As temperatures in London soar to 36°C, millions of commuters will be dreading their journey home.
But just how hot is your commute?
As the mercury crept up earlier today, the Daily Mail’s Senior Science & Technology Reporter, Wiliam Hunter, set out along some of the most popular London Underground lines and bus routes, armed with a thermometer.
And the results might make you wish you had worked from home – particularly if your journey includes a trip on the Victoria Line.
This line – which includes popular stops such as Brixton, Oxford Circus, and Victoria – topped the chart in our tests, clocking in at 35.2°C – around 4°C hotter than it was above ground.
For comparison, government legislation rules that you can’t legally transport cattle when temperatures climb above 30°C – a limit exceeded by almost every line in our tests.
And it isn’t just the heat you need to worry about.
We also found that the humidity inside a busy carriage ranged between a swampy 60 per cent and a borderline tropical 80 per cent.
As the mercury crept up earlier today, the Daily Mail’s Senior Science & Technology Reporter, Wiliam Hunter, set out along some of the most popular London Underground lines and bus routes, armed with a thermometer
The fact that the Victoria Line came out on top as the hottest line on the London Underground might surprise some commuters, but it does match Transport for London’s (TFL) expectations.
The Victoria Line has the dubious distinction of being TFL’s ‘official’ hottest line, according to temperature measurements taken from stations.
TFL puts this record at 31.1°C in August 2024, but our measurement from on the tube itself was far hotter.
While the sweltering heat on the Victoria Line stood out as the most brutal, many other lines weren’t far behind.
Much to our surprise, we found that the Piccadilly Line came in a close second at a gruelling 34.4°.
That is a fact that will certainly irk some frequent commuters, since this line is still waiting for new air–conditioned trains.
This vital rail link, which accounts for 10 per cent of all journeys on the Tube each year, was supposed to receive a major upgrade as part of a £3.4 billion investment.
However, it now appears that commuters will still have to sweat out this summer, with the new trains not expected until December at the very earliest.
The Victoria Line was the hottest line during today’s sweltering conditions, reaching temperatures over 35°C
The Central Line, which is often alleged to be the hottest in the Tube network, was next on the list with a temperature of 34°C, ahead of the Bakerloo Line at 33.3°C.
The Northern Line, meanwhile, fared better than expected but still clocked in a toasty 33.1°C.
This was also by far the most humid of all the lines, with measurements sitting between 75 and 80 per cent while the train was in motion.
There are currently only four lines on the London Underground that have air conditioning – the Circle, District, Hammersmith and City, and Metropolitan Lines.
All of these use the larger, more modern S–Stock trains that began service in 2010.
Predictably, these were far more comfortable with their un–air–conditioned counterparts, coming in at a still fairly toasty 28.2°C.
And things weren’t much better if you plan on taking the bus, with the 328 hitting a sweat–drenched peak of 34°C.
That was almost 3°C hotter than it was outside at the time we started measuring.
Older lines such as the Bakerloo line proved to be the hottest, clocking temperatures in the mid-30s. By contrast, government legislation rules that you can’t legally transport cattle when temperatures climb above 30°C
Amber and red weather warnings are currently in place for Wednesday and Thursday across the UK, with temperatures climbing to 36°C
However, the best way to get around in the heat was the newest addition to London’s transport network: The Elizabeth Line.
While it isn’t technically part of the London Underground, the ‘Lizzy’ Line was built from the ground up to be as cool as possible for London’s warming weather.
Every train has powerful air conditioning systems, and the stations are built to help remove heat as quickly as possible using under–platform extracts.
Compared to the too–small, too–deep trains on the rest of the network, that difference is immediately noticeable.
The Daily Mail recorded the temperature on the Elizabeth line at just 25.6°C.
That was almost 10°C cooler than the worst line on the London Underground and about 6°C cooler than the temperature outside.
When the tube was first constructed, its location deep underground actually made it a refuge from the heat above.
That was largely due to the clay soil, which sits around the tunnels and soaks up around 80 per cent of the heat produced by the trains.
London’s coolest line was the Elizabeth line (pictured), which features modern air conditioning and cooling systems that kept temperatures at a refreshing 25.6°C
Scientists estimated that this clay would have been about 14°C when the first tunnels were dug, but it has since risen to between 19°C and 26°C.
Combined with poor ventilation on the older, deeper lines, such as the Piccadilly Line, this has meant the London Underground has been getting steadily hotter over the years.
TfL’s crack team of engineers have been working to find practical solutions to cooling the Tube for more than two decades – and as yet, nobody has a silver bullet.
Claire Mann, Transport for London’s Chief Operating Officer, said: ‘Given the red extreme heat warning issued by the met office for parts of the country including London, we have been working closely with other agencies, including Network Rail, the Greater London Authority and others to confirm our hot weather plans.
‘We encourage customers to think about their comfort and to consider whether their journey is necessary.’
And as temperatures rise across London and the South of England, health experts have warned that heatwaves could start to become dangerous.
Professor Jesus Lizana, of the University of Oxford, says: ‘The UK is particularly vulnerable to extreme heat because our buildings, infrastructure and cities were designed for a cool climate, not for prolonged periods of high temperatures.
‘That means even temperatures that might be considered normal in southern Europe can have significant impacts here.’
