Why can wet-bulb temperature be lower than dry-bulb temperature?

Correct answer: Because evaporation on the wet bulb takes heat

Direct answer: A. A dry-bulb thermometer measures the ordinary air temperature. A wet-bulb thermometer has its bulb covered with a wet wick. If the surrounding air is not fully saturated, some water from the wick evaporates. Evaporation requires latent heat, and that heat is taken from the water, wick and thermometer bulb. The wet bulb therefore cools and can show a lower temperature than the dry bulb. The difference is called the wet-bulb depression and is related to the moisture content of air: greater evaporation usually means drier air and a larger difference. B is incorrect because the dry bulb is not covered with ice. C is incorrect because the wet wick does not block sunlight; proper instruments are shielded from direct radiation. D is incorrect because completely moisture-free air is not required, and humid air actually reduces evaporation. Memory cue: wet surface plus evaporation means cooling.