Humidity, evaporation and heat loss

Basic Information

When working or exercising in hot conditions it is essential to be aware of the difference that humidity can make. It increases the risk of heat stroke very considerably because it reduces the amount of beneficial sweat evaporation on your skin.

Further it can make subsequent cooling of a heat stroke victim much more difficult.

What is humidity ?

Humidity is the name for the amount of water vapour [ gaseous water ] in the atmosphere. 0% humidity is when the air is completely dry , and when the air is full of as much water vapour as it can carry that is termed 100% humidity, also called saturation.

What happens in humid conditions ?

When water evaporates from any surface it removes heat quite efficiently which is why you feel cool especially in a breeze if you have sweat or water on you.
As humidity increases it is more and more difficult for water to evaporate, and so higher humidity reduces the amount of evaporation and hence the amount of cooling possible.
For example: If an average man was standing nude completely covered in water [ or sweat ] in still air at 0% humidity he would cool rapidly from water evaporating all over his body. The average male surface area is 1.8 square meters [ approx 18 square feet ] and this could theoretically give cooling of 1300 watts.
However if the external temperature was about blood temperature i.e. 37 oC and 70% to 80% humidity then cooling would drop to about 300 watts.

Practical Implications

The following chart based on information from "Performing in Extreme Environments" shows the level of danger, even if you are acclimatised, when exercising at different combinations of temperature and humidity.

The risk expressed is that of suffering Heat Exhaustion or Heat Stroke

Relative Humidity

Dry Bulb Temperature
20% 30% 40% 50% 60% 70% 80% 90% 100%
o C o F
41 105
38 100 Very High
35 95
32 90
29 85 High
27 80 Moderate
24 75
21 70 Low
18 65