Cold Water Acclimatization

This post was a companion to HABITUATION, both of which I wrote in early 2010. Since I revisited and largely rewrote that as Cold Water Habituation, my plan was to do the same in this post also.

Acclimatization (acclimatisation for those of us who forego the use of the z)  is a different factor to habituation.

While habituation is simply the process of adapting to getting into cold water, acclimatization is about a person’s ability to stay in cold water for longer.

(Acclimation is the same process but done in controlled or lab conditions).

In brief, as every open water swimmer knows, the more you train in cold water, the better you will be able to tolerate the cold, and the longer you will be able to swim in the water.

Acclimatization is a more difficult and often almost mysterious process than habituation. It takes longer to develop and longer to lose. It tests one far more, requiring a greater willingness to push ourselves.

I’ve luckily gotten to know a lot of cold water swimmers, originally through the Sandycove swimmers group, many of whom say you can think your way through cold, at least up, to a certain point. I know one swimmer and psychologist who helps people in this area, and stress overcoming the fear, that the swimmer should tell themselves that they are warm when they feel the cold, or to focus on different subjects, or to imagine they swimming in warm water, etc. These are classic sports visualization methods that are used to transcend different problems.

Guillamenes platform during winter storm, long exposure

The Guillamenes platform during a winter storm, (long exposure)

I have certainly found for myself that even getting into  6 or 7 º C., after the first minutes of pain, that I now have a definite whole-body feeling of warmth, (excepting feet and hands).

However, there is the problem that physics and the laws of thermodynamics are absolute. A favourite quotation of mine is  “eventually the dead hand of the Second Law will hold sway over everything”. (Yes, I have a melancholy bent). However, this alludes to the fact that entropy increases and heat is lost in everything in the universe. As open water swimmers we are affected by such facts as:

  • One loses heat in water at 30 times the rate in air (thermal conductivity).
  • Heat loss is slower on sunny calm days than overcast windy days which strip body heat away even more quickly.
  • You lose 10% of your heat through your head, (in proportion with the rest of your body).
  • The ratio of heat loss is proportional to the volume and surface area, so larger people lose heat more slowly as the ratio of volume to surface area is increased.
  • Fat is an insulator and slows heat loss.
  • Insufficient food and fluids, alcohol intake, illness or not enough sleep all make one feel colder.
  • Pockets of changing water temperatures have a significant effect.
  • The Second Law of Thermodynamics: In a closed system, entropy increases. In the case of swimming, the closed system is the body, the air and the water. heat will flow from the warm body to the cooler water. You lose heat unless you input sufficient heat energy.
  • No-one is immune to heat loss or hypothermia.

Put all that together and all you get is what you already know. You get colder quicker in water, but the rate of change is dependent on a range of factors.

One factor I didn’t put in there is the mental aspect, because it’s difficult to see how thought (Werner Heisenberg & Quantum Mechanics aside 🙂 ) can have any effect on the rate of change of the system, i.e. how can thought slow your cooling rate? Many experienced swimmers will say you can think your way into extending your time in the water. I’d never been able to say this. I do believe that you can stay calmer, and accept what’s happening, which makes it feel easier.

I think that you get more used to being in cold, and you recognise your early hypothermia indicators better so you can push your limits more. You learn to swim further into your own cold experience. You get better at preparing and recovering. Some of those very experienced swimmers I know have learned to accept and box off the cold, realise it’s there, know the efficiency is decreasing but at the same time know there can be a long gap between the early hypothermia indicators and remaining period during which much swimming can still be done.

The simple positive feedback process of improving cold ability

The simple positive feedback process of improving cold ability

There is also the case that with improving  habituation, that heart rate and stress hormones decrease, and therefore the person feels better about getting into cold water and less nervous. Less heat will be lost in the initial minutes, which also leads to greater capability. This is the positive adaptive feedback system that I wrote about a couple of weeks ago.

The small improvements drive confidence, the confidence allows the swimmer to push themselves while staying more relaxed. The mental aspect of cold water swimming was the single thing I most struggled to understand in my first few years of winter swimming. It seemed too trite, too easy, without really saying anything useful. It is easy to say that mental attitude allows one to swim longer but it has taken me years of winter cold water swimming to really realise this, to integrate it and to try to convey it. To understand what it means and to comprehend the effect that thought has on my own cold acclimatisation and ability, and not least to be able to explain that better for myself and hopefully others.

