Portrait Of Tired African Male Athlete Finished Workout And Resting While Sitting On A Racetrack

Why it’s Harder to Exercise in Humid Conditions

BY Andy Blow

No, it's not just in your head—humidity really does make training more challenging. Here's how to cope.

Why does exercising in humid conditions feel so much more difficult than in hot but dry climates? It’s all to do with how human thermoregulation works…


One of the biggest challenges your body faces when exercising is the production of excess heat as a by-product of muscular contraction. The body is only about 20% efficient; meaning that if you output 200W on the bike, about 800W is also produced in heat!

Your core body temperature needs to be kept in check. A rise of only 5-6 F over baseline can be disastrous, so this excess heat needs to be offloaded to the environment quickly in order to stop your body ‘boiling over’. 

For humans, the main way we lose heat when exercising is via sweating, or to be more accurate, through the evaporation of sweat into the air from the skin, which takes a lot of heat away with it. 

And this is where humidity becomes a big problem.

When air is described as being 0% humid, it’s totally dry. If your body is trying to evaporate moisture into the air, the situation is a happy one for all concerned because there is a huge gradient from your skin to the atmosphere across which evaporation can take place.

But if humidity is up at 100% already (i.e. the air is as saturated as it can possibly be with water—think of a steam room), it’s basically impossible for sweat to evaporate, so it merely pools on the skin and drips off, taking very little heat with it in the process.

Of course, humidity levels of entirely 0% and 100% are rare, but there is a continuum between them. When the needle gets much above 40% (if accompanied by warm or hot air temperatures), it starts to become really tough for sweat to evaporate effectively—at that point, any level of intensity starts to feel rough because heat dissipation is poor.

This is why the average October temperatures in Kona – 80-90 F (26-32C) – don’t sound totally horrendous on paper (if you’re acclimated to the heat), but when you get there and experience the 80-90% humidity that can accompany those temperatures, you quickly realize why competing in the IRONMAN World Championships there is so brutal.

The Heat Index

Because of the dramatic effect humidity has on the ‘effective’ temperature experienced by your body, there are numerous ways of combining humidity readings with air temperature that have been invented to provide a more accurate ‘feels like’ temperature.

A ‘feels like’ temperature can then be used to help make practical decisions, such as what kind of clothing to wear and whether it’s even sensible to undertake a sporting or physical activity in the first place.

The Heat Index is one of the simpler models as it just takes into account the air temperature and relative humidity (others like Wet Bulb Globe Temperature also include additional factors like sun exposure, wind chill factor and so on). But, it’s a very useful guide nonetheless and definitely helps to illustrate the dramatic effect that heat and humidity can have when combined.

For example:

According to the Heat Index calculation, if the air temperature is 80F (26.6 C) and the relative humidity is a relatively pleasant 35%, then the Heat Index temperature is the same as the actual air temperature (i.e. 80 F / 26.6 C).

If the humidity jumps up to 70% then the Heat Index temperature increases to 83F (28 C).

If it goes up to Kona-esque 90% then the Heat Index temperature hits 86F (30 C). A big difference over what the air temperature alone would suggest it should feel like.

This online heat index calculator is a pretty useful tool if you want to just bash some numbers in and get a relative Heat Index result. If you do play with it, you may notice that at very low humidity levels the Heat Index temperature drops below the actual air temperature. This reflects the fact that sweat evaporates really easily at low relative humidity so it can make it feel cooler as a result.

The Heat Index gives an idea of the temperatures which will require some level of caution or represent a real danger to humans, especially those undertaking physical activity.

While this is based on a number of assumptions and how it might affect an ‘average’ human body, it’s still a good rule of thumb to help evaluate the approximate risk level posed by different combinations of heat and humidity.

Heat Index Applied

As 2021 summer Olympic hopefuls probably already know, the average temperature and humidity readings for Tokyo in August are ~82F (27.7 C) and 77%, putting the Heat Index temperature up at 88F (31 C) – thus butting up to the ‘Extreme Caution’ area.

If it gets hotter and wetter (as it often can) the heat index in Tokyo could top 100 quite easily. It’s no wonder that many of the teams and athletes preparing for the games are doing everything they can to figure out how to mitigate the risks posed by these conditions, as they’ll surely have a huge impact on the outcome of many of the outdoor events.

How to Reduce the Effects of Heat and Humidity

If you know you’re going to have to train or compete somewhere with a high heat index or high humidity, then there are potentially a few things you can do to make your life a little more pleasant and assist your performance…


The first of these is to acclimatize. Your body is pretty adept at getting used to the heat if you give it some exposure. Adaptations to the heat include:

  • An increase in sweat rate (maybe more so in high humidity)
  • An increase in blood plasma volume (to assist with cooling and blood supply to the skin and muscles)
  • Start sweating earlier (i.e. after a smaller increase in core temperature)
  • Lower baseline core temperature
  • A number of other metabolic adaptations that help to ensure that you cope better with the heat

Full acclimatization to a hot environment generally takes about 14 days of prolonged exposure and the effects are magnified if you do some training in the heat too. This is why all the athletes for Tokyo (and everyone who wants to do well in Kona each year) will spend time on training camps in the heat and humidity during the weeks leading up to the event. 

