RELATION OF CONDITIONS TO HABITAT DISTINCTIONS. 43 



ment of this type were taken by Brown 1 at my Windward Ravine 

 station in the summer of 1910, extending through four weeks. 



In the still air of the floor of the rain-forest, where the temperature 

 ranges through less than 10 a day, the atmometer is in effect a 

 hygrometer, registering the cumulative evaporation of the longer inter- 

 vals when the humidity falls below the prevalent high percentages. 

 The ratio of the rate of evaporation from a free water surface to that 

 from a standard cup has been found to be 0.76 at Cinchona as compared 

 with 1.15 at Tucson. This points to a difference in the character of 

 the evaporating water film under the two diverse climates, the film 

 probably being discontinuous in the drier climate, occupying only the 

 pores of the cup, while it is continuous in the moist climate, occupying 

 the entire surface. The difference between the dry look of the surface 

 of cups in operation at Tucson and their moist look when in operation 

 at Cinchona corroborates this explanation. The existence of a greater 

 surface film would have the effect of increasing the evaporating surface 

 of the cup, and would accordingly lower its ratio to a free water surface 

 as compared with this ratio determined in an arid climate. While 

 these considerations make it necessary to apply a considerable correc- 

 tion to atmometer readings from widely diverse climates before com- 

 paring them, they do not at all invalidate the comparableness of 

 readings taken under similar humidity conditions. In rain-forest 

 ravines the atmometer is subject to the condensation of moisture onto 

 its evaporating surface, whenever evaporation cools this a few degrees 

 below the air temperature. The condensation stops evaporation and 

 cooling, and permits the surface of the cup to warm up again and 

 presently to resume evaporation. The low rates of evaporation obtain- 

 able with the atmometer in Windward Ravines are undoubtedly some- 

 what lower than they should be for this reason. 



After repeated observations with the hygrograph and sling psy- 

 chrometer, I am convinced that saturation, or even humidities as high 

 as 97 to 99 per cent, are extremely transitory states of the atmosphere 

 in the most moist situations in the rain-forest. Saturation must pre- 

 cede precipitation; and the condensation of moisture on the foliage of 

 plants often takes place in the deep forest. As soon as precipitation 

 or condensation occurs, there is a fall in the humidity and it naturally 

 rises again but slowly, for although the extent of wet surfaces capable 

 of adding by evaporation to the moisture of the air is very great, the 

 high humidity itself retards such evaporation. Cloudiness is an impor- 

 tant factor in influencing humidity as well as is fog. The passage of 

 small clouds over the face of the sun causes immediate and pronounced 

 rises in humidity, due in great measure to the sudden fall of temperature 

 which may be too transitory to affect the sluggish thermograph. 



W. H. Evaporation and Plant Habitats in Jamaica. Plant World, 13 : 268-272,1910. 