It has not been a short journey. If I could do it, so can you.


26 thoughts on “Cold Water Acclimatization

  1. Hi Donal

    I enjoyed this post. I’ve been swimming in a river in Cavan the last two winters with a group of others.We generally do 1k but with wetsuits, boots, gloves, a couple of hats. That said, the water can get down to 4 degrees and on one occasion earlier this year we were pushing ice out of our way.
    Since May I’ve been in the lake without a suit for anything from 750m to 2k. I still have the suit for longer swims. Now it’s getting cooler but I’m in every day, morning or evening, for 15-20 minutes. It’s not about distance as I’m on my own and I’m it’s fairly new to me. I just want to swim around in a wide arc close to the shore. I really want to keep it up. The water is still around 14 degrees but I’m wondering how I’ll fare when it drops further and further. It’s very subjective but what length of time can i expect to stay in for 1, 2, 3, 4 months from now? And then if I want to start to doing some distance, any distance?

    Getting out is my biggest problem. I just need to organise myself better and stop trying to put 2 or 3 layers on at the same time and having to stop and start all over again….




    • Hi Barry,
      Start getting your post swim routine right now. It should be second nature. Too many people spend time talking to the people who want to know “if it’s cold”.
      The lake will get cold faster than the sea. What I ca’t tell you is how long you will endure. Your body weight, height, shape, wind etc all play a factor, along with wind, air temp, and simple desire or drive to stay in. Daily swimming will make you much better able at handling cold.  But it also varies by day. It’s a bit like medical advice on the radio. Anything I could tell you would be a ridiculous guess, that might cause you to some thing you shouldn’t. For me, I am not in favour of targets in cold, but to take it day by day. There really shouldn’t be any competition in the stuff, it’s too dangerous.
      Enjoy the winter! Donal


    • I have been swimming every day for the last three years no wetsuit. I have missed about 7 days over the 3 yrs due to illness. It was hard not to swim on those few days. I recommend you use a fish tank thermometer (3 quid on ebay) and have a water resistant watch. I have a Casio 200m watch with a 7yr battery life. Thermometer and a watch are the most important pieces of kit for winter swimming. At the lower temperatures I used to multiply the temperature of the water in degrees by a factor (that I was comfortable with) and that would give me my maximum time in minutes. For my first winter it was 1, second winter the factor was 4. But I agree with Donal its better not to have targets. I emphasize the equation was for maximum time and most days I would just get in and swim 10 minutes and get out. This morning I managed about 13 minutes at 7.5 degrees…the river was surprisingly cold for this time of year…


  2. Hi Donal, Thank you so much for all this information. I have been busily reading through all your posts since I found your site. I live in The northeast of the US, and I swim in a nearby river daily. I normally switch to the pool sometime in October, but I am excited about the possibility of extending my outdoor swim season. I wasn’t aware this was possible. So hear is my question – the water here isn’t even that cold yet, but after swimming for 15 minutes yesterday, I had mild to moderate hypothermia. I was shaking to the point I couldnt drink the hot tea out of my thermos without spilling half of it. I felt fine in the water, after the initial pain wore off, and it wasn’t until I got dressed that the after drop hit me. I had experienced this before, but didn’t know the term or the science behind it until I read about it here. Generally I take these symptoms as a sign that the water has gotten too cold, but it sounds like you guys just train through it.
    So my question is, in the process of training for acclimatization, is it necessary or safe to experience an after drop like that, or should I try to get out early enough to avoid it?
    Thanks again,


    • Thanks Dan.
      The safety question here is the most important one and that divides into safety/risk to your health while swimming, and after swimming.
      I advocate a very gradual increase in time of cold exposure, (remembering that you will not improve every day) always erring on the side of caution as you build up the ability to evaluate your own responses so that you have a margin for error and the ability to respond to some unanticipated requirement like an unusual current or changing wind.

      There are risks and possible side effects of cold water swimming that you have to evaluate for yourself but I do not think that simply getting hypo is in itself dangerous. (Unless the water is under 5C when risks are greater and more extensive).

