Why is dew seen on grass during a cool night?

Correct answer: Water vapour condenses on cool grass

The governing concept is condensation, the change of water vapour into liquid water when air or a surface cools sufficiently. During a clear, cool night, grass loses heat by radiation and its surface temperature may fall to the dew point. Air touching the grass is then cooled; if it becomes saturated, some water vapour changes into tiny liquid droplets on the blades. These droplets are called dew, so option D is correct. Grass does not manufacture water, air does not disappear, and smoke is unrelated to the process. Dew is different from rain because it forms directly on a surface rather than falling from clouds. The exact amount depends on moisture in the air, cooling, wind, and the surface temperature.