EVAPORATION AS A CLIMATIC FACTOR. 45 



with rapid evaporation the surface layers of the soil often dry 

 out so rapidly that the moisture of the lower layers does not 

 have time to diffuse upward to the surface before a functional 

 dust mulch is formed, which almost completely checks evapora- 

 tion. We are thus confronted with the seemingly paradoxical 

 fact that a high evaporation rate acts to really conserve water in 

 the deeper soil layers. Neither does the amount and distribution 

 of the rainfall furnish evidence by which evaporation conditions 

 may be surely predicted, for a time of heavy thunder-showers 

 may show a great precipitation for a particular week when the 

 evaporation rate was uniformly high throughout the period, ex- 

 cepting for a few hours before and after each shower. 



Doubtless one reason for the neglect of evaporation as a 

 climatic factor is that the evaporimeters which have been devised 

 are unsatisfactory in one way or another, so that the weather 

 services of the world have been able to do but little with this 

 subject. In weather prediction relative humidity has come to 

 be regarded as giving the same information as would rate of 

 evaporation, but it leaves out of account the factor of wind, 

 which is extremely important in determining the evaporation 

 conditions. Numerous as have been the efforts of various 

 workers to establish a formula by which the evaporation rate 

 may be derived from the data of the ordinarily observed cli- 

 matological elements, the suggested formulas are all as unsat- 

 isfactory, if not more so, than the different forms of evapo- 

 rimeters which have been devised. These formulas differ 

 markedly from one another, and the best, could one but select 

 it, must be regarded as only approximate. 



With a suitable instrument, evaporation is far better suited 

 to the needs of the botanist and agriculturist than is relative 

 humidity. In the first place, humidity variations affect the plant 

 only through their effect on the evaporation rate, so that the 

 evaporimeter gives direct information regarding the physical 

 conditions to which the plant is subjected. In the second place, 

 the evaporimeter is a self-integrating instrument, like the rain 

 gauge, but unlike the thermometer or hygrometer ; for any given 

 period a single reading gives the sum of all the evaporation in- 



