

 own water surface. Eeadings must be stated as from a cer- 

 tain instrument, in any event, and the application of an areal 

 coefficient only complicates matters. To avoid the continua- 

 tion of this fallacy, as much as may be, it is highly desirable 

 that all atmometric readings be stated in terms of weight or 

 volume, even though they were originally obtained by measure- 

 ments of depth. 



The worst feature of the use of depth units in pan atmome- 

 try is that it has led to another fallacy, by which these depth 

 units are taken to be equivalent to the other depth units that 

 are employed in the measurement of rainfall. The two classes 

 of units look alike but they are widely different in their mean- 

 ings. An example may illustrate this very important point. 

 Suppose that the rainfall for a certain place is found to be 

 75 cm. (of depth) for a certain year, and suppose that the 

 observer states that the evaporation from a Weather Bureau 

 pan for the same period was 90 cm. (also of depth). In such 

 a case students of climatology have been led to say that evapo- 



