Final answer:
Evaporation of sweat from our skin absorbs heat to change liquid molecules into a gas, leading to a cooler skin temperature. This process is more efficient in low humidity, making us feel cooler on hot, dry days. The high energy requirement for evaporation at body temperatures contributes to this cooling effect.
Step-by-step explanation:
Evaporation of a liquid from our skin makes us feel cooler because it is an endothermic process, meaning it absorbs heat. When sweat evaporates from our skin, each water molecule requires a significant amount of energy to change from a liquid to a gas state. This energy is taken in the form of heat from our skin, which decreases the skin's temperature and cools us down. On hot, dry days, this process is more efficient as lower humidity allows for a higher evaporation rate from the skin's surface. Conversely, on humid days, the atmosphere already contains a high level of water vapor, which slows down the evaporation process, making us feel hotter as less sweat can evaporate and remove heat from our bodies.
It's important to understand that the temperature of a liquid can decrease even without reaching the boiling point due to evaporative cooling. For instance, the evaporation of sweat, which happens around body temperature, requires more energy compared to the latent heat of vaporization at 100°C. The heat input required for the evaporation of sweat is about 2428 kJ/kg, which is roughly 10 percent higher than the latent heat at boiling point. This energy comes from the skin, and as it is used to evaporate the sweat, the skin's surface feels cooler.