Cloud occurrence frequency (COF, the ratio between the number of records with detected clouds with respect to the total available records) and the distribution of COF within one day below 8 kilometers in summer from 2007 to 2009 over Beijing is studied. Cloud base height (CBH) of one layer clouds, the characteristics of convective CBH and the relationship between convective CBH and relative humidity (RH) measured on the surface are further studied. Results show that cloud occurrence frequency is about 12% in summer over Beijing. Clouds are more likely to occur at night than in the afternoons. The distribution of CBH has one peak at 700 (±50) meters and low clouds (ranging up to 3000 meters) account for 82% of all clouds. Convective CBH ranges from 400 to 2000 meters for the cases studied, but do not vary much within each day. CBH can be conveniently derived from linear relationship between CBH and surface RH based on Adiabatic Parcel Model. Mean relative standard deviation of CBH is 0.321.