BrightKidz Library
Subjects
Two mountains of different heights and the snow line they share A ground line runs across the bottom of the picture with two mountains standing on it, drawn as bold outlines. The one on the left is short and the one on the right is much taller. A long dashed line runs level across the whole picture at one height. The short mountain stops below that dashed line altogether. The tall mountain carries on past it, and the small triangle of the tall mountain that stands above the dashed line is filled in solid to show snow that never melts.

Why a Mountain Top Is Colder, Not Warmer

About 14 minutes

Climb high enough on almost any big mountain and you reach snow. It is there in summer. It is there on mountains that stand in hot countries, with people in shirtsleeves at the bottom.

Every child who thinks about this hits the same wall. Up is towards the Sun. The Sun is what makes things warm. So the top of a mountain ought to be the warmest part of it, and instead it is the one place that stays frozen all year.

The first half of that is true. The top really is nearer the Sun. It is the second half that goes wrong.

Nine kilometres is nothing

Mount Everest stands about 8.8 kilometres above the sea. The Sun is about 150 million kilometres away.

Those two numbers are so far apart that it is hard to feel them. So shrink the whole thing. Suppose the Sun were only 150 kilometres away — an hour or two down a motorway. On that scale, climbing all the way from the sea to the summit of Everest would bring you about nine millimetres closer.

Nine millimetres. Less than the width of your fingernail, on a journey of 150 kilometres.

Whatever is making that summit cold, it is not the distance. Something else is going on, and it starts with a fact about sunlight that almost nobody is told.

Sunlight goes straight through air

Sunlight barely warms air on the way through. Clear air is nearly invisible to it — the light passes through and keeps going, and the air is hardly touched.

Sunlight only turns into heat when it hits something solid. Ground, rock, roof, sea, the back of your hand. That thing warms up, and then it warms the thin layer of air lying against it, and that warmed air spreads and mixes upward.

You have felt this without noticing. On a bright day the pavement is hot enough to be uncomfortable through your shoes while the air over it is pleasant. Sunlight falling on a windowsill makes the sill warm, not the room. A sunny wall is warm to the palm long before the garden is.

Sunlight passing through air and turning into heat only at the ground Four long straight rays come in from the top left and run down across the picture without stopping, passing small circles that stand for the air they travel through. Every ray ends with an arrow head exactly where it meets the solid ground, which is drawn as a thick bar and labelled as the only thing the sunlight actually warms. To the right of the rays, three wavy arrows rise up off that bar, showing the warmed ground passing its heat into the air lying on top of it. A scale on the far right is marked warm at the bottom, beside the ground, and cold at the top. the light goes straight through the air this is the only thing the sunlight warms cold warm
The light goes through the air almost untouched and stops at the ground. So the ground is the heater, and the air is warmed from underneath.

Which turns the whole picture upside down. The atmosphere is not heated from above by the Sun. It is heated from below, by the ground. And that means the warmest air is always the air nearest the ground — so going up is going away from the heater, not towards it.

It gets colder at a steady rate

Air thins out as you go up, too, and thinning air cools itself. (That is the same step that decides how high clouds sit, and it has its own article: why clouds have flat bottoms.)

Put both together and you get one of the most reliable numbers in geography. Climb a kilometre and the air is roughly 6.5 degrees colder. Climb another and it drops again.

Walking up a mountain and reading the air temperature at each kilometre A mountain stands on the left of the picture, reaching exactly as high as the scale beside it. That scale runs up the right hand side, ticked once for every kilometre from the ground at the bottom to eight kilometres at the top. A level line stretches from the scale across to the mountain and can be raised, marking the height reached. As it passes each tick, the air temperature at that height is written in beside the scale. The readings start at twenty five degrees at the ground and fall steadily all the way to minus twenty seven at the top. Nothing else in the picture changes at any point. 8 km 25°C 19°C 12°C 6°C -1°C -8°C -14°C -21°C -27°C about 6.5 degrees colder for every kilometre Same day, same weather, same Sun. Only the height changed.

Start at the beach on a warm day and walk uphill. Nothing about the weather changes — only your height.

Sea level to the summit of Everest is close to nine kilometres, so it is close to sixty degrees of cooling. A pleasant 25 degrees at the beach becomes something around minus 32 at the top, and no weather had to change for that to happen. The mountain did it all by being tall.

A mountain top has a second problem, as well. It is a thin spike sticking up into open sky, with hardly any warm ground around it and wind pouring past on every side. Whatever heat it does collect is carried off almost at once.

Why is the air near the ground warmer than the air high above it?

The cold is what carved the mountain

There is a height on every mountain where the year balances out — where the snow that falls in winter is exactly as much as the summer can melt. Below it, everything melts. Above it, some snow is always left over.

That line is called the snow line, and above it the leftovers stack up year after year. Buried snow is squeezed by the weight of the snow on top of it, the air is pressed out, and eventually it becomes solid ice. Ice thick enough starts to creep downhill under its own weight, and a creeping mass of ice with rock frozen into its underside is a very effective file.

So the shape of a high mountain is not a coincidence. Those knife-edge ridges, the bowl-shaped hollows scooped out below the summits, the broad valleys with sides like a letter U rather than a letter V — all of them were cut by ice that only existed because of the cold this article has been explaining.

The temperature is not just the weather up there. It is the tool.