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Londoners are bracing themselves for sweltering Tube journeys today with the mercury forecast to hit record breaking temperatures.

The London Underground is notorious for being incredibly hot, as anyone who's travelled on the baking Central line in summer can testify to.

Temperatures over 40C have been reported on Underground in the past, which makes for pretty uncomfortable journeys when you're stuck on a busy train on the way to work.

But just what is the hottest Underground line? According to data released by TfL last year the Bakerloo line - one of the oldest on the network - saw temperatures reach as high as 31.04C in 2017 while the Central Line was the second hottest with highs of 30.47C.

The figures included the average hottest recorded temperatures for the Bakerloo, Central, Jubilee, Northern, Piccadilly, Victoria and Waterloo and City lines in August 2017.

While this may be good news for those looking to spend their afternoon out in the sunshine, travelling to and from sunbathing spots - or work - won't be quite as pleasant.



The coolest line without air conditioning is Waterloo and City with the hottest temperature recorded at 25.95C.

London's hottest lines Bakerloo 31.04C Central 30.47C Jubilee 26.57C Northern 27.98C Picadilly 27.3C Victoria 28.03C Waterloo and City 25.95C

Although it feels hotter when there is more people on your carriage, it actually contributes very little to how toasty a journey can be, according to experts.

All of the heat is said to come from the actual movement of trains themselves.

Most of this heat is said to come from the trains slowing down and the conversion of movement turning into heat.

Fortunately, for commuters there is the welcome cooling down from wind passing through the tunnels as the Tube approaches.

It is estimated that pushing air ahead onto platforms and sucking air behind removes just over ten per cent of heat from the tunnels.

That is why it may feel hotter on the older tunnels, as those were not built with ventilation in mind because it was not thought to be as necessary as it is now.

The Victoria line, by contrast, is cooler as by the time it came along, engineers were aware of how sticky it can get.