The diurnal variation of predisposition for precipitation strengthens and weakens frontal effects periodically and gives rise to a diurnal variation of precipitation. The predisposition is an effect of stability conditions In the atmosphere. Blue Hill Meteorological Observatory, near Boston, Massachusetts, is taken as a station characteristic of the coast, and Jasper, New York, 332 miles west of Blue Hill, is taken to represent inland conditions. Hourly frequencies of rainfall are taken from four years of record. The discussion singles out frequencies of slight (<0.05 in/hour) and heavy (≥0.10 in/hour) rainfall intensities. In winter a whole‐day wave predominates for both heavy and slight rainfall intensity, but heavy rainfall, inland, shows a rather well developed semidiurnal wave superposed. In summer, semidiurnal waves predominate both at the inland and the coast stations for the two intensities of rain. Numerical tables and diagrams facilitate understanding and provide rapid reference. Apparently instability showers prevail at the coast station, and cyclonic precipitation prevails at the inland station. In summer, at the coast station, a morning maximum of both heavy and slight precipitation‐intensity frequency exists, caused by instability showers while an afternoon or evening maximum is of convectional nature (semidiurnal wave). Probabilities are important to the forecaster. For example, the average probability of a heavy snowfall at the coast station during night is 4.3 times as great as in da