1913.] OF THE UNITED STATES. 273 



normal frostlcss season rarely begins or ends with a month, it has 

 been generally necessary to interpolate values for fractions of a 

 month at the beginning and end of each season. The evaporation 

 indices obtained are in terms of mean daily loss in depth from a 

 small pan of water, in hundredths of an inch. When these indices 

 of daily evaporation are plotted on the map and the isoatmic lines 

 are drawn, there results the chart which appears in full lines in plate 

 X. Here, as in the case of the precipitation chart, we observe a 

 marked tendency of the lines to take a north-south trend and thus to 

 cut the temperature lines so as to make of the country a climatic 

 mosaic somewhat similar to that presented by the preceding chart. 

 The range of daily evaporation appears here to be from less than 

 ten to more than thirty-two hundredths of an inch. 



4. ^Moisture excess or deficit during the frostless season. — Deter- 

 mining, for each station considered, the difference between the in- 

 dices of precipitation and of evaporation, it is found that these 

 differences are approximately zero for some stations and are either 

 positive or negative for others. If the differences thus obtained are 

 placed upon a map it is possible to draw isoclimatic lines again divid- 

 ing the country into areas (full lines, plate XI.). As has been 

 mentioned, these areas or zones may be tentatively taken to be char- 

 acterized by the conditions of the plant water relation. The data are 

 again in terms of hundredths of an inch per day, during the frostless 

 season. They range from a negative value of 30 to a positive one 

 of more than 5. Almost the entire country is seen to have a 

 moisture deficit (i. c, evaporation is greater than precipitation, as 

 here measured). Only the extreme northwest, a small area in Mis- 

 souri, and a narrow zone near the eastern half of our northern 

 boundary, continued southward along the Atlantic seaboard and 

 westward along the margin of the Gulf of Mexico as far as Texas, 

 exhibit a moisture excess. Of course the highest deficits occur in 

 the most arid areas. These lines of moisture excess or deficit are 

 seen also generally to possess a north-south trend. Here again the 

 country is subdivided into areas by the crossing of temperature and 

 moisture lines and the various areas are susceptible of definition bv 

 means of these lines. The unsatisfactory condition of the evapora- 



