The dew is formed when the objects radiate heat thoroughly so that the moist air coming into contact with them may be sufficiently cooled and the water vapour is condensed into water. The clouds act as a blanket for the surface of the earth. They check the radiation of heat so it does not radiate heat thoroughly. Hence, the dew is not formed in the cloudy night and in shady places.
Similarly, the dew is not formed on the windy night, because the winds blow fast and the moist air does not come along enough into contact with cold objects to get cooled. Hence the condensation does not take place and no dew can be formed.
