Explanation:

Dew forms when water vapor in the air around cooling objects condenses into small water droplets. This happens because colder air can't hold as much water vapor as warm air. On clear nights, exposed surfaces lose heat to the sky through radiation, causing the surface to cool. However, on cloudy nights, clouds absorb and reflect the Earth's radiation back to the ground, preventing the ground from getting cold enough for dew to form.

The ideal conditions for its formation are clear sky, calm air, high relative humidity and cold and long night.