It’s something everyone knows from when they’re young: when a summer day is more humid, the heat feels worse. This isn’t because high humidity makes the day hotter. When it’s 85°F outside, it remains 85°F no matter if the relative humidity is a bone-dry 10% or a sopping wet 90%. What changes is how that 85°F feels. With more moisture in the air, it becomes more difficult for the human body to release heat through the skin. Perspiration slows down because there is too much water in the air. The result is more heat is trapped inside the body, making it feel hotter. It’s as if you decided to put on a coat on a hot day!