252 ENVIRONMENTAL CONDITIONS. 



indices for this by summing the normal daily precipitation values 

 within the period of the average frostless season (Bulletin R, U. S. 

 Weather Bureau) and then dividing the result by the number of days 



represented in the period. These averages may be represented by the 



p 

 symbol — , where P is the total rainfall for the average frostless period 



S 



and S is the number of days in that period. The values of this ratio, 

 for the stations considered, are given in the third column of table 11, 

 the values of P being placed in the second column. The precipita- 

 tion data for a few of the stations listed are not from Bulletin R : they 

 are either from Bulletin Q (in which case the station name is followed 

 by an H in parentheses), or they are from the Summary by Sections 

 (in which case the name is followed by an S and the number of the 

 section of the Summary in which it is listed, in parentheses). Table 11 

 also includes evaporation data, and where two stations are given for 

 the same data, the first (not in parentheses) is the one to which the 

 precipitation data refer. The second one (in parentheses) is the one 

 referred to by the evaporation data. 



Where the precipitation data are not derived from Bulletin R (those 

 marked H or S), the total precipitation for the period of the average 

 frostless season was approximated by calculation from the normal 

 monthly precipitation data as given by Henry (Bulletin Q) or in the 

 Summary by Sections. To accomplish this approximation the monthly 

 normals for all whole months included in the period of the average 

 frostless season were added together, and to this sum were added a 

 fractional part of the next preceding and of the next following monthly 

 normal, these fractional parts being, in each case, that part of the value 

 for the whole month that is represented by the number of days of 

 that month included in the frostless season. Thus, if the average frost- 

 less season extends from June 5 to September 6 and if the monthly 

 normal precipitation values for the months involved are a, h, c, and d, 

 then the approximate total precipitation for the period of the frostless 

 season is 



30 30 



Here S, the number of days in the average frostless season is 25+31 + 

 31+6 = 93, and 



P_ 5/6a+6+c+l/5(i 

 S 93 



