Appearance
2.1 — The Atmosphere
If you could drive straight up at 100 kilometres per hour, you would leave most of the atmosphere behind in about ten minutes. Half of all the air on Earth is below 5.5 kilometres. Ninety percent is below 16 kilometres. The layer that holds all weather, all clouds, all rain and every living thing is thinner, relative to the planet, than the skin on an apple.
Everything in the next eight chapters happens inside that skin.
What air is made of, and why that matters
Dry air, by volume:
| Gas | Share |
|---|---|
| Nitrogen | 78.08% |
| Oxygen | 20.95% |
| Argon | 0.93% |
| Carbon dioxide | about 0.042% |
| Neon, helium, methane, others | traces |
Water vapour is left out of that table on purpose, because it is the one component that varies wildly — from almost nothing over a desert or at the poles, to about 4 percent of the air in a hot humid place. It is also, as the next chapters show, the most important single ingredient in weather.
Three observations about this list are worth stopping on.
Nitrogen dominates but does almost nothing. The nitrogen molecule is two atoms held by a triple bond, which is among the strongest bonds in ordinary chemistry, so nitrogen is nearly unreactive. It is bulk. Its main role is providing the pressure that lets liquid water exist at the surface at all — reduce the atmosphere's mass enough and the oceans would boil away at ordinary temperatures.
Oxygen is chemically aggressive and should not be there. Free oxygen reacts with everything: iron, rock, wood, flesh. If life stopped producing it, it would be consumed by weathering and oxidation within a few million years. Its presence is a continuously maintained disequilibrium, and it is the strongest single sign of life on this planet. It is why astronomers looking for life on other worlds look for oxygen out of chemical balance in an atmosphere.
Carbon dioxide is present in tiny quantities and controls the temperature. At 0.042 percent it is a rounding error by volume, and the reason it matters so much is covered below and again in Part 14. A trace gas can dominate a system if it is the only thing doing a particular job, and here the job is absorbing infrared radiation, which nitrogen, oxygen and argon cannot do at all.
Why pressure falls with height
Air has weight. Atmospheric pressure at any point is simply the weight of all the air above that point, pressing down over a unit of area. At sea level that is about 101,325 pascals — roughly ten tonnes on every square metre, which is also roughly a tonne pressing on the surface of your body, balanced by the pressure inside you.
Go up and there is less air above you, so the pressure is less. But it does not fall in a straight line. It falls exponentially, because air is compressible: the air at the bottom is squeezed by everything above it and is therefore denser, so more of the total mass is packed into the lowest few kilometres.
P = P_0 \, e^{-h/H}
Read aloud: the pressure at height h equals the sea-level pressure times e raised to the power of minus h over H. Here e is the number 2.718, the base of natural growth and decay, and H is the scale height — the distance over which pressure drops to about 37 percent of its previous value. For Earth's atmosphere H is roughly 8.5 kilometres.
In practice: pressure roughly halves every 5.5 kilometres. At the top of Everest at 8,849 metres it is about a third of sea level. At the cruising altitude of an airliner, around 11 kilometres, it is about a quarter — which is why the cabin is pressurised, and why it is pressurised only to the equivalent of about 2,000 metres rather than to sea level, since holding a bigger difference would need a heavier fuselage.
This is also why there is no sharp top. The atmosphere does not end; it thins out indefinitely. The 100-kilometre Kármán line used to define "space" is a convention, chosen roughly as the altitude where an aircraft would have to fly at orbital speed to generate lift.
The layers, and what defines them
The troposphere — surface to 8–18 km. Temperature falls with height. All weather.
The name comes from the Greek for "turning", and it is exactly right: this is the layer that overturns. It is heated from below, because sunlight passes through mostly transparent air and is absorbed by the ground, which then warms the air in contact with it. Heat a fluid from the bottom and it convects — warm parcels rise, cool, sink, and the layer stirs itself continuously. Everything in the following chapters happens here.
