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Heat, Humidity and Sweat (Revised)

NOTE: Revised to correct some mistakes regarding dew point and wet bulb temperature. 

The last few evenings, my runs have ended in a total sweat bath. It started out fairly cool, so how did I finish totally overheated?

Before I left my home, I checked the temperature outside, a very comfortable 21 C. Surely some warmer air had come through during my run. On my return a quick check of the thermometer revealed nothing had changed, still 21 C outside. In fact, it was also the same temperature inside my home.

I quickly threw off my soaking shirt and hat. Within minutes I was freezing. The sweat had dried off my skin too quickly. Another dry shirt, finally comfortable again. As a biomechanical engineer, I understood very well what was going on. Let me explain how we keep our cool in hot humid weather.

Our bodies have two related mechanisms for maintaining core temperature. The first mechanism is a combination of conduction and convection. When we get a bit too hot, the blood vessels near the skin open up to permit more circulation. This circulation warms the skin (conduction) where air currents (convection) then take the heat away. Let’s call this “dry cooling”. The second mechanism is of course sweating.

The capacity of dry cooling improves with several factors: temperature difference, wind velocity, surface area and the thermal conductivity of our skin. The core temperature of your body must be 37 C. As a result, the closer the air temperature gets to 37 C, the less this cooling mechanism works. Also, the faster the air currents move over your skin are, the better the cooling.  This is one reason we love to stand in front of a fan when we are hot.

A related factor is how much of our skin is covered. We can improve heat rejection by exposing more of our skin to the air, or cover up when we get too cold. In addition, we can also improve our heat rejection by reducing fat build-up near our skin. Fat is an insulator and impedes conduction. At some point, though, our dry cooling mechanism becomes ineffective at maintaining our core temperature. The tipping point is usually how hard we are exerting ourselves. With sufficient exertion, we will require more heat rejection, more than dry cooling alone can provide – the extra heat rejection is then provided by sweating.

Sweating can be thought of as an add-on which our bodies call into action, but only when necessary. We still need circulation to bring warm blood to the skin surface to be cooled. However, instead of wind alone taking the heat away, evaporation assists. Water in liquid form has a lower energy than water in vapour form. Changing water from liquid to vapour therefore requires energy (heat) and your circulation provides this heat. The more liquid that can be evaporated, the greater the cooling effect will be.

Evaporation of moisture from your skin depends mainly on how much moisture is already in the air. The more humid the air, the less sweat will evaporate and the less we will be cooled. The mechanical concept that controls our evaporation rate is known as the dew point. It is the air temperature where the air can hold no more moisture; it is 100% relative humidity. Another measure is called wet bulb temperature. It is the temperature our skin will be when it is wet, and this again depends on how humid the air is. There is a fixed relationship between air temperature, wet bulb temperature, dew point and humidity. Know any two, and the other ones can be figured out.

So, once we begin sweating to keep cool, it is not so much the air temperature but rather the dew point (or relative humidity) that determines if we are comfortable, and to some degree how much we will sweat to keep cool. But here is the thing: Whenever conditions are near 100% relative humidity, i.e. whenever the dew point temperature is very close to the air temperature, sweating will not be very effective. Yes we will sweat, but instead of evaporating, the liquid simply drips off. In such conditions, our bodies can only reject heat by convection and this is a big problem – the whole reason we had started to sweat was that convection alone was not enough! It is therefore easy to see why we can quickly overheat when it is very humid, regardless of the air temperature.

So how did it get so hot on my run? A quick check of 24 hour weather history shows the air temperature was 21 C, and the dew point was 20 C, when I went for my run around 7:00 pm last evening.  This is around 95% relative humidity! In other words, my skin temperature, soaked with sweat could be no cooler than 20.5 C (the wet bulb temperature).  As it turns out, a dew point of 20 C is just above the upper limit for comfort.

Pickering Hourly Weather

Ideally the dew point temperature should be 18 C or less to ensure comfort while we exercise and sweat. (see Wiki One difficulty is that weather forecasts rarely give dew point, but they do provide relative humidity. Below are some typical conditions that all have the same dew point of 18 C and, in theory, should feel the same once we start to sweat:

22 C, 80% RH, 19 C wet skin temperature
24 C, 70% RH, 20 C wet skin temperature
26 C, 60% RH, 20.5 C wet skin temperature
29 C, 50% RH, 21.5 C wet skin temperature
33 C, 40% RH, 22.5 C wet skin temperature
36 C, 35% RH, 23.5 C wet skin temperature

The above air temperature and humidity combinations should represent upper limits for most exercise that involves sweating. As you can see, it does not necessarily have to be hot to create uncomfortable (even dangerous) conditions for running. In fact, cooler humid conditions can be quite deceiving. If you had not warmed up prior to a race (or checked the forecast), you might not be prepared with all the necessary hydration. Conversely, it can actually be quite hot yet remain comfortable so long as it is relatively dry. In such conditions, we can safely handle the heat, provided we practice very good hydration.

What also makes sense, is that when we start sweating, our wet skin temperature does not change all that much for the same conditions of comfort. The list above shows a change in dry temperature of 14 C, yet our wet skin temperature only changes 4.5 C over the same range. That is how we can cope with excercise in fairly hot conditions. The key is to remain hydrated because if we could not sweat, our skin temperature would return to dry air temperature and the consequences could be fatal. 

And finally, how did I get so cold in my house? The answer was my air conditioning system had removed a lot of moisture from the air. It was 21 C but only 40% RH inside, which is equivalent to a wet bulb temperature of 13 C. Until my skin had dried of sweat, 13 C was also my skin temperature! Yes, indeed, it felt very cold.


Born and raised in Hamilton & Stoney Creek. Ran X-Country in high school, but not really special at it - a middle of the pack finisher. But then again, really didn't know how to train. Didn't run after Gr 12 due to nasty shin splints. Really never ran in proper shoes back then. Didn't try to run again until age 30. Then tried. And tried. And tried. Shin splints every time. Finally got it going for good at 38 in proper shoes and I have vowed never, ever, to stop running again.

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