260 



ENVIRONMENTAL CONDITIONS. 



sidered here as a semirainy region. There are thus four precipitation 

 zones or provinces roughly marked out on this chart, which may be 

 defined as in table 12. 



Table 12. 



The four provinces thus indicated will be repeatedly referred to in 

 our further discussion of moisture conditions. 



(3) Total Normal Precipitation for Period of Average Frostless Season Plus 

 Preceding 30 Days, Divided by Number of Days in Average Frostless 

 Season (tt/S). (Table 11.) 



This rather artificial index of precipitation intensity is based, as has 

 been mentioned, upon the consideration that some of the precipita- 

 tion occurring just before the opening of the frostless season is still 

 effective in the early part of that season. In many places the first few 

 weeks of the average frostless season are normally more or less dry, and 

 yet plants may be able to begin their activities with the advent of 

 frostless weather, on account of soil-moisture left over from the latter 

 part of the preceding frost season. The length of the additional period 

 of 30 days was chosen quite arbitrarily, in an attempt to bring these 

 considerations into the index, which we term tt. Bigelow's precipita- 

 tion normals (Bulletin R) were again used. The values obtained are 

 given in column 4 of table 11, and these totals divided, in each case, 

 by the corresponding number of days in the period of the average frost- 

 less season (tt/S) are given in column 5 of the same table. 



The chart obtained from these averages (ir/S) shows no pronounced 

 differences from that presented in plate 46, and it is not reproduced 

 here. This chart is mentioned, since the method by which it was 

 obtained is new and may be of value in the future, for special studies 

 of certain regions. The values of tt will be otherwise employed below. 



(4) Number of Normally Rainy Days in Period of Average Frostless Season. 



(Table 13, Plate 47.) 



This kind of index of precipitation intensity is frequently employed 

 by climatologists, though for other duration factors than the one here 