Temperature falls at an average of about 6.5 °C per kilometre. The top of the layer, the tropopause, is a lid: above it temperature stops falling, so rising air is no longer buoyant and stops. This is why thunderstorm anvils spread out flat — the storm has hit the ceiling. The tropopause is higher over the equator (about 18 km) than over the poles (about 8 km), because more vigorous convection pushes it up.
The stratosphere — up to about 50 km. Temperature rises with height. Ozone.
Here the temperature increases upward, and the reason is ozone, a molecule of three oxygen atoms. Ozone absorbs ultraviolet light, and absorbing it releases heat, so the layer is heated from above. Warm air on top of cool air is stable and does not overturn, which is why the stratosphere is calm, why airliners cruise near its base for smooth air, and why volcanic aerosols injected into it linger for years (Chapter 1.7).
The ozone layer is not a layer of pure ozone. Ozone is a few parts per million even at its peak. If you brought all of it to sea level pressure it would be a shell about 3 millimetres thick. That is what stands between the surface and the ultraviolet.
Why ozone matters, precisely. Ultraviolet radiation has enough energy per photon to break chemical bonds, including bonds in DNA. Shorter wavelengths do more damage. Ozone absorbs almost completely in the most damaging bands and partially in the next band up. Without it, land life as it exists would be impossible.
And the ozone hole is the one large environmental problem the world actually solved, which is worth stating in a book that will later be pessimistic elsewhere. In 1974 Mario Molina and Sherwood Rowland worked out that chlorofluorocarbons — the inert, non-toxic, wonderfully useful gases in refrigerators and spray cans — drift up to the stratosphere intact, are broken by ultraviolet light, and release chlorine atoms that destroy ozone catalytically. Catalytically means one chlorine atom destroys ozone molecules over and over without being consumed — estimates run to tens of thousands of molecules per atom. In 1985 British Antarctic Survey scientists reported a genuine hole over Antarctica. The Montreal Protocol was signed in 1987, ratified universally, and the chemicals were phased out. The ozone layer is now measurably recovering and is projected to return to 1980 levels around the middle of this century. Chapter 14.5 asks why this worked and climate policy has not, and the answer is instructive.
The mesosphere — 50 to about 85 km. Temperature falls again. Where meteors burn.
Little ozone, so no heating from above; too thin to hold heat. The coldest place in the atmosphere is at its top, around minus 90 °C. This is where meteors burn up — not because the air is thick but because they are moving at tens of kilometres per second and compressing the air ahead of them violently.
The thermosphere — above 85 km. Temperature rises steeply, and the number is misleading.
Individual molecules here can be at 1,500 °C or more, because they absorb the most energetic solar radiation. But there are so few of them that there is almost no heat to transfer. A thermometer in the thermosphere would read cold, because it radiates away more energy than the rare molecule collisions deliver. Temperature measures the average energy per particle; heat is about total energy, and there is not much. The International Space Station orbits in this layer.
The ionosphere overlaps the upper mesosphere and thermosphere. Solar radiation strips electrons off atoms, leaving charged particles, and that charged layer reflects radio waves. This is why shortwave radio can be heard across oceans — the signal bounces between the ionosphere and the ground. It is also why AM radio reception improves dramatically at night: the lowest ionospheric layer, which absorbs those waves during the day, disappears after sunset and the waves get a clean bounce off the higher layers. Auroras happen here.
The energy budget, which decides everything
The whole of Part 2 and much of Part 14 comes down to one accounting exercise. Energy arrives from the Sun, and the same amount must eventually leave, or the planet heats or cools until it does.
Of the sunlight arriving at the top of the atmosphere:
- About 30 percent is reflected straight back to space — by clouds mostly, and by ice, snow, desert and the surface. This fraction is called the albedo, from the Latin for whiteness.
- About 20 percent is absorbed within the atmosphere itself by ozone, water vapour, dust and clouds.
