ISTew York Weather Bureau. 517 



The probable error of a 35 years record is 1.78 per cent, of the 

 annual amount. 



The probable error of a 30 years record is 2.26 per cent, of the 

 annual amount. 



The probable error of a 25 years record is 2.75 per cent, of the 

 annuial amount. 



The probable error of a 20 yeans record is 3.24 per cent, of the 

 -annual amount. 



The proibable error of a 15 years record is 4.75 per cent, of the 

 annual amount. 



The prohable error of a 10 years record is 8.22 per cent, of the 

 annual amount. 



These values were generally found to include both irregular 

 and possible secular variations of rainfall. As the uncertainty of 

 monthly averages is not given, a rough estimate is here attempted 

 by the method of variability, as explained in the case of tempera- 

 lure normals. The average deviations of the single monthly 

 values from their means were found to be as follows for the 

 thirty-eight years record of Cooperstown 



The average variability of rainfall for January=34 per cent, of 

 the total. 



The average variability of rainfall for Februar3'=42 per cent. 

 of the to/tal. 



The average variability of rainfall for June=36 per cent, of 

 the toial. 



The average variability of rainfall for July=32 per cent, of 

 the total. 



Mean for the four m'0nths=35 per cent, of the total. 



From the mean variability, 35 per cent., the following probable 

 errors of the average monthly values are found by the formula 

 of page 513: 



For a 40 years record the probable error of monthly means 

 :^6 per cent. ^ 



For a 30 years record the probable error of monthly means 



=6 per cent. 



For a 20 years record the probable error of monthly means 



=8 per cent. 



