Final answer:
Sweat disappears through evaporation, which cools the body by transferring heat away from the skin. This process is less efficient in humid environments, where the air's saturation with water hinders evaporation and can lead to increased body temperature.
Step-by-step explanation:
Sweat generally disappears when it evaporates from the skin, which occurs when the body temperature rises and the body needs to cool down. Evaporation effectively cools the body because it requires a significant amount of heat transfer—specifically, a heat input of 2428 kJ/kg at body temperature, which is approximately 10% higher than the latent heat of vaporization at 100°C. However, in a humid environment, this cooling mechanism is less efficient due to the saturation of water in the air, making it more difficult for sweat to evaporate and potentially leading to a dangerous increase in internal body temperature.