If the temperature difference between dry and wet bulbs is large, what does it generally indicate?

Correct answer: Low relative humidity

Correct answer: A, low relative humidity. The wet bulb is cooled by evaporation from its moist covering. When the surrounding air is dry, it can accept more water vapour, so evaporation proceeds strongly and the wet bulb cools considerably. The dry-bulb reading therefore remains much higher than the wet-bulb reading, producing a large wet-bulb depression. This condition generally means that relative humidity is low. Option B is incorrect because saturated air permits little or no further evaporation; the two bulb temperatures would then be nearly equal. Option C is incorrect because a large difference points to strong evaporation, not necessarily continuous condensation. Option D is the opposite of the observation: evaporation is not zero in dry air. Remember: a large dry–wet difference means dry air; a small difference means moist or nearly saturated air.