What is the primary cause of dew formation?

Dew formation primarily occurs due to condensation of water vapor on surfaces as temperatures drop. When the temperature of a surface, such as grass or a car windshield, cools down overnight, it can reach the dew point temperature. At this point, the air in contact with the cool surface can no longer hold all of its moisture, resulting in condensation. This process is crucial in understanding dew and other forms of moisture that can form when the air cools at night, especially under clear skies when radiative cooling is most effective.

Other processes like evaporation add moisture to the air but do not directly cause dew. Similarly, while deposition involves the transformation of water vapor to ice, it is not typical of dew formation, which occurs as small droplets of liquid water. Finally, the formation of clouds at ground level, while related to moisture and temperature, represents a different phenomenon and is not the mechanism by which dew specifically forms.