Four Routes Out
Heat leaves the body by radiating to cooler surroundings, by convection as air moves across the skin, by conduction into cooler objects in contact with it, and by evaporation as sweat turns to vapour. All four run at once, in proportions that shift with conditions.
Why Hot Days Change the Balance
Radiation, convection and conduction all depend on a temperature difference between skin and surroundings. As air temperature climbs toward skin temperature, that difference shrinks and those routes fade. On a very hot day they can approach nothing, or reverse.
Evaporation Does the Heavy Lifting
That leaves evaporation, and evaporation depends on how much water vapour the air can still absorb. In dry heat it works well. In humid heat it slows, which is why a humid 30 degrees can feel far harder than a dry 35, and why sweat that drips rather than evaporates has done nothing to cool you.
What Follows For Training
Airflow helps, because moving air carries vapour away from the skin. Clothing that lets sweat evaporate helps for the same reason. And on humid days, pace expectations are worth adjusting rather than pushing through, because the cooling route you are relying on is working at reduced capacity.
Frequently Asked
Why does humid heat feel worse?
Evaporation is the main cooling route in the heat, and humid air absorbs vapour more slowly. The same effort produces less cooling.
Does sweat that drips off cool you?
Not much. The cooling comes from the change of state as sweat evaporates. Sweat that runs off has cost you fluid without the benefit.
Does a fan help in extreme heat?
Moving air aids evaporation, which usually helps. In very high temperatures the picture is more complicated, and shade and reducing effort matter more.
