As we approach summer, you’ll hear us begin to talk more about how moisture content in the atmosphere make it feel warmer than it really is. If the temperature outside is 85 degrees, it will feel hotter if it is more humid. This is because evaporation is slower when the air is closer to being fully saturated.
Evaporation is a cooling process. When you get out of a pool, you feel cold. That’s because as water on your skin changes phase from a liquid to a gas, energy is absorbed by the water vapor during the phase change.
Dry and unsaturated air allows for rapid evaporation and faster cooling. Humid and more saturated air causes evaporation to take place at a slower rate, causing less cooling. Adding a breeze can enhance the evaporation process as well.
If you visit a pool after a cold front has moved through and a cool and dry Canadian airmass is in place, you’ll feel the rapid evaporation effect on your skin once you get out of the water. You’ll either want to get back in the water to stop the evaporational cooling or grab a towel to get the water off and end the chilling effect.
So the bottom line is that it feels hotter on humid days because moisture that is always on the surface of your skin cannot evaporate efficiently. When the air is dry, that moisture evaporates faster and you feel cooler. We can speed up the evaporation effect by pushing more air molecules past our skin. That’s why a gentle breeze or a fan helps to battle hot conditions.