Why is moisture alone not enough for fog formation?

Correct answer: Air must also cool to saturation

Correct answer: A. Fog forms when moist air becomes saturated and water condenses into tiny droplets suspended near the ground. Moisture is necessary because there must be water vapour in the air, but moisture by itself does not guarantee saturation. The air must usually cool to its dew point, the temperature at which it can no longer hold all its water vapour under the existing conditions. As cooling continues, excess vapour condenses on tiny particles and produces fog droplets. Option A is correct because it gives the second essential idea: cooling to saturation. Option B is meaningless and unrelated to condensation. Option C is incorrect because lava is not required for fog. Option D is incorrect because atmospheric pressure does not need to disappear; fog can form under ordinary pressure when moist air cools sufficiently. A useful warning is that “humid” does not always mean “foggy.” Humid air may remain clear if it has not cooled to saturation.