Why is it colder at the top of a mountain, if you’re closer to the Sun?

Surely it should get hotter the closer you are to the Sun?

21st January 2016
Why is it colder at the top of a mountain, if you’re closer to the Sun? (iStock)

Asked by: Jonathan Lloyd, Telford

As the Sun is around 150 million kilometres away, even being on top of Everest only brings you 9km closer – far too small a difference to make you feel any warmer. The distance effect is totally overwhelmed by that of having less atmosphere around you as you climb. This leads to a steady fall in atmospheric pressure, and – as the air isn’t so compressed – a fall in temperature as well. The rate of decline is surprisingly fast: around 1°C for every 100m, and continues all the way up to the so-called tropopause around 12km above the Earth.

At these altitudes, barely 10 per cent of the atmosphere remains, and the air pressure is so low that the temperature falls to a lethally cold -55°C. The threat is not academic, either: at any given time, hundreds of thousands of people are being transported at these altitudes aboard aircraft. Passengers and crew are kept warm using hot air taken from the compressor stages of the engines before it’s mixed with fuel. This, combined with insulation in the walls and heat generated by the passengers themselves, ensures the cabin can be kept at room temperature.

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