Cold Water: A Temperature Guide

Swimmers experience cold water in bands from baltic to fresh. Kate Rew explains

Cold water temperature varies hugely, and while this isn’t scientific, anecdotally, open water swimmers experience it in these bands.

Compared to indoor heated swimming pools, which vary in temperature from around 26 to 31 degrees Celsius, swimmers are likely to be exposed to a much wider range of temperatures when swimming outside. In the UK, inland waters can be as low as zero in winter to as high as the mid 20s in peak summer. Coastal waters vary from low single digits to the high teens. The water temperature has a massive impact on how you swim and how long you can safely stay in the water.

0-6 DEGREES: Baltic

Jumping in is likely to impair breathing in the uninitiated, as breath comes in big jolting gasps and it feels like someone has clamped on an ice neck brace. Water has bite, skin smarts and burns. This is winter swimming. Limbs soon become weak – 25 metres can be an achievement – and only takes a minute or two at the lower end of temperatures before skin becomes a lurid purple-orange-red (for those with lighter skin) when you exit.

That said, the joy of swimming without a wetsuit at this end of the temperature spectrum is the cold water high: the pure exhilaration and rush of endorphins that you get from getting in. Winter swimmers frequently become addicted to it, and it is sufficiently powerful that a 1-2 minute swim can leave you feeling good all day. Anecdotally winter swimming clubs like Serpentine Swimming Club and Tooting Bec Swimming Clubs report increased immunity and fewer colds.

6-11 DEGREES: Freezing

Much like baltic, but not quite so painful, or breathtaking.

12-16 DEGREES: Fresh

At this temperature triathlons start operating. In a wetsuit you may find you can swim comfortably for a while, outside of one the water is fresh, doable for the brave, and not a problem for hardened open water lovers.

17-20 DEGREES: Summer swimming

Lakes and more mature rivers reach this temperature over summer, during hot spells. Still fresh on entry, but comfortable picnic lazy-hazy summer swimming.


You’d think that’d be a good thing, but on the rare occasions that river pools and shallow lakes reach these temperatures during hot spells, there is the odd sense that there’s something missing…. the exhilarated feeling when you get out, that cold water ‘tang’. On the plus side, some of you will be able to spend hours swimming without a wetsuit.

30 DEGREES: Pool temperature

Arguably unpleasant. Plus, as Rob Fryer comments, ‘the sun is not invited’.

More on Risk

When embracing cold water please be aware of the following – wearing a wetsuit keeps you warmer for longer, but will not protect you from these risks:

  • Cold Water Shock. Sudden immersion in cold water causes, among other things, a sharp intake of breath, an increase in breathing rate and an increase in blood pressure. It can also cause panic attacks, and asthma attacks – see Risks of Cold Water and Will I Get Too Cold?
  • Swim Failure. As blood rushes to vital organs in the core, blood flow to the limbs is reduced – which, if it reaches extreme levels, can weaken the arms and legs to a point you can’t swim. Staying close to shore assists safety. If you feel arms and legs weaken, get to the shore the get out. See ‘Will I Get Too Cold?
  • Afterdrop. You will typically be at your coldest 10 minutes after you exit the water, as blood begins to return to your skin and cools. See ‘The Subtle Art of Warming Up‘ for tips on avoiding afterdrop, such as putting on warm layers and having a warm drink.
  • Hypothermia. Hypothermia is a drop in core body temperature and can be dangerous, leading to violent shivering, loss of consciousness and heart failure. Learn how to acclimatise, and if your stroke rate slows down or you start to shiver, get out and warm up.
  • Read more in ‘How to Acclimatise To Cold Water.



wild swim and the outdoor swimmers handbook by kate rew

Kate Rew is the author of The Outdoor Swimmers’ Handbook (Rider, 2022, available internationally and signed in The OSS shop) and Wild Swim (Faber). Instagram: @kate_rew.

Kate Rew