Humidity

Air temperature is only one factor in human comfort : humidity is also important.

We cool by perspiration - the body produces water as sweat, which evaporates from the surface of the skin, or from clothes. When the water changes from a liquid to a gas or vapour, it absorbs a lot of energy, cooling the surface.

If there is no fan blowing hot air from indoors to outside, then not only will heat build up, but the humidity will, too. If the air is more humid, more sweat will dribble off the skin instead of evaporating. This removes much less heat from the body.

The worst case you may have experienced is in a 'sauna'. If you pour water on the hot rocks, it instantly increases the humidity so that you suddenly feel much hotter, even though the temperature changes little. The high humidity removes the body's ability to cool itself.

Another situation is at weekenders : sometimes we use 'evaporative coolers'. They blow air over a wet surface, which cools itself by evaporation. In an empty shop, where people are just sitting or walking around, without having to sweat, this can feel cooler. But when people are exercising so they have to sweat to cool down, you have already used the dry air's capacity to cool by evaporation to cool the air down : sweating will no longer have much effect in cooling the body down. The air is already nearly saturated with humidity.