384 ENVIRONMENTAL CONDITIONS. 



the publications of the United States Weather Bureau, but it appears 

 to be of comparatively little use in interpreting climate in connection 

 with the physiological activities of plants and animals. 



B. MOISTURE CONDITIONS. 



The moisture conditions with which we have dealt are those of (1) 

 precipitation, (2) evaporation, (3) aqueous-vapor pressure, (4) rela- 

 tive humidity, (5) wind, and (6) sunlight. As far as the environment 

 above the soil-surface is concerned, precipitation is the condition that 

 determines the supply of water to plants, but indices of precipitation 

 are also indirect indications of the evaporating power of the air, atmos- 

 pheric humidity, and sunlight intensity; for abundant precipitation 

 is generally accompanied by high air-humidity and much cloudiness. 

 But no methods are as yet available for obtaining an index of the power 

 of the soil to supply water to plants, which is the subterranean moisture 

 condition that corresponds to atmospheric evaporating power, and we 

 have employed several precipitation indices as representing either the 

 moisture conditions in general or else the moisture-supplying power of 

 the climatic complex. 



We have employed eight different precipitation charts, seven of 

 them original and the remaining one (normal annual precipitation, 

 plate 52) after Gannett. ^ Probably the most valuable single criterion 

 for the water-supplying power of the climatic complex is the normal 

 mean daily precipitation for the period of the average frostless season 

 (plate 46). But the precipitation of the United States can not be 

 adequately interpreted by any single criterion ; using the observational 

 data that are at hand several different indices are required. Only 

 when data for soil-moisture conditions shall have become available 

 can a true study of precipitation as an influence on plant growth be 

 undertaken. 



For the various precipitation indices the country appears to be made 

 up of a series of more or less parallel precipitation zones, which may be 

 represented by four precipitation provinces. These we have termed 

 humid, semihumid, semiarid, and arid. The same terms are applied 

 to the corresponding four provinces based on the desiccation features 

 of the climate, evaporation, aqueous-vapor pressure, relative humidity, 

 etc. The exact extent of each province of course depends upon the 

 nature of the climatic index employed, but there is generally shown an 

 eastern and northwestern portion of the humid province, while the arid 

 province lies in the region of the southwestern desert and semi-desert. 



Our evaporation charts represent three different climatic indices 

 derived from Russell's data for a single year (1887-88), and also 



' A more satisfactory chart of this featuie, by Kinccr, has recently appeared, too late to be 

 included in our study. See Kincer, J. B., Average annual precipitation in inches [for the United 

 States], Based on Records of About 1,600 Stations for the 20-year Period 1895-1914, and 2,000 

 Additional records, from 5 to 19 years in length, uniformly adjusted to the same period, ad- 

 vance sheet 1, Atlas of Amer. Agric, U. S. Dept. Agric, Weather Bur., Jan. 1917. 