Heat adaptations come quickly but also disappear relatively quickly too, which is why acclimatization camps should come almost immediately before the focal event so that the results are not lost by the time race day arrives.

If you’re pressed for time and resources (like most of us who are not full-time athletes), then the good news is that about 70% of the adaptations you can achieve through full heat acclimatization can be attained in as little as 5 days, or possibly even just 5-8 prolonged exercise sessions in the heat.

So, it can be helpful to invest a little bit of energy into finding a heat chamber to train in a few times or just crank the heaters up when you train indoors. Even wearing some extra layers of clothing when doing training in a more temperate environment in the last week or so before you have to compete in the heat can provide you with significant benefits.

One key thing about acclimatization is that the optimal plan to follow would be to match the training conditions to those you’ll be competing in (in terms of both temperature and humidity). Unsurprisingly, you tend to get the most relevant adaptations from the most specific type of preparation. 

A final word to the wise here on heat training (because athletes do tend to love pushing the limits): Always be safe and be cautious with heat acclimatization training. It’s remarkably easy to overcook it and heat illness is a very serious, potentially life-threatening scenario if you’re not careful. 

Always moderate your pace (at least 15-20% slower than you would normally go for a given workout), start with shorter and lighter sessions to begin with, don’t push yourself too hard and make sure someone is either supervising you directly or at least aware of what you’re doing so they can keep an eye on you.


Clearly, when you sweat more in hot and humid conditions, hydration becomes more of a performance-defining issue. Whenever you’re training to acclimate or planning to compete in a situation with a high Heat Index, it’s extremely wise to start properly hydrated and to drink adequately to help mitigate fluid loss, especially in the early stages of a longer race or session. It’s also a time when fine-tuning sodium intake according to your own individual needs is very important.

How to start to work out how much (and what) to drink to optimize your own hydration plan can be found in this blog, so it’s well worth taking a look at that for more detail. It’s not a topic where there is a quick ‘one size fits all’ answer, that’s for sure.

As an interesting aside; I have read speculation that in high humidity specifically, your total sweat rate can increase over and above your rate in hot and dry conditions as your body up-regulates sweating responses in areas of the skin that typically don’t perspire as much in dry weather.

The theory is that this helps to increase the overall ‘wetted area’ on your skin, leading to more chance for sweat to evaporate if it can. Whilst I’m not sure of the magnitude of this effect, it seems plausible that heat + humidity would equal more sweating than heat alone causes and it certainly feels that way when you’re running in Florida!

Pace yourself

One final thing that’s utterly critical when facing hot and humid conditions is to be very realistic about pacing.

As described earlier in this article, when you’re exercising a huge proportion of the energy produced gets ‘wasted’ as heat, causing the body to warm up more rapidly the faster you go.

When going hard in cold conditions this is not a big issue as it’s relatively easy for the body to shed heat into the external environment. It’s why when researchers studied a host of marathon races around the world they essentially found that somewhere between ~39 F (~4 C) and ~49 F (~10 C) seems to be the optimal ambient temperature for running fast over that distance, with performance being compromised as the temperature creeps up above that.

When conditions get ‘super hot’ and humid, offloading heat via the evaporation of sweat is very inefficient so there’s often a terrible penalty to pay for aggressive pacing, especially early on in a race. It can cause overheating that’s very difficult or impossible to reverse without significantly slowing down or stopping altogether.

It means that being more ‘tortoise’ than ‘hare’ is the order of the day.

I can recall seeing a brilliant interview with Jan Frodeno (Olympic Triathlon champion and IRONMAN World Championships winner) a few years ago saying that his rule of thumb is to take 15-20% off his normal wattage on the bike in Kona to account for the high Heat Index that’s so often a feature of the race out there.

He likened it to racing at altitude in that you just have to learn to accept that you’re not going to put out the same kind of power and pace as you can in more accommodating conditions. Being humble enough to respect this is a huge part of being successful in very tough environmental conditions.

So, it’s a simple fact that exercising in the heat and humidity presents a particularly challenging scenario for the human body. It’s quite a bit more challenging than just working out in dry heat.

That said, with a sensible, pre-meditated approach to acclimatization, hydration and pacing, you can certainly limit your losses and hopefully leave a few less prepared competitors in your wake as a result.

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About Andy Blow

Andy Blow has a few top 10 Ironman and 70.3 finishes and an Xterra World Age Group title to his name. He founded Precision Hydration to help athletes solve their hydration issues. He has a degree in Sport and Exercise Science and was once the Team Sports Scientist for Benetton and Renault F1 teams.

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