The Atmosphere

From the ground the sky seems to go on forever. From orbit it is a startlingly thin blue line hugging the curve of the planet. The atmosphere, the layer of gas that makes Earth habitable, is only about as thick in proportion to the planet as the skin on an apple. Everything that breathes depends on it, and almost everything about it is the work of living things.

What air is

Dry air is about 78 percent nitrogen, 21 percent oxygen and just under one percent argon. Everything else, including the carbon dioxide that governs the climate, amounts to a few hundredths of a percent. Water vapour varies from almost nothing over deserts to about four percent over tropical seas.

That mixture is unlike any other planet's. Venus and Mars have atmospheres of almost pure carbon dioxide, which is probably what Earth started with too. The oxygen is the product of photosynthesis. For the first two billion years of Earth's history there was essentially none; then microbes in the oceans learned to split water using sunlight and released oxygen as waste. Around 2.4 billion years ago it began to accumulate in the air, and it reached modern levels only about 400 million years ago. Oxygen is so reactive that without constant replenishment by plants and plankton it would disappear from the atmosphere within a few million years. A planet with free oxygen in its air is a planet with life.

Layers

The atmosphere has no sharp top; it simply thins out until it merges with space. Half its mass lies below about 5.5 kilometres, and 99 percent below 30. Scientists divide it into layers by temperature.

The troposphere, the lowest layer, holds nearly all the water vapour and all the weather. It is heated from below by the warm ground, so it gets colder with height, about 6.5 degrees per kilometre, which is why mountaintops are cold and why airliners cruising at 11 kilometres fly through air at minus 55 degrees. Its top, between about 8 kilometres at the poles and 18 at the equator, is the ceiling that flattens the tops of thunderstorms into anvils.

Above that lies the stratosphere, dry and calm, rising to about 50 kilometres. Here the temperature increases with height, because the stratosphere contains the ozone layer. Ozone is a form of oxygen with three atoms instead of two, and it absorbs almost all the Sun's ultraviolet light, the radiation that causes sunburn and skin cancer and would otherwise sterilise the land surface. In the 1980s scientists found that chemicals used in refrigerators and spray cans were destroying it, opening a hole over Antarctica. The world banned those chemicals in 1987, and the ozone layer is slowly recovering, the clearest example of a global environmental problem solved.

In the mesosphere, 50 to 85 kilometres up, the air is thin and bitterly cold, and this is where most meteors burn up. Grains of dust and rock from space hit the air at tens of kilometres per second and glow as shooting stars, protecting the ground from a constant bombardment of about a hundred tonnes a day.

The thermosphere above that is where the aurora glows and where the International Space Station orbits, at 400 kilometres. The air there is so thin it would count as a vacuum in any laboratory, yet it is still enough to drag satellites slowly down over years. Beyond about 600 kilometres the exosphere fades into space.

Weight and pressure

Air has weight. The whole column above a square metre of ground at sea level weighs about ten tonnes, which is what we call atmospheric pressure. We are not crushed because the pressure inside our bodies pushes back equally. Climb a mountain and there is less air above you; at the top of Everest the pressure is a third of that at sea level, and each breath delivers a third as much oxygen, which is why climbers carry tanks. Pressure differences are what make the wind blow: air flows from where there is more of it to where there is less.

Why the sky is blue

Sunlight contains all colours. As it passes through the air, molecules of nitrogen and oxygen scatter the shorter blue wavelengths far more than the longer red ones, so blue light comes at us from every direction in the sky. At sunset the light travels through far more air to reach you, the blue is scattered away before it arrives, and what remains is orange and red.

Why it matters

The atmosphere keeps the surface about 33 degrees warmer than it would otherwise be, through the greenhouse effect of water vapour and carbon dioxide; without it the oceans would freeze. It blocks ultraviolet light and cosmic rays. It spreads heat from the equator to the poles, and water from the sea to the land. And it is the one part of the planet that every living thing shares. A molecule of oxygen you breathe out today may be breathed in next year on the other side of the world.