Rainless days of London

The incidence of the 12,616 rainless days which have occurred at London (Kew) during the 70 years, 1878 to 1947, has been examined, a rainless day being one on which less than 0·1 mm. of rain has fallen. The average number of rainless days is greatest in June and least in November. On the average just under half the days in the year are rainless. For the 70 years, the frequency of rainless days on each day of the year has been found, that on May 21 being the greatest, that on October 28 the least. Many of the days when this frequency is exceptionally high or low are associated respectively either with the anticyclonic or stormy “singularities” of C.E.P. Brooks, or with large positive and negatives departures from the mean daily pressure at Kew. The shortest rainless spells (1 and 2 days) are most frequent in October, the longest (exceeding 14 days) in July. Spells of more than 14 days have not been recorded between October 28 and January 22. Spells exceeding 6 days' duration occur at all seasons of the year. They are to be expected most frequently in June and least so just after Christmas. The odds to one against the occurence of a rainless day being included in a rainless spell exceeding 14 days are about 10 in June and July and 118 in January. An examination has been made of the synoptic succession which marks the onset, course and termination of the nine longest rainless spells and a brief summary given.