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The London Underground is essential for getting around the capital, sparing millions of people from the congestion on the streets above. While it certainly has its flaws, most Londoners would agree that moving across the city would be far more difficult without it.
One of the most common complaints about the Tube is the intense heat, which persists even during the winter months. Many people assume this is simply due to crowded carriages and the heat produced by constantly running trains. However, according to one scientist, that explanation only tells part of the story.
Professor Hannah Fry, a mathematician, explained on The Rest is Science podcast with YouTuber Michael Stevens that the Underground was once much cooler. When it first opened, temperatures in the tunnels ranged between 10 and 15 degrees Celsius. At the time, the clay beneath London’s streets was able to absorb the heat generated underground.
But after around 150 years of trains running through the tunnels and millions of passengers passing through, that clay has reached its limit. In a clip shared on Instagram, Professor Fry explained that the clay has become “saturated” with heat and can no longer absorb any more. As a result, the heat now remains trapped within the Underground system.
She added that temperatures in the Tube can reach up to 40 degrees Celsius even in winter, and surprisingly, more people faint underground during colder months than in summer.
London’s clay has a high thermal mass, meaning it can store large amounts of heat energy. However, since it can no longer take in additional heat, the warmth produced by trains and passengers has nowhere to escape. This causes the heat to linger throughout stations, tunnels, and trains, making journeys increasingly uncomfortable.
Many social media users were fascinated by the explanation. Both Londoners and visitors said they had always noticed how hot the Underground felt but never understood why.
One person recalled visiting London in October 2011 when temperatures above ground were minus 10 degrees, yet their Tube journey was delayed because someone had fainted. Others shared similar experiences, saying they were shocked by how hot and cramped the platforms and tunnels felt, even compared to warmer countries.
Several commenters pointed out that winter clothing plays a big role. Passengers often enter the Underground wearing heavy coats and scarves to protect against the cold outside, and when they don’t remove them, they can quickly overheat in the stifling conditions underground.
As one person summed it up, standing on a packed Tube train in thick winter clothing can be extremely uncomfortable—and it’s no wonder people sometimes faint.
