If air shows a very small difference between dry-bulb and wet-bulb temperatures, what can be concluded about humidity?

Correct answer: The air is close to saturation.

The correct answer is C. A wet-bulb thermometer is cooled by evaporation of the water covering its bulb. In dry air, evaporation is rapid, so considerable heat is removed and the wet-bulb temperature becomes much lower than the dry-bulb temperature. In humid air, the air already contains much water vapour, so evaporation from the wet bulb is limited. Consequently, the wet-bulb temperature falls only slightly and the wet-bulb depression—the dry-bulb temperature minus the wet-bulb temperature—is small. C is therefore correct: a small depression indicates air with high relative humidity, often close to saturation. A and B are wrong because dry air causes a large, not small, depression and usually permits more evaporation. D is wrong because a small difference indicates moisture is already present, not that vapour is absent. The instruments do not directly prove that the air is exactly saturated unless the temperatures are equal, but they show that it is near saturation. Memory cue: small wet-bulb depression means moist air; large depression means dry air.