170 Vapor Tension Deficit [368 



air may be very different in two locations only a few centi- 

 meters apart. The differences here encountered are much 

 greater than the similar ones met with in thermometry and 

 the general exposure of the instrument needs to be stated in all 

 climatological studies of atmometry. The readings obtained 

 are taken as averages for the time period of operation and are 

 stated with reference to a shorter time unit. 



To summarize the points brought out above, every atmo- 

 metric measurement should be formulated so as to include all 

 the five features indicated by letters in the following state- 

 ment, which is given as an illustration. The atmometric in- 

 dex for location A, for the period of operation E, is found to 

 be G units of water lost per time unit D from an atmometer 

 of type E. Filling in the features represented by these let- 

 ters, to render the illustration more concrete, we may say : 

 The atmometric index for a place 1 meter above the ground in 

 the center of a large field of clover in northern Ohio, for the 

 period of operation from May 1 to May 10, 1916, was found 

 to be 12 grams of water lost per day from a standard white 

 spherical atmometer. If any of these five features is omitted 

 from the statement, the meaning is rendered vague and un- 

 certain. 



THE VAPOR TENSION DEFICIT AS AN INDEX OF THE 

 MOISTURE CONDITION OF THE AIR 



By BURTON E. LIVINGSTON 



Studies on the manner in which external conditions con- 

 trol the activities of animals and plants must deal with the 

 moisture conditions of the air in all cases where the organ- 

 isms considered are aerially exposed. While atmospheric 

 evaporating power (measured with reference to some standard 

 evaporating surface) furnishes an index of the air conditions 

 that influence the rate of water loss from aerially exposed 

 organisms, it is frequently desirable to analyze this complex 



