The effect of cloudiness on the occurrence of atmospheric new-particle formation events at two measurement stations, SMEAR II in Hyytiälä, Finland and San Pietro Capofiume (SPC) in Italy, was investigated. As an indicator of cloudiness, we use the relative radiation intensity (I/Imax) defined as the ratio of measured global radiation and the modeled clear-sky global radiation. We studied the relationship between the occurrence of new-particle formation (NPF) events and I/Imax using multi-year data sets. The results showed that, at both sites, the radiation intensity (I) should be at least about 50% of its maximum possible value (Imax) for a clear NPF event to occur. In SPC, clearly higher relative radiation intensity was typically required for the occurrence of an NPF event than in Hyytiälä. Also, the features of anomalous days, i.e either NPF events that occurred in cloudy conditions or non-events that occurred in clear-sky conditions, were explained using the environmental variables sulfur dioxide and condensation sink.