The average or mean total monthly precipitation is calculated as
Rmon=yrs∑d=1N∗Rday,mon
where Rmon is the mean monthly precipitation (mm H2O), Rday,mon is the daily precipitation for day d in month mon (mm H2O), N is the total number of records in month mon used to calculate the average, and yrs is the number of years of daily precipitation records used in calculation.