Observations at Dnbuque, Iowa. at 7 a. m., 90th meridian time, for June and July, for the 20-year period, 1901 to 1920, inclusive. are tabulated with reference to the occurrence of precipitation within the following 12 and 24 hours, and classified according to the following elements: A further classification according to (a) wind direction, and (b) kind of clouds is based on a 30-year period for the four months, May to August, inclusive. The results are shown in a series of tables and curves, based on the relation which the number of observations followed by rain within 12 or 24 hours bears to the total number of observations within the group. These tables and curves are intended to show the more important relationships existing between the data of the morning observation and the rain frequency during the following 12 and 24 hours, and it is suggested that a Series of simple classifications, such as these, could be made at numerous stations throughout the United States, and would furnish material of much value.