Why does hot weather cause travel chaos?

Rail operators have warned that intense heat could cause tracks to buckle, leading to disruption for scores of passengers. (PA)

Forecasters have warned train passengers and drivers to prepare for travel chaos amid searing 39C temperatures across the UK.

Rail operators told customers to bear in mind possible speed restrictions when planning journeys due to the risk of tracks buckling.

Earlier on Thursday, travel experts also warned ‘roads could become like chocolate’ as they said drivers should take care on their commutes.

Rail executives fear tracks could buckle under the intense heat, causing nationwide disruption.

Embargoed to 0001 Sunday April 14 File photo dated 27/5/2016 of traffic queues along the M25 in Dartford, Kent. Drivers are being warned they could face severe queues next week with more than 25 million leisure car journeys expected on UK roads.

Around 20,000 miles of tracks are said to be susceptible to buckling when the mercury climbs above 27C (80.6F).

Overhead lines can also expand and sag in extreme heat, bringing a risk of passing trains pulling them down.

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More than three-quarters of the Network Rail tracks are on concrete sleepers, which, when 'fully compliant' with its standards, can withstand rail temperatures of more than 59C (138.2C).

But some sections of track are not designed to cope with that level of heat and are at risk of buckling.

People enjoy the hot weather in Hyde Park, London, as the UK could encounter the hottest July day on record later this afternoon. (PA)

Drivers, meanwhile, have been told the potentially record-breaking heat could cause roads to soften and bubble beneath them.

In some parts of the UK authorities are so worried about potential road degradation they are preparing to use gritters to keep traffic moving.

Dave Rigby, chief technical officer at the Road Surface Treatments Association (RSTA), said: ‘During very hot weather roads could reach 50°C at the surface and at these temperatures can start to soften.

‘Dark asphalt on the road surfaces absorbs a lot of heat and is like chocolate, it melts when it is hot and goes brittle when it cools down. ‘I would not be surprised to see gritters out over the next couple of days, they are used to spread dust and sand which absorbs the soft bitumen stabilising the road surface and making it less sticky.’

A yellow weather warning is in force for most of England, except the south-west, and parts of Scotland from 3pm on Thursday until 4am on Friday, while the Met Office has said there is a 60% chance the 38.5C current record could be beaten.

Meteorologist Matthew Box said it is likely both the 36.7°C July record and the all-time maximum of 38.5°C will be broken in the coming days.