Pluviometric coefficient

From Glossary of Meteorology

pluviometric coefficient

(Also called hyetal coefficient.) For any month at a given station, the ratio of the monthly normal precipitation to one-twelfth of the annual normal precipitation.

Seen collectively, the 12 pluviometric coefficients describe the normal month-to-month distribution of the normal annual precipitation in terms of each month's "share" of the annual amount.
See isomer.