Explanation:

The ground becomes much cooler, causing the water vapour to condense. This condensed vapour is dew. On a cloudy night, the clouds send the heat back to the ground so the ground never gets cold enough for the dew to be formed.

The dew formation is more when the sky is clear and less when it is cloudy. When the sky is clear and the trees and plants are cooler at nights, there is more evaporation of water and hence more dew formation. But when it is cloudy, trees and plants do not get cool in the night and hence there is less dew formation. As the sun raises high in the sky, these dew drops evaporate into air.