We analyze a series of daily 8-term observations of the total cloudiness cover index n over St. Petersburg and over a small village Belogorka (countryside). The distribution function of the difference Dn between of the cloud cover index values in St. Petersburg and Belogorka is constructed for two seasons (summer and winter). The maximum of Dn occurrence is at Dn = 0 being equal to 68% in winter and 40% in summer. The occurrence of both synchronous and time-separated Dn values has been calculated. The probability that the cloud cover index remains the same or changes from one cloud gradation to another one is studied. From analysis of diurnal and monthly mean Dn values, we conclude that: (1) the cloud cover index of low-level clouds under urban conditions is a few tenths larger than under the rural ones; (2) dynamic factors are major contributors to the cloud formation process; and (3) thermal factors contribute no more than 10-20%.