In official terms a London heatwave starts when the temperature reaches 28°C on three consecutive days, which is the Met Office threshold for Greater London and the south-east. In practical terms the city starts to struggle several degrees below that, because almost nothing here (not the housing, not the deep Tube, not the railway) was built for it.
That is the honest summary. London is a rain city with a mild average and a long, slow autumn, and the equipment matches. Air conditioning is the exception in hotel rooms rather than the rule, shade is concentrated in the parks and absent from the shopping streets, and the older parts of the transport network lose capacity as soon as the rails get hot.
Our guide to London in summer covers the season as a whole, including which lines are cooled and what the long evenings are for. This page is narrower: what to change on the day the forecast says 34°C.
A London heatwave in numbers
- Official threshold
- 28°C on three days in a row, for London and the south-east
- Average July maximum
- 23.89°C at the Heathrow station (Met Office, 1991–2020)
- Heat-health alert season
- 1 June to 30 September, run by UKHSA with the Met Office
- Alert colours
- Yellow, amber, red — based on likely health impact, not on one temperature
- Rail speed restrictions
- Imposed in the hottest part of the day when forecast air temperature passes 36°C
- Track temperature
- Steel rail can run up to 20°C hotter than the air
- Hottest London night on record
- 25.8°C minimum at Kenley Airfield, 19 July 2022
What an official alert actually changes for a visitor
Two separate warning systems run in parallel, and they are easy to confuse. The Met Office issues extreme heat warnings through the National Severe Weather Warning Service, coloured yellow, amber or red according to how severe the effects could be and how likely they are. Heat-health alerts come separately from the UK Health Security Agency, in the same three colours, aimed at the health and care sector but published openly.
Read the colour as a statement about consequences rather than temperature. In the UKHSA's own wording, a red alert means a significant risk to life for even the healthy population, and an amber alert means effects are likely to be felt across the entire health service, with everyone, not only the vulnerable, exposed to some degree. Neither colour closes anything by itself.
What they change for you is timing. Both agencies say the same thing about the middle of the day: stay out of the sun between 11am and 3pm, plan physical activity for the morning or the evening, and keep the sun-facing windows and curtains shut in whatever room you are sleeping in. If you take one instruction from this page, take that one.
The railway slows down before the Underground does
This is the part visitors do not see coming, because it affects the trains that leave London rather than the ones inside it. Network Rail explains the mechanism plainly: steel absorbs heat easily and track can run up to 20°C hotter than the air, the rails expand, and if there is nowhere for them to go a rail can buckle and the line has to close.
To prevent that, speed restrictions go on during the hottest part of the day when the forecast air temperature is over 36°C. Overhead power lines are the other weak point; they sag when hot, and trains are slowed to avoid pulling them down. Older fixed-tension wiring is the vulnerable kind.
The consequence for a traveller is that a day trip to the coast or a cross-country connection can take considerably longer than the timetable says, with no disruption visible at the London end. Check the operator's live information before you set off on a 35°C day, and do not book a tight connection. Our guide to UK train tickets explains which fares let you travel on a later service.
Water refill points matter more than a bought bottle on a delayed train. Network Rail provides refill stations at all the stations it manages, which includes the big London termini. Fill up before you board rather than relying on a trolley service that may not run. Where else to fill a bottle in the city is set out in our guide to drinking water and refill points.
The Piccadilly line's air-conditioned trains are not here yet
You will read in plenty of places that the Piccadilly line now has air conditioning. It does not. Transport for London's own page on the upgrade says the new Siemens fleet will be the first deep Tube train with air conditioning, that the first train arrived in London in late October 2024 for testing, and that TfL is working to a window of between December 2026 and June 2027 to bring them into service.
That matters because the Piccadilly line is the one most visitors use, and it is the line that runs to Heathrow. Until the new trains are carrying passengers, the run from the West End out to the airport in the middle of a heatwave is done in 1970s stock with forced ventilation and no cooling, with luggage. Allow more time, or use the Elizabeth line and change. Our comparison of which Tube lines have air conditioning has the full breakdown, and every Heathrow option compared covers the alternatives.
Where the shade and the cold water are
Shade in central London is not evenly spread. Oxford Street, the bridges and the big squares have almost none in the early afternoon; the royal parks, the garden squares and the tree-lined side streets of Bloomsbury and Marylebone have a great deal.
Water helps in three forms. The river is a few degrees cooler than the streets and moving air makes it feel cooler still, which is the real argument for the river bus over a Tube on a hot afternoon. The lidos and ponds are the proper answer, and our guide to swimming ponds and lidos covers which are heated, which are single-sex and what they cost. And the shaded, high ground of Hampstead Heath is reliably several degrees cooler than the centre.
Indoors, the large stone-built museums are the coolest free rooms in the city, which is why they fill up on the worst days. Go at opening time rather than at two o'clock; our list of free museums has the options, and a public library is the same trick with fewer people, as working from a cafe or library in London explains.
The night is harder than the afternoon
British buildings are designed to hold heat in, which is the right choice for nine months of the year and the wrong one for the other three. A brick terrace that has absorbed sun all day releases it into the bedrooms after dark. The worst version is the attic conversion, where the roof itself has been baking since dawn.
The July 2022 heatwave set a London record that tells the story better than any daytime figure: Kenley Airfield in Greater London recorded a minimum overnight temperature of 25.8°C, the highest ever measured in the UK. A night that does not fall below 25°C is a night almost nobody sleeps through without cooling.
