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Climb a mountain and you get nearer the Sun and colder at the same time. Both of those are true. Only one of them is doing anything.
Kilimanjaro sits about three degrees from the equator, in Tanzania, where the ground below is hot enough to walk on barefoot.
Its summit is 5,895 metres up, and there is ice on it.
Same country. Same afternoon. Same Sun. One place is baking and the other is frozen, and the only difference between them is height.
Ask why a summit is cold and you'll often get told the opposite of what you'd expect: you're higher up, so you're further from the warm ground.
And almost immediately somebody points out the awkward bit — going up also takes you closer to the Sun. Shouldn't that make it warmer?
Here is the strange thing. That objection is completely correct. You really are nearer the Sun on a summit than at the beach. It just does not matter at all, and it's worth seeing exactly why not.
The Sun is about 150 million kilometres away.
The top of Everest is about 8.8 kilometres above the sea.
So climbing the highest mountain on the planet — all of it, from the beach to the summit — moves you 8.8 kilometres closer to something 150 million kilometres away.
There's a second clue too, and it's a lovely one. Earth's path round the Sun isn't a perfect circle. In January we're about 147 million kilometres out; in July we're about 152 million. Five million kilometres of difference, every single year — and July is the middle of summer in the northern half of the world, the far half of the orbit.
So five million kilometres doesn't even decide the season. Nine is not going to freeze a mountaintop.
You climb from the beach to the summit of Everest — 8.8 kilometres up, towards a Sun that is 150 million kilometres away. What fraction of the journey to the Sun have you just done?
It isn't the Sun that changes. It's the air.
Air has weight. The atmosphere presses down on every square metre of ground with roughly the weight of ten tonnes of air stacked above it — you don't notice because it presses from every direction at once, including from inside you.
Now climb. Every step up leaves some of that air below you, so there is less of it left on top pressing down. By about 5,500 metres the pressure is only half what it is at sea level. On Everest's summit it's about a third.
That is the real difference between the beach and the summit: not the Sun, but how much air there is.
Going up doesn't move you meaningfully nearer the Sun. It moves you out from under most of the air.
Take a bicycle pump and squash the air inside it. The pump gets warm — you can feel it in the barrel. Squeezing air heats it.
Run that backwards. Let air spread out into a bigger space and it cools. It's the same effect a can of compressed air has when it goes frosty in your hand as you spray it.
Now put those two facts together. Air near the ground is thick. Push some of it up a mountainside and it arrives somewhere with much less pressure squeezing it — so it swells. Nobody took any heat away from it. It cooled purely by getting bigger.
The average across the world works out at about 6.5°C lost for every kilometre you climb.
This is the reference model aviation and weather services use — 15°C at sea level, and 6.5°C off for every kilometre. Read it against a mountain you know: Kilimanjaro is a little under 6 km, Everest a little under 9.
| Height above sea level, in kilometres | Standard atmosphere temperature |
|---|---|
| 0 | 15 |
| 1 | 8.5 |
| 2 | 2 |
| 3 | -4.5 |
| 4 | -11 |
| 5 | -17.5 |
| 6 | -24 |
| 7 | -30.5 |
| 8 | -37 |
| 9 | -43.5 |
Notice it is a straight line, not a curve. Each kilometre costs the same 6.5°C as the one below it.
A beach at a pleasant 15°C and a summit six kilometres up on the same day: the line says about 24 degrees below freezing. That is a jacket-and-then-some difference, and none of it came from the Sun moving.
Not a person climbing — a lump of ordinary valley air, shoved up the slope by the wind. Press through and watch what is done TO it, and what is never done to it.
Seal some valley air in a balloon so nothing can get in or out. Carry it — don't push it, carry it — to the top of a ridge. Nobody chills it, nobody heats it. It still arrives colder inside. What happened?
Sunlight mostly goes straight through clear air. It doesn't warm it much on the way past.
What it does warm is the ground — soil, rock, roads, the sea. And the ground then warms the air sitting on it, from below, like a hob under a pan.
So the air is heated from the bottom up. Which means the further you get from the bottom, the further you are from the heater. Height takes you away from the only thing in the sky that is any good at getting warm.
And if you ever wondered why climbers carry oxygen: that thinner air is not just colder, there's less of it in every breath. Around a third as much at the top of Everest. That part is a grown-up's problem to plan for, not a puzzle to solve.
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Temperature falls with height through the troposphere at an average of about 6.5°C per kilometre — the environmental lapse rate used in the International Standard Atmosphere, which fixes sea level at 15°C. Every chart point in this episode is T = 15 - 6.5h with h in kilometres. (ICAO Standard Atmosphere; NOAA/NWS JetStream, 'Air Pressure'; UK Met Office glossary.) The physical cause is adiabatic expansion: air lifted to lower pressure expands, and the work of expanding is paid for out of its own internal energy, so it cools even though no heat is removed. The dry adiabatic rate is about 9.8°C per kilometre; when condensation releases latent heat the rate is roughly half that, and the observed average sits between them at about 6.5. (NOAA JetStream, 'Air Parcels and Adiabatic Processes'; American Meteorological Society Glossary, 'lapse rate'.) Standard sea-level pressure is 1013.25 hPa, which is 101,325 newtons on every square metre — about 10.3 tonnes of mass per square metre, since 101,325 / 9.81 = 10,329 kg. (ICAO Standard Atmosphere.) Pressure with height, from the standard barometric formula p = 1013.25 x (1 - 0.0065h / 288.15) ^ 5.2559 with h in metres: 505 hPa at 5,500 m, which is 49.8% of sea level, and 314 hPa at Everest's 8,849 m, which is 31%. The episode rounds these to 'half' and 'about a third'. Mount Everest's summit is 8,849 m (the 2020 joint China-Nepal survey). Kilimanjaro's Uhuru Peak is 5,895 m, at about 3 degrees south of the equator, and carries ice fields at the summit. (Survey publications; Tanzania National Parks.) Earth's distance from the Sun averages about 150 million km, varying from roughly 147.1 million at perihelion in early January to 152.1 million at aphelion in early July. Northern hemisphere summer therefore falls at the FAR end of the orbit, which is why distance does not explain the seasons. (NASA, 'Earth's Orbit and Seasons'.) 8,849 m against 150 million km is 5.9 x 10^-8 of the distance — under one millionth, and about one seventeen-millionth. Sunlight passes largely unabsorbed through clear dry air; the surface absorbs it and heats the air above by conduction and convection, which is why the troposphere is warmest at the bottom. (NOAA JetStream, 'The Transfer of Heat Energy'.) This episode explains atmospheric physics. It gives no mountaineering or altitude-sickness guidance, which is a grown-up's responsibility and a different document.