Why is dew more likely on a clear calm night?

Correct answer: Radiational cooling is stronger and air mixing is weaker

Correct answer: A. Dew forms when a surface cools to its dew point and water vapour condenses into liquid droplets. On a clear night, the ground loses heat efficiently by outgoing terrestrial radiation because clouds do not act as a covering layer. The surface can therefore become colder. Calm conditions are also important because they prevent strong vertical mixing; moist air remains close to the cooled grass, leaves, or soil. When that nearby air reaches saturation, dew forms on the surface. A is correct because it combines both necessary ideas: stronger radiational cooling and weaker air mixing. B is wrong because solar radiation is absent or very weak at night. C is wrong because strong wind usually mixes air and can prevent a thin layer near the surface from cooling sufficiently; it does not always heat the surface. D is wrong because clouds generally reduce net surface heat loss by absorbing and re-radiating terrestrial radiation. Memory cue: clear sky cools, calm air keeps moisture near the ground.