Evidence of a tidal influence from luni-solar gravitational fields has been highlighted by analysing daily precipitation data collected at many stations in the Alps and the Po Valley (Italy) over the 1970--81 period. Tidal influence as the possible result of a modulation effect caused by luni-solar cycles having similar lengths, does not appear to be strictly constant over time. Time variations in daily precipitation data as a function of some particular cycles show that gravitational tides affect heavy precipitation more than mean precipitation values. ''Semimonthly'' oscillations have been derived from trends in daily precipitation data grouped according to the age of distinct lunar months. The investigation of the combined effects of different luni-solar components, relating to years of particular climatological interest, might assist in the prediction of extreme precipitation. ¿ American Geophysical Union 1987