How Cold Water Feels

An introduction to temperature

© Vivienne Rickman Poole

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.


For the inexperienced, the biggest danger from sudden immersion in water that’s significantly cooler than you’re used to is cold water shock. This is the body’s initial and automatic response to rapid change in skin temperature. It causes, among other things, a sharp intake of breath, an increase in breathing rate and an increase in blood pressure. It typically lasts up to a couple of minutes. For the unwary, cold water shock can be deadly, especially if that sharp intake of breath occurs under water. In addition, if you have an underlying heart condition or hypertension then the sudden change in blood pressure may cause complications. Therefore, enter the water slowly and keep your face clear until your breathing is under control. The cold water response decreases with swimming experience and being mentally prepared.


The second problem with cold water is that it can result in swim failure. To protect vital organs in the core, the body restricts blood flow to the limbs when in cold water. If this reaches extreme levels the arms and legs no longer function properly and you can’t swim. If you feel yourself slowing down or struggling to swim, get out.


The next risk is hypothermia. This occurs when you suffer a drop in core body temperature and can eventually lead to loss of consciousness and heart failure. The amount of time you can swim in cold water without suffering from hypothermia is determined by the temperature, your body size and shape and your experience, among other factors. Start with short swims to learn what your limits are. Always swim with other people. If your stroke rate slows down or you start to shiver, get out and warm up.


When you finish swimming, you also need to concern yourself with something called ‘after drop’. This happens when you exit the water and cool blood from extremities starts circulating through your body again, lowering your core temperature, which is why you often start to shiver a few minutes after you finish swimming. To minimise the risk, dress immediately starting with the top half of your body. Put on a hat and gloves and have a warm (non-alcoholic) drink.

A wetsuit will not prevent cold water shock nor stop you from suffering hypothermia. However, it will help you to stay warmer for much longer, keep you afloat and, in most cases, allow you to swim faster.

Also bear in mind that the water near the surface, especially on hot sunny days, can be much warmer than the water below. See Thermal Layering.


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

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 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’.

Kate Rew & Jonathan Cowie