Why does sweat dry slowly in moist air?

Correct answer: Because the evaporation gradient becomes weak

Answer: A. Sweat cools the body only after liquid sweat evaporates from the skin and carries energy away. In moist air, a relatively large amount of water vapour is already present. The difference between the vapour concentration at the wet skin and in the surrounding air—the evaporation gradient or vapour-pressure gradient—is therefore smaller. With a weaker gradient, fewer water molecules leave the skin per unit time, so sweat dries slowly and cooling becomes less effective. A is correct because it identifies this reduced driving force. B is wrong because vapour does form when sweat evaporates. C is wrong because the body has temperature and supplies energy for evaporation. D is wrong because moist air may be warm or cool; it is not always icy. Memory cue: humid air accepts less additional vapour, so sweat remains longer.