Why does sweat cool the body less in humid weather?

Correct answer: Because high humidity slows evaporation of sweat

Direct answer: A, high humidity slows the evaporation of sweat. Evaporation requires some liquid water at the skin to change into vapour and leave the surface. This change uses energy, much of it taken as heat from the skin, so evaporation normally produces a cooling effect. In humid weather, the surrounding air already contains a large amount of water vapour and is closer to saturation. The difference between the vapour concentration at the skin and in the air is smaller, so sweat escapes more slowly. Sweat may remain on the skin instead of evaporating efficiently, and the body feels warmer or more uncomfortable. A is correct. B is wrong because sweat contains water. C is wrong because the relevant process here is evaporation, not condensation. D is wrong because humidity does not reduce air temperature to zero. Memory cue: dry air accepts vapour more readily; humid air makes sweat stay on the skin.