Why dew appears on cool surfaces
After sunset, the ground and objects exposed to the open sky lose heat. Grass blades, car roofs, and windshields can cool faster than the air around them, so their surfaces may become colder than the air even on a mild night.
Air contains invisible water vapor. The dew point is the temperature at which that vapor begins to condense into liquid water. When a surface cools below the dew point of the nearby air, water molecules gather on it as tiny droplets. The air need not cool to the same temperature; a warmer layer just above the grass can still supply the vapor.
Droplets grow where the cooling happens
A surface provides a place for condensation to begin. Small irregularities, dust, and other particles help water molecules collect into visible beads. Because each blade or panel cools differently, dew can form in scattered patches rather than as an even coating.
This also explains why dew is common on clear, calm nights. With little cloud cover, surfaces can radiate heat away more readily; with little wind, warmer air is less likely to mix down and warm them. If the surface stays above the dew point, no dew forms, even if the night feels damp.
The droplets are therefore not rain arriving in miniature. They are water drawn from the air and assembled on whatever has cooled enough—sometimes leaving a windshield wet while the air beside it remains noticeably warmer.
