The temperature gauge by the side of the pool says 1C but we are assured it's wrong – the water is actually 0.1C. The snow on the ground is burning my bare feet as more than 100 of us prepare to take the plunge. As we're given the whistle, I lower myself slowly into the water.
My entire body is instantly numb. I'm in shock, as I gasp for air. After five or six strokes, my hands have frozen into two gnarled claws. My only thought is of reaching the other side. As we clamber out quickly, we're as pink as beetroots. And suddenly I'm incredibly warm, my skin tingling as if with static. Giddy and grinning, we get dressed quickly, stamping our feet and sipping rum.
I was taking part in the outdoor swimming society's annual fundraising dip at the Parliament Hill lido. I caught the outdoor bug swimming all summer in the unheated lido on London's Hampstead Heath, which dipped to 13C in early October before I called it a day. But what am I doing back here, in the grip of a big freeze? Looking for adventure, for good health – and vanity.
The anecdotal benefits of regular cold-water swimming are well known: shinier hair, fewer colds, glowing skin, more energy, a tighter bottom. It boosts your immune system, they say, and releases adrenaline and endorphins, for a natural buzz. It improves circulation and flushes out impurities like a detox, resulting in a great complexion. It burns more calories than warm-water swimming, and is said to boost testosterone and oestrogen, enhancing libido.
Can it really be this good for you? Not necessarily, says GP and Guardian columnist, Tom Smith. "Whether a sudden rush of adrenaline is good for you depends on how healthy you are. If you have a heart condition, it can kill you. And the danger is, you might not know you have one. But it's a small risk." However, he warns against staying in the water too long. "After three or four minutes, the nerves in your skin stop working, which means you won't realise how cold you are as your core temperature drops – below 35C or 34C, you're hypothermic. Warm up properly afterwards. Putting your clothes on is no use – you have to apply heat. A hot shower will do it."
And the benefits? "To boost your immune system, you'd have to increase white blood cells, and I don't know of any proof this happens. And there's no relationship between healthiness and colds, as they're viruses. But people who have regular cold dips do seem happy. And if you exercise regularly, you're unlikely to be obese – so more likely to have a higher sex drive."
Before my plunge, I call swim coach and cold-water fan, Alex Davis. "If you want to swim, wear a wetsuit and at least two swimming hats," she says. "If it's the thrill of cold water you're after, I wouldn't advise jumping in as it can stop your heart. Wade in and get your hands wet first – it helps you acclimatise – until you're ready to go." Davis usually swims for around 15 minutes, with no wetsuit. The last time, the water was 7C. "I wait until I start to shiver, then get out," she says. "Or you could do the hypothermia test: touch your thumb and little finger together on one hand. If you can't do it, get out fast." Why does she swim in these conditions? "It's an addiction, exhilarating. You feel fresh and tingly all day."
At the Heath, Bernadette Saglio has just taken a plunge in its 4C waters. She swims several times a week and has bright eyes, clear skin and an easy laugh. "Afterwards, you feel full of energy, relaxed but buzzing at the same time," she says. "While you're swimming, it's a burning sensation, but you can't tell if you're feeling hot or cold. You turn a good colour afterwards. Swimming in cold water focuses the mind so utterly that it becomes a mindful, meditative experience. It completely clears your head. And I never catch a cold."
Don't let the near-freezing temperature put you off, says Gill Russell, standing nearby in her swimsuit. "Freezing water is easier than, say, 7C, because your mind doesn't try to convince your body the water's OK. When it's this cold, your brain switches off."
If you're thinking of taking the plunge, go with friends – it's more fun and you can egg each other on. Better still, there are festive group swims nationwide,, particularly on Boxing Day and New Year's Day. And me? I'm off again this weekend.
For more information and a swimming map see outdoorswimmingsociety.com/index.php?p=swimming_map.