Dew forms when the air just above the ground is cooled to its dew point. The air above the ground is cooled because the ground radiates heat into space. If there are clouds overhead the clouds reflect the heat back to the ground so it does not cool and o dew forms. If it is windy then the air just above the ground to the dew point and no dew forms. Thus to form dew you need a still clear keeps on getting mixed with the air higher up and the air does not cooled to the dew point and no dew forms.
