Heatwave: Why is Heathrow so hot?

By Tom Edgington
BBC News

Boeing 747-400 landing on the runway at sunset with runway-lights on.Image source, Getty Images

It's Europe's busiest airport, and as well as attracting millions of passengers could Heathrow also be a magnet for the sizzling heat?

Reaching 36.9C, Heathrow briefly broke the UK record for July's hottest day ever - before Cambridge achieved 38.1C.

That beats July 2015's previous record of 36.7C, which was also at Heathrow.

So why does this west London airport often top the temperature charts?

How is temperature measured?

To get a standardised temperature, a weather station, known as a Stevenson Screen, is used.

These white boxes, which contain a thermometer, are installed 4ft (1.25m) above the ground and are dotted all around the UK.

The weather station at Heathrow is located very close to the northern runway, so with aeroplanes constantly landing and taking off, does it make a difference to the temperature?

Not according to Paul Williams, Professor of Atmospheric Science at the University of Reading.

Image source, Paul Williams
Image caption,
Heathrow's Stevenson Screen (the white box) is used to measure the temperature

"Planes make a negligible difference," says Professor Williams.

"Every time you use energy - whether it's from a plane's engine, or even just switching on a light bulb or taking a shower - it's eventually turned into heat.

"But all of that is a minor influence compared to the effect of the urban heat island."

The urban heat island is, Prof Williams explains, the process where buildings absorb more sunlight than open fields.

Cities tend to hang on to the heat for longer, which can push up temperatures by a few degrees, he says.

Image caption,
Heathrow's weather station is located close to the northern runway

Heathrow - with its large black asphalt runways and airport buildings - will naturally absorb more heat.

But London is very built-up, meaning surrounding areas should also be affected in a similar way.

This can be shown by comparing the average monthly temperature of Heathrow to nearby Kew, eight miles away.

The temperature graph above shows there is hardly any difference between Heathrow and Kew - but both areas are hotter, on average, than the rest of the UK.

That suggests that it is the buildings, rather than planes, contributing to the higher average temperatures.

But what about CO2 gas levels expelled by the planes?

Prof Williams says CO2 is a greenhouse gas and it does trap heat but, because it mixes very quickly with the air, it warms the entire climate, not just Heathrow.

"If you measure the CO2 levels above Heathrow they wouldn't be any higher than other parts of the UK because it spreads so quickly," he says.

Media caption,
Measuring temperature

The Met Office told us that their weather stations are built to very specific standards and any biases that could affect temperature records are taken into account when taking down readings.

The Met Office also pointed out that Heathrow is many miles from the sea, which means it does not benefit from a cooling effect that many coastal areas receive.

It says if you look at overall temperature records, there is a pattern between high temperature and the distance from the sea.

Image source, Rufus Woodwark
Image caption,
Heathrow holds the record for the all-time highest July temperature

And even the soil can be a factor, according to Gareth Harvey from the BBC Weather Centre.

"Take, another very warm spot, like Wisley - located in the Surrey heathland and typified by sandy soils," he says.

"Sand is a natural insulator and so the heating effect of sunshine is stored in the top layer only, which gets very hot and then warms the air."

In summary, the overall temperature of any particular weather station is likely to be affected by several factors - such as the geographic features, the wind and the soil.

So, there's more to Heathrow's hot spot than its proximity to roaring jet engines and all that tarmac.