CLIMATIC CONDITIONS OF THE UNITED STATES. 267 



an index of raininess, this may be taken as an index of dryness or 

 aridity. The vahies are given in column 3 of table 13 and are shown 

 graphically on plate 48. This chart shows a total range for the country 

 of from 294 days (Los Angeles, California) to no days (Cape Hatteras, 

 North Carolina). The isoclimatic lines here again represent incre- 

 ments of 25 days each, full lines being shown for the values 50, 100, 

 and 200. In a general way, the climatic zonation of the country is 

 similar to that of plates 46 and 47, but this chart is markedly different 

 from the others in certain details, and of course the actual values are 

 different. Some of these differences will be considered below. 



(6) Percentage of Days in Period of Average Frostless Season that are Dry Days 

 (with Normal Precipitation of 0.10 Inch or Less). (Table 13, Plate 49.) 



This index of precipitation intensity is obtained simply by expressing 

 as percentage each value of the third column of table 13, in terms of 

 the corresponding length of the period of the average frostless season. 

 These percentages are given in column 4 of table 13. They express 

 the relative frequency of dry days in the period. 



These values are shown graphically by the chart of plate 49. The 

 total range for the country is from nil (Cape Hatteras, North Caro- 

 lina) to 100 per cent (various stations in the arid region). The lines of 

 the chart are drawn at intervals of 10 per cent, those for 20, 50, and 100 

 being full lines, and the zonation is once more similar to that of the 

 other precipitation charts already mentioned. 



(7) Length of Longest Normally Rainy Period in Period of Average Frostless 



Season. (Table 14, Plate 50.) 



In many regions the duration factor for the favorable range of 

 moisture conditions is not as great as that for the corresponding range 

 of temperature conditions, and the former thus becomes the main 

 duration factor influencing plant activities. In such cases only a 

 portion of the period of the average frostless season is suitable 

 for active plant growth. In southern Arizona, for example, the 

 normal frostless season is very long (241 days at Tucson, from March 

 26 to November 22), but all of this period is practically without rain, 

 excepting only a portion of the summer. The summer rainy period at 

 Tucson extends from about July 1 to about September 15, but there is 

 also a spring period of general plant activity, extending from the 

 cessation of frost to about May 1. The latter period is nearly rainless, 

 but the soil-moisture content is high, due to the residual effects of the 

 winter precipitation. Thus there are here two periods of general plant 

 activity within the period of the average frostless season, one from 

 about March 26 to about May 1 (at which time the winter moisture 

 is about dried out of the soil) and the other from about July 1 to about 

 October 15 (when the summer moisture has largely disappeared). It 