      The after swimming dangers are more likely to be with impaired cognition afterwards and doing something dangerous while driving etc.


  3. Hello! I am on my second winter of swimming daily. Your website has been very helpful to clarify almost all of my questions, but I was wondering about acclimatization. It is true what you say that it seems a mysterious process. Last year I was staying in the water for whatever the temperature was (C) in minutes (hardly nothing, but I was on my own). This year I find I can manage triple the degrees in minutes (in good company). I was just wondering how many winters it takes to be able to do an ice mile (considering it takes me 40 minutes to swim a mile). Am I right in thinking that acclimatizing takes years? My swimming buddy seems to think you can acclimatize to do an ice mile in one winter and lose your tolerance by the summer. Any thoughts? Diolch yn fawr (many thanks)


    • Hi Meilyr, I am a strong critic of Ice Mile swimming, so I’m not in favour of encouraging it. I would not like to give an answer to this question, because one of the many reasons I am critical is that people with little or no experience like your friend don’t take full measure of how dangerous the stupid pursuit is. Otherwise I hope you will find the cold water swimming resources here of use.


  4. I had never heard of exostosis before. I’m glad to have found out about it. I almost never cover my ears in winter, as it rarely gets uncomfortably cold here in Toronto. But I also went for a bike ride in a t-shirt in the blustery 0° weather today, and I gather I’m a little different in finding that comfortable.

    I am not a habitual cold water swimmer, but it has an appeal to me. I went for a swim at Reykjavik mid-June this year, and the 8° water was a bit of a shock when first going under, but also exhilarating. I also walked into the Arctic at the one sandy beach on Magerøya (71.010249, 25.894861) a week or so later, but I didn’t have it in me to go all in. I was already cold before walking in, and I had only the heater in the car to warm me. It was cloudy and 3°, and they say the ocean averages 6° that time of year. I had one bought of hypothermia in the winter a few years back (from sweating through my clothes at -15°, ironically enough), so I know to not push my luck too far. It’s been on my bucket list to swim in the Arctic, and I’m kicking myself for not doing it, so I have to go back one day.

    Your Reddit post from today brought me here. I’m really enjoying your site so far. It’s inspiring.


    • Thanks Mark, glad you are enjoying it and finding it useful. I know one long-term swimmer & organiser who doesn’t bother wearing earplugs, thinks it’s an overreaction and yet freely admits his hearing is now compromised. And he lives and swims in southern California, not the wild shore of Canada or Ireland.

      I’m well south of the Arctic , 52N, but I think the wind simulates another (fake) few degrees. I’d guess sea average, (which is interestingly a thought I never had, and must work out from my notes) is somewhere around 10C. 6C is about the coldest on a normal winter, worst is about 4.5C.

      There is a huge benefit to cold water swimming. That shock of cold, the rush of warn blood returning afterwards.. these are sensations that unlike many others, don’t attenuate with time and experience, and always remains vivid, and thus we often describe an addiction.


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  6. Hi Donal,

    Apologies if it’s something you’ve covered elsewhere on the website, but I have a question about cold water tolerance and circadian rhythm. Basically, do you find it noticeably easier/harder to deal with cold water exposure at different times of day? I might be imagining it, but I’m sure I find it significantly easier to stay in the water in the late morning and afternoon, rather than my usual morning swims (~08:00). This would seem to fit with the idea that the body’s core temperature fluctuates over a 24hr period and will be colder soon after waking up.

    Was curious if you had noticed anything similar?




  7. Cold water is mainly mentally ( in the range of non-harmful condition ). In my region 24° Celsius is consider cold water. The minimum temperature of the water is in February and it is about 17°. Very few enter the water in the winter. 99% of the swimmer here stops entering the water on September. (The water is about 22°)
    In our region the air temperature can be 10° in the morning on January February, in this condition it is much difficult to swim because of the time you spent between you go outside the water and go back to your car.
    We small group swim in the winter every Saturday and are consider crazy!


