Why is the London tube so hot?

It is because the tube tunnels are deep and poorly ventilated.

Also, 80% of the heat is produced by train operation, 15% from other equipment and 5% from people.

In some of deeper lines, there is no air conditioning due to the lack of space for equipment on trains and the problems of dispersing the waste heat these would generate.

Read: http://en.wikipedia.org/wiki/London_Underground_cooling

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