In the present paper the tendency of the persistence of weather was ascertained using the days of fair weather (cloud amount less than 8) and cloudy weather (cloud amount 8 or more than 8) in the interval from 1905 to 1929 at Tokyo. The ratio of the observed mean duration of any weather to the mean duration of the same weather calculated from the probability of that weather was studied. And it was found that the persistence factor f_f, f_c could be derived from the ratio and that there was a relation between the persistence factor f_f, f_c and probability.
The monthly values of the persistence factor and the ratio of the meau duration were calculated. The value of the ratio is large for July and August, and small for Jan., Apr. and Nov., but it is confined within the limits of 1.2 and 1.6. The annual change of f_f is almost parallel to p_c, while the change of f_c is antiparallel. For this point an explanation was given by the relation between the persistence factors f_f, f_c and the ratio of the mean duration.