Dewdrops are formed when the temperature drops to the dew point and the air becomes saturated with water vapour.

On a clear night, the Earth's surface loses heat through radiation and can reach a temperature low enough to form dewdrops.

However, on a cloudy night, the clouds act like a blanket, reflecting back the Earth's radiation. This keeps the temperature of the air near the ground warmer, preventing it from reaching the dew point and thus, dewdrops are not formed.