If the air is dry, then perspiration evaporates rapidly. This cools our bodies because evaporating water requires heat, and that heat is removed from the surface of our skin. If, on the other hand, the air is very moist, then perspiration tends to cling to us without evaporating and we lose the benefit of this cooling. Air at a particular temperature can hold only so much water. Air holding the maximum possible amount of water is said to be saturated .
Warmer air can hold a lot more water than cooler air.
U.S. weather reports traditionally include the relative humidity , telling us how much water there is in the air as a percentage the maximum possible amount. It doesn't tell us how the air feels . If you were outside on the afternoon of July 28, you know it felt terrible . The heat index , which combines the effects of temperature and humidity into a single number. The dewpoint , the quantity meteorologists actually measure to determine the humidity.