CLIMATIC FACTORS : THE AIR 85 



respond similarly to the external factors affecting the latter, evap- 

 oration is taken as indicative of potential transpiration. 



Evaporation is measured by the United States Weather Bureau 

 by means of large open tanks of uniform size and depth, but this 

 method is quite unsatisfactory for most ecological purposes. The 

 bulkiness of the equipment, the necessity for frequent checking, 

 and the probability of disturbance and of contamination are all 

 against it. 



Various compact evaporimeters have been devised primarily for 

 ecological use. Of these the now well-known Livingston atmom- 

 eter has been most widely used. It consists of a porous clay sphere 

 or cup connected to a reservoir by means of a tube. Water evap- 

 orates from the clay surface and is constantly replaced from with- 

 in. If the sphere and tube have been filled with distilled water so 

 that no air bubbles are present (most easily done under water), 

 the water will be drawn from the reservoir through the tube. An 

 additional small-bored tube passed through the stopper of the res- 

 ervoir will permit equalizing of pressure but negligible loss of 

 water by evaporation. The reservoir is marked near the top and 

 filled to this mark by lifting the stopper. Subsequent fillings made 

 at regular intervals indicate water lost to the air by evaporation 

 over the period of time involved. 



The simplicity of this device has been in its favor, and it has 

 other advantages. Before they are sold, all atmometer cups are 

 checked against a standard and marked with a correction factor 

 which, when applied, permits direct comparison of results obtained 

 with every instrument wherever it is used. If, as is frequently true, 

 the cups become dirty or accumulate a film of algae, they must be 

 restandardized. If algae and fungi tend to accumulate in the reser- 

 voir or on the cup, they can usually be controlled by a small piece 

 of copper sulfate in the water. The error produced by the solute 

 is negligible as compared to that caused by a film of algae. 



The spherical form of the atmometer cup gives it the advantage 

 of exposing half its surface to the sun regardless of the sun's posi- 

 tion. Other evaporimeters with different shapes have been less 

 useful for this reason alone. Black cups can be used in combination 

 with white and the increased evaporation resulting from their 

 greater heat absorption may be used as a measure of relative light 

 intensity in different habitats. 



