The warmest place in Britain is never where they say it is. The hotspot could be anywhere, whereas the Met Office only takes temperature readings at certain predefined locations. Each approved site has a Stevenson screen, a louvred wooden box which protects from direct sunlight, positioned precisely 125cm above the ground. Thousands of these exist across the country, many in the hands of willingamateurs, but the Met Office only collects observations in real time from around 270 synoptic stations [map], seven of which are in Greater London. Where are they? And what was the maximum temperature there yesterday?
» Heathrow(weather station 03772, altitude 25m) 33.2°C
The hottest place in London yesterday, or at least the place with the highest officially-recorded temperature, was Heathrow Airport. You might not be surprised by this, given that aeroplanes have enormous jet engines pumping out heat, but rest assured this is of minimal influence compared to the blazing rays of the sun. Heathrow is a good spot for a weather station for two reasons, the first historical. A large proportion of official weather stations are at properties once owned by the Air Ministry, an up-to-date knowledge of current meteorological conditions being essential for flight, hence there are three airfields in London's list, each with detailed records going back decades. Secondly a weather station needs to be located at a suitable distance from anything that might interfere with readings, hence an open space without trees, buildings or bodies of water is ideal.
Heathrow Airport has an area of five square miles, most of it empty, so it may be a surprise to learn that the weather station is right up against the edge beside a busy road. Specifically it's in the middle of the northern edge, alongside the Northern Perimeter Road, very close to where the vehicle tunnel plunges underneath the runways. Intriguingly it's very close to General Roy's Cannon, one end of the baseline which kickstarted the triangulation work of the Ordnance Survey, and a completely different fascinating tale to boot. These days you drive in from the roundabout on Nene Road, or walk in if you dare - this peripheral freeway isn't designed for those on foot. Although there's a pavement on the airport side a succession of red KEEP OUT signs is affixed to the razorwire fence, and the police drive by just often enough that poking a camera through feels somewhat unwise. But the wind vane is easily seen from the bus shelter across the road, and there's the excuse of waiting for a Heathrow-bound service to explain why you're hanging around.
The UK's warmest ever July day was recorded right here at Heathrow, last year, that's 36.7C. The record-breaking weather station is screened off within a rectangular compound, barely 100 yards from the main north runway, from this angle with the main terminal buildings on the horizon immediately behind. The passage of planes depends on which directional mode the airport's in, but while I watched only the very largest aircraft made it far enough down the runway to pass by. All the whining and braking took place almost out of sight, with British Airways and Etihad A380s eventually lumbering past. More easily seen is a squat concrete building beside the fence which now belongs to a fire and security company, but used to belong to the Met Office, confirming the former importance of this key airside location.
» Northolt(weather station 03672, altitude 37m) 32.5°C
Another weather station, another airfield. Northolt opened in 1915 as home to No. 4 Reserve Aeroplane Squadron, and has developed over the last century into the airport of choice for the armed services and certain private jets. Its weather station is in the centre of the southern perimeter fence, which runs immediately alongside the A40 dual carriageway. Walking this way is not recommended, I tried it once and it took half an hour of verge-scrambling beneath half-height lampposts, but rest assured that the location is ideally situated within a large circle of non-built-up area.
» St James's Park(weather station 03770, altitude 5m) 32.3°C
The Met Office's only regularly-reporting climatic station in Central London is in St James's Park. Specifically it's in the northeast corner, close to the point where Horseguards Parade meets The Mall, immediately alongside a brightly bordered tarmac path. A small square of grass, about five metres long, has been railinged off to prevent public access, with the Stevenson screen towards one corner and the open funnel of a rain gauge in the other. A bank of trees shields the site from the early morning sun, but sightlines are rather more open from breakfast onwards, which makes this a popular spot for sprawling when the weather's good. Yesterday afternoon a touchy-feely couple were frying nicely on a towel by the fence, while a de-flip-flopped cyclist lay head-deep in a book beside a Brompton while his back slowly bronzed.
» Kew Gardens(altitude 8m) 32.0°C
You can see this one for £15, it lies within the Botanical Gardens, alongside the Broad Walk close to the Orangery restaurant.
» Hampton Waterworks(altitude 30m) 31.6°C
You can't see this one, it's within the Thames Water Treatment Works off the Upper Sunbury Road, and very nearly in Surrey.
» Hampstead(weather station temporarily closed, altitude 175m)
This station near the summit at Whitestone Pond holds a peculiar weather record - the UK's highest 155-minute rainfall total. On 14th August 1975 an astonishing 169mm of rain, that's more than six inches, fell in less than three hours during a cloudburst over the Heath. The weather station is on Lower Terrace, sharing the top of an underground reservoir with Hampstead Observatory, although both are currently closed and out of action (until at least March) while Thames Water install a waterproof membrane across the site.
» Kenley(weather station 03781, altitude 170m) 29.4°C
This is the third and final airfield on the list, on the Downs to the south of Purley, and again very nearly in Surrey. To show the difference that a few miles (and a change in altitude) can make, this station recorded a temperature four degrees cooler than Heathrow yesterday, indeed Kenley's often the London site to watch in winter rather than at the height of summer.
One site that's not in this list of Met Office weather stations, but used to be, is the roof of the London Weather Centre. This has moved around in its time, including a spell in South Kensington, but spent the majority of its existence in and around the Holborn area. In 1919 readings were taken on the Air Ministry Roof in Kingsway, before shifting to nearby Victory House in 1938 and Princes House in 1959. In 1965 it moved to Penderel House on High Holborn, where an under-exposed roof led to measurements eventually moving to the top of State House across the road. A single snowflake here meant a White Christmas, a target which in 1992 moved to the western end of the Clerkenwell Road. By now renamed Met Office London, the service was closed for good on 12th September 2006, and that's why you never hear about the London Weather Centre any more. For a full detailed history, read this.
Oh, and if you'd ever like to keep an eye on the actual temperature at various sites around the country, the Met Office has a live map with a 24 hour slider. Here it is focused on London, but you can recentre and rescale to wherever. It won't be quite so shirt-drippingly hot today.