  8. I am a regular cold water swimmer down the road from you. i do most of my swimming on the hook peninsula.. regarding ear plugs i had to have an operation for ostheoma of the ear which basically means having your ear drilled out as the bone of the ear develops growths. my right ear was 90% closed. i was also told i would have to have my left ear done within 2 years. that was twelve years ago and i still havent had it done but swim now more than i ever did. i used silicon ear plugs initially than paid 50 euro for custom made ones but lost two sets of these coming in on waves. Now i use blu tack for my ears. Cheap give a great seal and easily got. some local surfers laughed at me initially now i see a couple wearing them .but anyone reading WEAR EARPLUGS of some sort. Believe me the pain of the operation is very severe.


    • Thanks Paul, I of course agree completely. I’ve been tempted more than once by custom plugs, but the current cost of almost €100 per pair is too risky when a rough day could see them lost. Silicon plugs do me quite well, I go through a couple of pairs a year. Whereabouts on the Hook do you swim?


  9. Forgive me if I’ve missed it, but do you have, or will you have, an article on post cold water swim recovery techniques? I find I’m still very low body temp up to six hours later. I think I’m doing something wrong.


    • Hi Maureen, Six hours seems very long. Two or three hours for me at the coldest usually. I can’t remember if I did a post specifically on rewarming, but I don’t think so, partly because I don’t access to a post swim sauna to be able to include that in any writeup. But I have included rewarming in many articles, and always the same advice; get dressed quickly, use multiple layers, some walking, no showers or warm water.


  10. Hi Donal. I have a few questions.

    I read somewhere that it is not recommended to go straight to the warm shower after swimming in cold water. When I have done this, it does feel like the hot water does not do much. I assume that this is because the blood is still mostly concentrated in the core and has yet to flow to the body surface. However, the other day I went to the sauna right after swimming, and that felt great. my first question is: Is the sauna OK after cold water swimming?

    Second question…On my drive before swimming, I normally turn the heat on to the max in order to get my core as warm as possible before the swim. I feel like the more heat I retain in my core, the longer it will take for that heat to be lost (the longer it will take for me to get cold). Sort of like a thermostat. Does this make any sense?

    Finally, ear plugs. I’ve never used them. I read in your blog that I should. And other swimmers have told me the same. Is the reason related to the coldness of the water or is it advisable under any water temperature??



    • Hey Mauricio,

      It’s funny you should comment on this very post, I’d just been thinking about repost this and the one on habituation for this week.

      1) I have no access to a post swim sauna, I think I’ve only ever had one once after a cold swim. However, many experienced cold water swimmers do it, notably Dolphin and SERC in SF and SLSC in the Tooting Bec lido. Sauna warms the peripheral blood much quicker than a shower, so yes, it should be fine.

      2) That’s absolutely right. Generally you want to be as warm as possible before getting in the water. Inexperienced swimmers think you might be better off being cold, because the shock would theoretically be lower, but all that does is accelerate heat loss. I put on my coat and woolly hat for the 100 metre walk from the car to the water’s edge where I change.

      3) Ear plugs are to prevent long term ear and hearing damage. Exposure to cold water or cold wind causes the bone at the exterior of the ear to grow to protect the inner ear, until over a long period the only remedy is surgery. The condition is called exostosis and occurs mostly in surfers, farmers and swimmers who don’t protect their ears. The cold required to initiate exostosis isn’t as low as we experience when swimming. I wear mine year round. The water would need to be above 18 to 20C before I’d consider not using them. I was discussing this a couple of months ago with a well-known California swimmer who’d written his cold water advice and recommended not wearing plugs as it helped swimmers adapt better to cold. I said that I felt it was incorrect advice given the possibility. He still disagreed, but happened to mention that himself had noticed a deterioration in his hearing! I would love custom ear plugs for myself and if cost is not an issue, I’d recommend them, they work very well, with better hearing and less likely to get lost but the silicon plugs have worked very well for me for the past years of surfing and swimming and are cheap to replace.


      • Great, thx a lot. I knew I was forgetting another question that I’ve been meaning to ask you. Compare a 4km workout in 14C water and in 28C water. Is it more of a workout in 14C, do you burn more calories? Intuitively, it seems like the body would need to “work harder” to keep warm. OK, this really is my last question for a while. I promise!


        • Hey, always good questions. Undoubtedly. Cold water requires more calories. The week I go from mostly pool to mostly sea swimming, I call transition week. And it’s very tough. I get more tired, more hungry and by the end of that week can be rightly exhausted. It’s like doing 10k a day in the pool.


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