This December is set to become one of the mildest on record with temperatures reaching an unseasonable 16C across parts of the country over the weekend of December 19-20. It's weather you would normally only see in spring, perhaps explaining why social media is awash with photographs of daffodils blooming early.

As scientists point to climate change and the effect of El Nino to explain why the UK has had such unusual weather, one group of Londoners has probably noticed the difference more than any other.

Saturday (19 December) saw the reopening of the men's outdoor swimming pond on Hampstead Heath in north London, after a period of maintenance work. One of three swimming ponds on the Heath, they are the only lifeguarded open-water spots in the UK to allow swimming all year round.

It means winter swimming fanatics come in the droves to get the adrenaline kick that comes with jumping in icy water. Except so far this year, the ice hasn't come. Lifeguards told IBTimesUK the water temperatures had stayed at "unheard of" levels this December, hovering around 10C when they should be in the low single digits.

Footage captured in December 2010 shows the stark contrast to what conditions were like in previous winters. Swimmers were arriving to water surrounded by snow and covered in a thick sheet of ice, with temperatures so low many were able to withstand the water for just minutes.

Hampstead Heath pond
The difference between the 1C water swimmers at Hampstead Heath's men's pond faced on 19 December 2010 (left) and the 'balmy' 9C water they faced on 21 December 2015 (right)YouTube Gemeadow/IBTimesUK

While some swimmers this year were relishing being able to stay in the water longer, others complained the mild weather had robbed them of the adrenaline fix they had come for...

For anyone thinking of taking a dip, the City of London Corporation, which manages the Heath, has issued advice on the dangers of winter swimming.