Podcast, Science

Heat, humidity, wind, and slush drinks: cool down your body to go harder for longer with Andrew Buckrell | EP#160

 December 17, 2018

By  Mikael Eriksson

​​​Heat, humidity, wind, and slush drinks: cool down your body to go harder for longer with Andrew Buckrell | EP#160

Andrew Buckrell, co-founder of STAC, discusses heat. Our bodies do a great amount of work to get rid of excess heat generated when training, and strategies for maximising the body's ability to get rid of heat, and minimising heat build-up in the first place, are critical for optimal endurance performance. 

Discuss this episode!

  • Let's discuss this episode and the topic in general. Post any comments or questions in the comments at the bottom of the shownotes. Join the discussion here!

In this Episode you'll learn about:

  • How much heat do our bodies generate in triathlon and how does that impact performance.
  • The effect of ambient temperature.
  • The effect of speed and air flow.
  • The effect of sweating and humidity.
  • Strategies for minimising or reducing heat buildup in racing.
  • Strategies for minimising or reducing heat buildup in training. 
  • What's the deal with slush drinks?

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Andrew's background in this field 

6:15  -

  • Andrew is probably best known from this podcast as the co-founder of Stac Performance. 
  • My academic background is mechanical engineering.

    I'm currently working on my PhD which is focused on heat transfer, modelling of through flow and heat transfer. 
  • I believe that this is going to be one of the next frontiers of performance. 

    Heat plays such a significant factors particularly in very hot races, so learning how to manage it will help athletes at any level perform better. 

What is heat transfer?

7:35 -

  • Heat transfer can be broken down into 3 major types: conduction, convection and radiation. 

    They all have different characteristics and some are more relevant than others.
  • For example, radiation is something that can travel through a vacuum so if you're sitting around a campfire, that's how you can feel heat. 

    This happens at the speed of light, and it's dependent on the temperature of the emitting object. The hotter this is, the more heat transfer you'll get. 
  • An example for conduction would be if you pick up a hot cup of coffee, you will feel the conduction of heat into your hands. 

    It's not as relevant for the athletic side of things as there is usually a limited area for heat transfer and surfaces that are usually insulated. 
  • Convection is what dominates. This is where you have a fluid blowing over top of you that essentially removes heat from the surface. 

    The faster you are travelling (e.g. on a bike), the more heat you can sweep away. As soon as it's removed from the surface it is replaced by colder fluids, so you're keeping what is next to the surface of your body cool.

    This is why when you're travelling quickly, hotter environments often don't feel as hot. 
  • N.B. 'Fluid' here doesn't have to be a fluid in the practical sense, air would also be considered a fluid as it behaves similarly to water. 

How much heat can someone generate?

10:58 -

  • The harder you exercise, the more heat you're going to generate. 

    The efficiency of the human body is not bad, but we still generate a lot of waste heat. 
  • You can compare the amount of wattage between mechanical work and thermal work.

    If you're exercising at a rate of 150 watts, a typical person might output 600 watts of waste heat. 

    If you scale this up to a rate of 300 watts in an Ironman, you will output around 1200 watts of waste heat. 
  • A household space heater are often 1200 - 1500 watts, if you imagine how quickly they heat up a room it gives perspective of the heat output of an athlete. 
  • When you're cycling you need to be able to get rid of this heat effectively. 

    If you can't transfer the heat out of your body it has to go somewhere, and it ends up as storage. 

    This leads to your core temperature increasing, which ultimately leads to your body have to pull back and slow down. 

    It's these protective mechanisms in the body we need to avoid as they force us to slow down. 
  • If you were to fully insulate your body so all the waste heat remained, for a 77kg person it would take around 10 minutes exercising at 300 watts for them to overheat. 

    The criteria for overheating is a 2 degrees celsius increase in body temperature. This is where your brain chemistry starts to have problems, and heat stroke is likely. 

Heat transfer practical example

14:59 - 

  • Looking at radiation, if you're exercising on a hot day you will feel hotter.

    If you're wearing a white shirt, you would gain around 200-210 watts. If you're wearing a black shirt, you'll gain around 250 watts. This is a 25% increase in the amount of heat you now need to get rid of. 

    This is why on a hot day, it's good to stay in the shade because you're reducing the amount of heat you need to get rid of, and will lead to a lower core body temperature. 
  • Convection can also play a large role, and a lot of it depends on the temperature different between you and the surroundings. 

    Comparing a temperature difference of 5 vs 10 degrees, you would get twice the heat transfer in 10 degree difference. 
  • Your skin surface temperature will range from 32-35 degrees. If you're exercising in a 30 degree environment, your temperature difference is maximally 5 degrees. However in a 20 degree environment, there would possibly be a 15 degree difference. 

    You're getting three times as much heat transfer in the second one. 
  • Example calculation (considering 10 minute time to exhaustion insulation example)

    In 30 degrees with no wind, you would only lose 40 watts of heat transfer which would increase your time to heat exhaustion by 30 seconds.

