264 LIVINGSTON— CLIMATIC AREAS [April i8, 



been compiled, as the author states, wholly from the observations 

 of rural stations. The requisite stations must be, however, in the 

 open country, and not even in small towns. 



Furthermore, the geographical distribution of Weather Bureau 

 stations in the United States is anything but rational. Being located 

 mainly in large cities, these stations cluster thickly east of the Mis- 

 sissippi River and are widely separated in the western half of the 

 country. Such an arrangement has, no doubt, its political, com- 

 mercial, financial and historical reasons ; nevertheless, it is scien- 

 tifically quite the opposite of rational, for climatic gradients are 

 gentle in the east and very abrupt in the west. 



For the purposes of the student of vegetational-climatic relations, 

 the actual observations might be greatly improved. As far as the 

 temperature conditions are concerned, the observational methods are 

 fairly well worked out for the present. In the future we shall need 

 a thermo-integrator, the indications of which may bear some at least 

 empirical relation to plant growth, but such instruments remain to 

 be devised. As has been pointed out, the moisture conditions of 

 the environment afifect the activities of a plant through their influ- 

 ence toward increasing or decreasing its water content. Now, most 

 plants — and all agricultural plants — derive water mainly from the 

 soil and lose it mainly to the air. It is thus clear that, with proper 

 consideration of soil conditions, the data of precipitation should 

 furnish us with a valuable criterion for comparing climatic areas. 

 Precipitation is easily measured and our information in this connec- 

 tion is fairly satisfactory. For the other factor of the moisture rela- 

 tion of plants, however, namely the power of the aerial surroundings 

 to extract water from the plant, the climatic data which have been 

 accumulated in this country furnish practically no information. 

 The available measurements and averages bearing on this point are 

 those of relative humidity (a somewhat artificial abstraction), pres- 

 sure of water vapor, wind velocity, temperature and sunshine inten- 

 sity. While the present method of measuring rainfall is self- 

 integrating and leaves little to be desired in the way of improve- 

 ment, the methods employed in measuring the water-extracting fac- 

 tors just mentioned all involve artificial manipulations before any 



