It is easy to feel sorry for commuters on the London Underground at this time of year, crammed into stuffy carriages with the temperature rising. But some passengers, it appears, are more deserving of pity than others.

A map compiled by Transport for London (TfL) has revealed the hottest spots on the Tube network, notorious for its lack of air-conditioning.

The map, which covers most of the Underground lines in zones one and two, was compiled by TfL officials to identify areas most in need of cooling, but it will be a handy aid for travellers anxious to avoid the worst spots.

For many, it proves that London has indeed become a melting pot. The Central Line — which serves some of the most popular tourist spots such as Oxford Street and St Paul’s Cathedral — is one to avoid on a hot day.

Appropriately coloured red on the London Underground map, the Central Line boasted some of the highest temperatures when the heat map was compiled on July 28 last year — the hottest day. The section covered by the map was above 30C (86F).

The Bakerloo Line, which runs deep under ground, did not fare much better. Between 4pm and 7pm, the majority of its stations were in the highest temperature band, although the discomfort eased as the trains moved south. But even in the coolest stations the temperature hovered at about 25C. In previous tests, temperatures in some carriages during the summer have exceeded 35C, which would make the network officially unfit for transporting cattle.

At the beginning of June, the Mayor of London, Boris Johnson, announced the first trial of air-conditioned trains, which should provide welcome relief for commuters on the Metropolitan Line next summer. By 2015 they will be running on the District, Circle, Metropolitan and Hammersmith and City lines at a cost of £1.56 billion for 191 trains.

But TfL faces a bigger problem when it comes to the rest of its network, which is built far deeper under ground — more than 220ft in the case of the Northern Line. The new trains, which have air-conditioning units installed in the roof, do not fit into the smaller tunnels of the deep-level network.

TfL has set up a Cooling the Tube team to look at options for air-conditioning its deep-level trains. A long-term solution has proved elusive so far, however, and efforts have been limited to expanding ventilation shafts and installing industrial fans.

A TfL spokeswoman said that the London network was at a disadvantage because there was “no concept of air-conditioning” when construction began in 1860. “Our team is working hard to find a solution,” she said.

Times recommends

• Overlanding then and now
• Live Q&A: ski fitness and injury
• Get the right payment card and deny Ryanair its hefty fee