350 ENVIRONMENTAL CONDITIONS. 



vapor-pressure of water for the given air-temperature and the actual 

 vapor-pressure of water in the air. As has been pointed out, this 

 deficit should be an index of the relation of air humidity to evapora- 

 tion; it should measure that portion of the atmospheric evaporating 

 power for any given time, which is not related to wind-movement. 

 The arguments of air-temperature and air moisture-content are thus 

 combined in a single function, which becomes the more significant 

 when it is pointed out that different parts of the country do not 

 generally differ very markedly in relation to wind. The terms from 

 which the vapor-pressure deficit might have been obtained were at 

 hand when the relative humidity observations were recorded, but it 

 is not possible to deduce them from the recorded percentages, espe- 

 cially since Stockman's published data have been reduced to normal 

 monthly means. It is readily seen, however, that the higher the 

 temperature {i. e., the farther south is the location of a given station), 

 the more a given percentage of relative humidity is to be discounted, 

 as it were. Thus, if a northern and a southern station (as Duluth, 

 Minnesota, and Little Rock, Arkansas, for example) agree in having 

 the same normal mean relative humidity for the period of the average 

 frostless season — say 72 per cent — we are perfectly safe in concluding 

 that the normal mean evaporating power of the air at the southern 

 station is greater than at the northern, supposing the air-movement 

 to be alike in the two cases. But it is not possible in such a case to 

 weight the humidity records in a quantitative way. 



Before proceeding to our results in this connection it is necessary 

 to mention one other very important point, which requires s 

 attention frofn climatologists who hope to relate climate to plant 

 activities in a detailed way. As has been stated, the humidity data 

 of Stockman's table were derived from observations "made at 8 a. m. 

 and 8 p. m,, seventy-fifth meridian time." This is the regular practice 

 of the United States Weather Bureau in dealing with humidity, and it 

 will be seen that the longitude of a station determines the times in 

 its day when observations are to be obtained. Thus, the observations 

 for any day at North Head, Washington, come to us as though they 

 were made at about 4'' 40"" a. m. and p. m., while those for a day at 

 Eastport, Maine, are to be similarly considered as made at about 

 8*" 30™ a. m. and p. m. Now, humidity alters very rapidly, in most 

 cases, during an hour or two about sunrise and about sunset, and the 

 morning observation on the Pacific coast is usually made well before 

 sunrise, while that in the northeast is made well after sunrise. A simi- 

 lar consideration applies to the evening observations. It is clear that 

 neither the morning observations nor the evening ones, throughout the 

 country, represent the true humidity conditions for the given date, 

 and it remains to be determined what the mean of these two percent- 

 ages may denote. The question is, simply: Can the humidity con- 



