Why may less dew form on a cloudy night compared with a clear night?

Correct answer: Clouds reduce longwave heat loss from the surface

Direct answer: A is correct. At night, the land surface loses energy by emitting longwave, or terrestrial, radiation. Under a clear sky this radiational cooling can be strong, so grass, leaves, and other surfaces may cool to the dew point; water vapour then condenses as dew. Clouds absorb and re-emit longwave radiation and therefore reduce the net loss of heat from the surface. The surface often remains warmer than it would under a clear sky, so it may not reach the dew point and less dew forms. A states this heat-balance mechanism. B is wrong because clouds do not always make the surface colder than freezing; they often have an insulating effect at night. C is wrong because clouds do not destroy all water vapour. D is wrong because clouds do not reduce air pressure to zero. This is a tendency, not an absolute rule: local humidity, wind, and cloud thickness also matter. Memory cue: clear night, stronger cooling; cloudy night, weaker cooling and usually less dew.