How to acclimatise

The science and practise of adapting to cold water

Liz Seabrook

 ‘Acclimatisation’ is the process whereby the body is able to deal with longer periods of cold water without dangerous levels of suffering. Channel and ice mile swimmers can all survive in temperatures that would kill the un-initiated, while 10k swimmers and skin swimmers need to master cold as well as fitness when training to swim a distance. How is it done?

‘We all start out with different baselines of cold sensitivity,’ says OSS Founder Kate Rew. ‘Baselines are a mix of physiology, psychology and experience. On a physical level overall body mass is important: large objects take longer to cool than small ones, so a big person will take longer to cool than a small one. The ratio of surface area to volume is also important – a tall slender person will cool down faster than a shorter stockier one.

‘Body fat is relevant too – fat is an insulator, and when you get in cold water blood withdraws from the surface of skin, turning the fat layer into ‘bioprene’, which helps insulate your core.’

Life so far also makes a difference – anyone working outdoors, or with other cold-related hobbies or habits (mountaineering) is likely to have built up greater resilience to cold than someone used to warm environments.

But wherever you begin on the spectrum, how do you increase your tolerance to cold water?

‘The secret to acclimatising to cold water is just to swim in it, often – at least once a week, and preferably two or three, gradually extending the time that you stay in the water,’ says Dr Heather Massey a swimmer and a researcher at the Extreme Environments Laboratory at the University of Portsmouth. ‘Get out if you are not comfortable, and don’t set time goals for staying in the water.’

‘It is necessary for your core to cool in order to start acclimatising, but it is not necessary to push acclimatisation to the point of hypothermia,’ says Rew. ‘Regular exposure will lead to ‘hardening’, and increasing ability to judge and manage your own reaction to cold. Lynne Cox (who has swum English and Catalina Channels as well as the Bering Straits) recommends making the cold a way of life: wearing lighter clothing, having colder showers, training in cooler pools, sleeping with the windows open. Some swimmers at the Channel swimming, marathon challenge end of the spectrum go as far as sleeping without blankets or duvets, even in winter. This spartan approach is not for everyone but it is likely that all exposure to warmth (warm pools, warm houses, warm baths, warm jumpers) will counteract any cold acclimatisation work being done in cold water. Spring swimmers who want to get fit for outdoor summer events have a balancing act between using warm pools to achieve longer training times, and not being made soft by them.

The secret to acclimatising to cold water is just to swim in it, often – at least once a week, and preferably two or three, gradually extending the time that you stay in the water

Massey’s research indicates that acclimatised swimmers experience a more rapid reduction in skin circulation than others, moving blood supply to the deeper tissues of the body and retaining heat. This has the side effect that acclimated swimmers may take longer to rewarm hands and feet compared to friends who have not been cold exposed, as the blood supply is as slow to hands and feet.

5 tips on acclimatisation:

  1. ‘If you want to swim through winter it is easier to start your swimming career when the water is 16 degrees or above, and then keep on swimming as the temperature drops, than to begin when temperatures are low,’ says Rew.
  2. To wetsuit or not to wetsuit? That depends very much on the temperature. While wetsuits are very useful to extend time in the water during summer, autumn and spring, they arguably become less useful in winter when air and water temperature are very low. At this point, you may get colder changing in and out of your wetsuit than swimming in a costume and changing fast.
  3. Choose your mindset. ‘Whether you find cold water horrible or energising, whether you expect a winter swim to leave you buzzing or miserable, is a large determining factor in what happens when you enter the water,’ says Rew. ‘Some people really love swimming in wind, ice, and stinging rain and get high from it, some enjoy gaining mastery over the pain, and still more find the whole thing tiring and horrid. But one thing the Wim Hoff revolution has shown us is that people can train themselves to like it, if they believe.
  4. Choose your neoprene: going in to very cold water (typically less than 10oC) can cause numbness and pain, particularly in the extremities, such as the hands and feet. Neoprene socks and gloves can help protect your hands and feet, making them popular winter accessories, alongside long arm and neoprene swimming vests and costumes.
  5. Rehydrate afterwards! Ever noticed the urge to pee after (or during) a cold swim? In cold water your body retracts blood to your core, which it achieves by filtering fluid out of our blood into. Be conscious of this and rehydrate after a swim.

Further Reading:

  • For more on cold water and your physiological reaction to it, see Understanding Hypothermia and other features in our section on Cold.
  • Heather Massey’s research at the University of Portsmouth on cold water immersion and safety can be followed here.

Kate Rew is founder of The Outdoor Swimming Society and the author of The Outdoor Swimmers’ Handbook, with extensive chapters on Understanding Cold, Winter & Ice Swimming.

Kate Rew & Heather Massey