    To put this into perspective, it would be similar to exercising in a green house. Most people would agree that exercising in this environment would not be very efficient. 
  • When you increase the amount of air speed - e.g. adding 45km/hour wind, you would increase your time to exhaustion to 15 minutes. You still can't exercise at a steady state, but it shows the significant impact convection can have. 
  • If you go down to a 20 degree environment with no wind, it increases the time to overheating from 10 minutes to 11.5 minutes. 

    However if you then add the 45km/hour wind, your time to exhaustion goes up to around 2 hours. 

    In a practical sense, if you want to do quality training indoors, you need to do it with a fan. 

Impact of evaporation

24:22 - 

  • The other big factor is evaporation, which often makes the difference in places like Kona where there is 70-80% humidity.
  • Relative humidity is how much water the air is holding compared to it's overall capacity.

    If you have a low relative humidity the air is very willing to accept additional water. meaning your sweat will be very effective. 
  • Evaporation is so effective because of phase changes, which store a huge amount of energy. 

    A common phase change is ice melting - if you put ice in a drink it cools the drink down a lot without even changing temperature. It stores so much heat that the body has essentially evolved to use this as a heat release mechanism. 

    To put this into perspective: if you sweat 2 litres/hour, if you evaporate all of it, it would be around 1200 watts of cooling. So just the sweat alone would be enough to keep you cool. 
  • When you get to humid environments, you no longer evaporate the sweat - it often ends up in a puddle under your bike if you're training indoors! 
  • Evaporation is analogous to convection in terms of how heat and mass transfer are calculated the same way - they are almost identical equations. 
  • The faster you are travelling, the more air flow you have, the more you can evaporate.

    If you're sitting at home with a fan on you, you will evaporate a lot of the sweat, but if you don't have the fan it will just run off you. 

    So if you can travel quickly, sweat is very effective. On the bike if you're travelling 35-45km/hour you can evaporate a lot of your sweat. 

    However when you're running, you don't get the same amount of evaporate so a lot of it is essentially wasted. It isn't evaporating effectively and therefore isn't removing heat effectively. 
  • Heat acclimation protocols allow someone to start sweating earlier, which will allow your body to remove heat for a longer period of time. 
  • Sweat rate is generally just your own physiology, I don't think it can be influenced too much. 

    Stephen Cheung is probably the best expert to speak to about this! 
  • There are some people who have a condition that means they produce little to no sweat, and are therefore at a significant risk of overheating because you can't get rid of the thermal energy.

The impact of humidity

31:12 - 

  • Humidity is probably the largest single factor in determining how much evaporation occurs. 
  • The 2 lites/hour evaporation may be possible in Arizona because the air is dry. However, in Kona, the air is essentially saturated in water already.
  • Going back to the 10 minute baseline for over-heating, if you factor in the 2 litres/hour of sweat (Arizona conditions), you could exercise almost indefinitely. 

    If you reduce the amount of evaporated sweat by 50%, that goes down to 30 minutes. 

    If you've got 25% of your sweat being evaporated, which is closer to Kona conditions, you're only going to have 15 minutes time to exhaustion. 
  • This is why so many people have issues in Hawaii, because your bodies end up storing a lot of the heat and your body goes into a safety mode which limits your performance. 

How to keep your core body temperature down

33:21 - 

  • One of the most effective methods is taking in cold fluids. 

    Internally, you transfer your heat energy into this and it cools your body down. 
  • If you throw cold water on the outside of your body it is also pretty effective, but some of the heat is being lost to the outside environment. 
  • Before a race, freeze your transition water bottles - you want to be able to get some liquid out, but have as much cold fluid as you can. 

    I ride a Ventum bike, so I just put my bike water reservoir in the freezer the night before a race!

    At Ironman Mont Tremblent it ended up being 25-30 degrees, so having the ice water really helped. I also had a couple of other water bottles on the bike in case I needed liquid immediately. 
  • A lot of temperature control is within your brain, so if your brain starts to perceive that you are cooler, it will allow you to exercise at a higher level. 
  • There are some tricks people have done before, such as having menthol in the mouth which evaporates quickly. It doesn't actually cool you down but it makes your body think it is cooler.

    Using the same theory, when taking in very cold fluid you get the benefit almost immediately because your brain acknowledges you have access to heat removal. 
  • On the run it is much harder. If you have access to any ice (e.g. it's out on the course), putting it down your trisuit can be really helpful. 

    When I did this, my pace immediately picked up because it temporarily decreased my body temperature. 

    Additionally, having sponges placed in your trisuit can also help. 
  • You can see this with Patrick Laing - he has actually had pockets sewn into his trisuit so he can keep sponges where he likes them. 
  • Whenever you have the opportunity, dump ice or cold water over yourself. 
  • Looking for areas where you tend not to sweat, or all your sweat has evaporated can be a good place for sponges. You can also use sponges to redistribute your own sweat to make sure it's evenly spread across your body. 

    This may help drive up the amount of heat that is evaporating. 
  • These strategies are helpful to practice on a hot day so you're more likely to take advantage of it when training. 

