The old saw about it not being the heat but the humidity as the cause of summer misery is not the best summation. Dew point is the key to comfort.
“Dew point” is a phrase often seen on weather reports but little pondered. The words describe the temperature at which water vapor — which is measured as humidity — condenses at the rate it evaporates. As this is dependent on air pressure, this is not a constant.
If the temperature is below the dew point, the water stays rather than goes. High dew points mean discomfort. As it is the evaporation of perspiration that cools the skin, the slower the evaporation, the slower the cooling.
If you want to calculate the dew point yourself, this link explains how. We’ll stick to the weather report.
Dew Point Key To Comfort