Over the years, the heat soaking capacity of the London underground clay has decreased considerably, making the tunnels hotter. In recent summer times the tunnel inside temperature are recorded as 30°C, whereas the same were 19°C in the 1900s. The most important factors leading to this heat are the aerodynamic drag, frictional losses, motor engines and other electrical and auxiliary systems due to the moving trains.

Mechanical ventilation through output shafts fitted with huge ventilation fans reduces this heat considerably, by sucking the hot air out.