The London Underground is a great way to get around – but it can be quite a hot and sweaty experience!

Bex and Dan find out what can be done to cool things down…

Trains push air through the tunnels which helps to cool things down a bit, ventilation shafts suck out hot air and sometimes fans are used on hot stations to create a cool breeze… But why does the underground get so hot?

It’s all because of how the tube network was built!

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There’s three types of lines – lines that travel in the open, sub-surface lines which are just below street level, and the tube proper which runs through tunnels.

Now you won’t get too hot on the open lines – unless it’s a sunny day. And the sub surface tunnels tend to stay cool too.

That’s because as they were built for steam trains, they needed lots of ventilation to remove the smoke. Today, this ventilation brings fresh air in which helps keep things cool.

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These lines are also larger than the deep tube tunnels which means that trains on these lines can be fitted with air-conditioning, with the heat vented away thanks to the old steam train ventilation.

However, it’s the deep level tube tunnels that cause the biggest problems for passengers – and staff.

Most deep level tunnels were built through London Clay. When the tunnels were dug, they were so cool that the tube was seen as a respite from the summer heat on the surface!

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The problem is that over time heat from the trains soaked into the clay to the point where it can no longer absorb any more heat.

Tunnels that were a mere 14 degrees Celsius in the 1900s can now have air temperatures as high as 30 degrees Celsius.

So where is all this heat coming from? As you would expect, most of it’s from the trains themselves – all those moving parts generate a lot of energy – which results in heat, especially when trains are braking.

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Over the years engineers have tried to come up with new ways to get the temperatures lower. Regenerative brakes can convert heat back into electricity for use by other trains on the tracks.

Modern tube lines are built with considerably more ventilation shafts than older tunnels were. Whilst building more ventilation shafts on the older tube tunnels is an option, it can be complicated.

You have to think about what’s above the tunnels and the people who live nearby.

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Engineers have also looked at using cold water. An experiment at Victoria used water from the River Tyburn to cool the air in the station.

At Green Park, five boreholes drilled deep into the ground suck cool water to the surface to cool a separate water supply which is pumped down to cool the platforms.

As once the circulating water has warmed up, it’s pumped away.