Heat acclimation protocols 


  • Personally, I noticed that after a week in Hawaii I slowly become more accustomed to the heat - it didn't feel as problematic. 
  • I think just spending time in an environment makes your body try to adapt. 
  • One of the challenges is someone living in a Northern country like Canada or Finland who is training in their cold winters, and you want to race somewhere like Australia which will be a dramatic temperature shift. 
  • Using saunas and hot baths can help, as well as training without the fan. 
  • Most of your bang for your buck probably comes from heat acclimation, as there are so many things you don't have control over once the race starts. 

    E.g. your transition bag may be stored in the sun, so any frozen bottles may just be tepid water by the time you get them. 

Fan setup for indoor training

45:49 - 

  • I found a remote control system on Amazon that I plug my fans into, which allows you to switch them on and off from your bike.

    If you're going in to a hard interval you can switch them on, but in a rest where you're not generating much heat you can switch them off so you don't over-cool yourself. 
  • The fans I use are industrial fans, they're around 50cm diameter and around 200-300 watts each. 
  • It depends a lot on what you need individually - the higher output you have the more cooling you will need. 

    It also comes down to preference. You notice the effectiveness of evaporation quickly with these fans - and they may over-cool you to an uncomfortable level. 

Key takeaway

  • Realise how significantly heat build up and your core body temperature will affect your performance. 

    This should be considered in both training and racing environments - if you are racing indoors without a fan, you're probably not getting your best performance! 

Links, resources and contact

Links and resources mentioned

    Connect with Andrew Buckrell

    Connect with host Mikael Eriksson


    Hi! I'm your host Mikael,

    I am a full-time triathlon coach and an ambitious age-group triathlete. My goal is podium at the Finnish national championships within the next few years.

    I first started the website Scientific Triathlon in autumn 2015 as a passion project to share my learnings with a larger triathlon audience. Later on, in early 2017 I started the podcast That Triathlon Show. 

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    Let's discuss this episode and the topic in general. Post any comments or questions in the comments below. I'll be here to reply and take an active part in the conversation, so don't be shy! 

    Mikael Eriksson

    I am a full-time triathlon coach, founder of Scientific Triathlon, and host of the top-rated podcast That Triathlon Show. I am from Finland but live in Lisbon, Portugal.

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  • Thanks Mikael. I really enjoy the shows. And I especially enjoyed the episode from Andrew Buckrell re Heat, Humidity etc.
    Sometimes when I listen, I do not always understand the finer points being made by your guests and sometimes/quite often whilst I think the shows are interesting, I don’t always agree with them or I don’t think they are right for me. But they are all good learning.

    I am a 60 year old self-coached (+TrainerRoad) triathlete based in Hong Kong … I only started triathlon training 2 years ago. and completed my first 70.3 in Whistler, Canada last July (just over 6 hours).

    One of the “issues” that I have had with some past sessions is that I sense they are mostly applicable to Northern hemisphere/cold climate athletes. For example, I find I can hardly ever undertake zone 2 (by heart rate) training, which is often recommended.

    Currently in Hong Kong the temperature is lovely at low/mid 20s with humidity at 65-85%. But for much of the year it is 30c+ with humidity 85-95%. As soon as I start training, my heart rate increases to above zone 2. I have figured that this is in large part temperature and humidity related. The episode with Andrew Buckrell supports my view. But even this episode is mostly focused on giving tips to cold climate athletes on how to acclimatise to hot/humid weather.
    I think it would be really useful to do an episode on how to train regularly in hot/humid conditions – should training programmes be adapted and if so, how? Eg, at the moment, I just ignore the zone 2 and accept that my training will be at least zone 3. But is that sensible – what else can I do?

    On the upside, last July, the Whistler 70.3 I competed in was held in “hot” conditions (for most triathletes). But for me it was perfect – it did not seem hot as it was not humid. That is the benefit of my living/training in a hot/humid climate. But I do wonder if my training is less effective than it would be if I undertook the same training in a cooler climate. This seems to be what some of your guests have implied, but I have not heard a specific episode on this topic or suggestions as to how to mitigate this (if there are any).

    Thanks again for your diligent, thorough, informative and enjoyable shows.

    • Hi Lawrence,

      If you established your heart rate zones oudoors in similar weather conditions to what you’re normally training in, you shouldn’t have this problem of notbeing able to stay in Z2. Perhaps if you did it in a cool lab I understand why you have this issue, but then I would recommend that you instead do a field test outdoors to set your zones with the heat and humidity accounted for.

      For the question of whether your training is less effective in the hot/humid environment: I would say yes, if you always train outdoors it is. So I would recommend doing some (most, actually) of your hard training sessions indoors with great cooling and good fan(s). You seem to be doing that already on the bike, but consider also starting to do at least one hard run session per week on the treadmill, in a good cool gym with a fan.

      You shouldn’t use heart rate to guide these harder sessions anyway, but power/pace/RPE, so it’s not a problem that your HR zones are set for outdoor conditions.

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