As temperature drops, air cannot hold as much moisture. What happens when air contacts surfaces at or below its dew point?

• A. Humidity increases without condensation.
• B. Temperature rises to reach equilibrium.
• C. Condensation occurs on the first solid surface it encounters.
• D. Water vapor becomes airborne.

Answer: (C)

When air cools to its dew point, water vapor changes into liquid. If that air touches a surface that is at or below the dew point, the thin layer of air near the surface cools to the surface temperature and the vapor condenses on that surface. The first solid surface the cooled air contacts provides the place for condensation to start, so droplets form there—the familiar dew on grass or condensation on a window.

Other ideas aren’t what happens here: humidity isn’t simply increasing—it's leaving the air as liquid droplets. The surface is cooling the air to reach dew point, not causing the air to warm up to equilibrium, and the moisture isn’t just becoming airborne again.