If you are booking in July or August, filter for air conditioning explicitly and check the description rather than the star rating, because a great many converted townhouses have none; our guide to finding a London hotel with air conditioning goes through it. If you are already in an un-cooled room, close the curtains before you go out in the morning, open the windows only once the outside air is cooler than the room, and ask reception for a fan. Most hotels keep a small stock and give them out on request.
What closes, and what quietly stops working
Very little in London formally closes because of heat, which is part of the problem: the queue outside an attraction is unshaded whether the attraction is open or not.
The known exception is the cable car across the Thames, which may stop running in strong wind, lightning or extreme heat. Outdoor events and sports fixtures are sometimes shortened. Glasshouses and greenhouses in the botanic gardens can become dangerously hot and are occasionally shut. Lifts and escalators fail more often. And rooftop terraces, which sound like the right idea, are usually the hottest place in the building until the sun goes down.
The bigger risk is the queue. Anything with an outdoor line — the London Eye, a timed-entry gallery, a popular food market — becomes an hour standing in direct sun. Book a timed slot where one exists, take the earliest one, and carry a hat rather than an umbrella.
Heat exhaustion, and the point at which it stops being a nuisance
The NHS describes heat exhaustion through symptoms including headache, dizziness, nausea, heavy sweating, pale clammy skin and cramps. Its advice is to move the person somewhere cool, get them to lie down with their feet slightly raised, give them water, and cool their skin. Most people recover within half an hour.
Heatstroke is a different matter and the NHS treats it as an emergency: call 999. The service also says to contact NHS 111 for heat-related problems that are not emergencies. Older visitors, babies and small children, pregnant women and anyone with a heart or lung condition are at higher risk and should be checked on more often.
For how the NHS works for people who do not live here, see our guide to healthcare for visitors; for sunburn, rehydration salts and the rest, a pharmacy is the right first stop, as pharmacies and minor illness in London explains. This page reports what the official sources say and is not medical advice.
Rebuilding a day that was planned for 20°C
The shape that works is front-loaded and back-loaded, with the middle given over to a large cool building. Start at opening time with whatever is outdoors or ticketed. Be inside by 11am and stay there until the worst of it passes. Come out again at five for the water, the parks and the evening, which in July does not get dark until well after nine.
Cut two things from the plan rather than trying to keep all of them: walking tours in the middle of the day, and anything that involves standing still outdoors. Move the shopping to the evening, because the big stores are cooled and open late.
Travel against the crowd where you can. The combination of a packed carriage and an un-cooled tunnel is what actually makes people faint, and our guide to avoiding rush hour in London is worth more on a hot day than on any other. Families should keep the day shorter than usual, and build in two stops rather than one. And if you would rather know what the opposite problem looks like, we have a companion page on when it snows in London.
How hot it has actually been
Some perspective, because forecasts in a heatwave attract a lot of noise. The UK record is 40.3°C, set at Coningsby in Lincolnshire on 19 July 2022; the Met Office describes that day as the first time 40°C had been recorded in the UK, with 46 stations beating the previous national record of 38.7°C. It was also the first time the Met Office had ever issued a red warning for extreme heat.
Against that, the Met Office thirty-year average maximum for July at its Heathrow station is 23.89°C, and for August 23.40°C. A typical London summer day is warm, not dangerous. Two or three times a summer it becomes something else, and those are the days this page is for. Our guides to London weather and what to pack and to London in July cover the ordinary version.
Frequently asked questions
What temperature counts as a heatwave in London?
The Met Office threshold for Greater London and the south-east is 28°C, and a heatwave is declared when the daily maximum reaches or exceeds that on at least three days in a row. The thresholds vary around the UK and are based on the 1991 to 2020 climate averages.
Does the London Underground have air conditioning?
Only on the sub-surface lines, where a fleet of 192 air-conditioned S-stock trains runs. The deep-level lines are ventilated but not cooled. The Piccadilly line's new trains will be the first deep Tube stock with air conditioning, and TfL says they will enter service between December 2026 and June 2027.
Do trains get cancelled in hot weather in Britain?
Cancellations are less common than delays. Network Rail imposes speed restrictions during the hottest part of the day when the forecast air temperature goes above 36°C, because steel rail can be 20°C hotter than the air and may buckle. Overhead power lines sag in heat, which also forces trains to run more slowly.
What should I do if I am staying somewhere without air conditioning?
Close the curtains on sun-facing windows before you go out, and keep the windows shut while the outside air is hotter than the room. Open everything once the evening air is cooler. Ask reception for a fan, take a cool shower before bed, and if possible ask to move off the top floor.
Where is the coolest place to spend the afternoon in London?
The large stone-built museums and libraries, which stay several degrees below street temperature and cost nothing to enter. Outdoors, the shaded high ground of Hampstead Heath, the tree cover in the royal parks, and the river, where the moving air on a boat makes a real difference.
Sources
- UK Health Security Agency - Heat-Health Alerting system (checked September 2026)
- Met Office - Weather warnings guide (National Severe Weather Warning Service) (checked September 2026)
- Met Office - A milestone in UK climate history (July 2022 heat review) (checked September 2026)
- Network Rail - Hot weather and the railway (checked September 2026)
- Transport for London - Piccadilly line upgrade (checked September 2026)
- Transport for London - Four Lines Modernisation (checked September 2026)
- NHS - Heatwave: how to cope in hot weather (checked September 2026)