- About 50 percent reaches the surface and is absorbed.
The surface then has to get rid of that energy, and it does so in three ways, which matters because the proportions are unintuitive. It radiates infrared upward. It heats air in contact with it, which convects away. And — the largest single term over the oceans — it evaporates water, which carries away enormous energy as latent heat: the energy it takes to turn liquid into vapour, which is released again wherever that vapour later condenses. Evaporation over a tropical ocean and condensation in a cloud thousands of kilometres away is a heat pipe moving energy around the planet, and it is the engine of every storm in Chapter 2.4.
The greenhouse effect, stated correctly
This is where popular explanations go wrong, so it is worth doing properly.
The Sun is hot — about 5,500 °C at the surface — so it radiates mostly at short wavelengths, in and around visible light. The Earth is far cooler, around 15 °C at the surface, so it radiates at much longer wavelengths, in the infrared. Two different sets of wavelengths going in opposite directions.
Nitrogen, oxygen and argon are transparent to both. A molecule can only absorb infrared if its vibrations change the way electric charge is distributed within it, and a symmetric two-atom molecule of identical atoms cannot do that. Carbon dioxide, water vapour, methane and nitrous oxide can, because they have three or more atoms and asymmetric vibrations.
So the atmosphere is largely transparent to incoming sunlight and partly opaque to outgoing infrared. The infrared leaving the surface is absorbed by those gases and re-radiated in all directions, including back down. The surface therefore receives energy from two sources — the Sun and the atmosphere — and settles at a higher temperature than sunlight alone would give.
The size of the effect. Without any greenhouse gases the Earth's average surface temperature would be roughly minus 18 °C. It is about plus 15 °C. The effect is worth around 33 degrees, and without it the planet would be frozen. The greenhouse effect is not a problem; it is the reason there is liquid water. The problem, taken up in Part 14, is changing its strength quickly.
The name is wrong and it is worth knowing why. A real greenhouse works mainly by stopping convection — the glass physically prevents warm air from rising away. The atmospheric effect works by absorbing and re-emitting infrared. The mechanisms are different; the name stuck anyway.
Where this shows up in your life
Cooking at altitude. Water boils when its vapour pressure equals the surrounding air pressure, so lower pressure means a lower boiling point — about 93 °C in Shimla, about 86 °C in Leh. Boiling water is cooler there, so food genuinely takes longer, and rice or dal may simply never soften. This is why a pressure cooker is standard equipment in the hills: it raises the pressure inside and pushes the boiling point back up.
Your ears on a flight or a mountain road. The middle ear holds a pocket of air at the pressure it was sealed at; when outside pressure drops, the eardrum bulges outward until the Eustachian tube opens and equalises. Swallowing or yawning opens it.
Sunburn and sunscreen. UV intensity increases with altitude because there is less atmosphere above to absorb it — roughly 10 to 12 percent more per 1,000 metres — and snow reflects most of what arrives, so it hits from below too. This is why people burn badly while skiing in cold weather.
Why the sky is blue and sunsets are red. Air molecules scatter short wavelengths far more effectively than long ones — the scattering rate goes roughly as one over the fourth power of the wavelength. Blue light is scattered all over the sky, so the sky looks blue everywhere except directly at the Sun. At sunset the light travels a long slanting path through the atmosphere, so nearly all the blue is scattered out before it reaches you and what is left is red and orange. Dust and smoke lengthen the effect, which is why sunsets are spectacular after a volcanic eruption or over a polluted city — the same physics, an uglier cause.
What the next page covers
We have an atmosphere with a temperature profile and an energy budget. It is not sitting still. Chapter 2.2 explains why the wind blows at all — how uneven heating creates pressure differences, why the Earth's rotation bends every moving thing on the planet to one side, why there are three circulation cells in each hemisphere instead of one, why the great deserts sit at almost exactly the same latitude on every continent, and what the jet stream actually is.