If a desert night is very cold but the air is extremely dry, why will dew formation be limited?

Correct answer: The amount of water vapour is insufficient

Correct answer: B. Dew forms when a surface cools enough for nearby air to reach saturation and water vapour condenses as liquid droplets on that surface. A cold desert night supplies the cooling condition, but extremely dry air contains very little water vapour. With so little vapour available, the air may not reach saturation easily, and there may not be enough moisture to produce visible dew. Option A is wrong because condensation can occur on sand or other suitable surfaces when the necessary thermal and moisture conditions exist. Option B is correct because the limiting factor in the question is the shortage of water vapour. Option C is wrong: dry air can certainly cool at night, especially in a clear atmosphere; dryness does not prevent temperature from falling. Option D is plainly false because deserts do have nights. The important lesson is that dew generally needs both a sufficiently cool surface and enough atmospheric moisture. Memory cue: “Cold makes dew possible; moisture makes it visible.”