What you will learn in Atmosphere: Composition and Structure

1. 1. What air is

   Ten thousand molecules of dry air, counted. Nitrogen and oxygen fill almost all of them, water vapour has to be counted separately, and carbon dioxide gets the few squares its abundance earns — which turns out to be the wrong thing to measure it by.

2. 2. Why there are layers at all

   The four layers are not boundaries somebody drew. Each is where temperature changes direction, and each change has a cause. Remove ozone and the stratosphere is not cooler — it is gone.

3. 3. Pressure with height

   Half the air is below 5.5 km, lower than Everest. Pressure falls by more than six powers of ten on the way to the Kármán line and never reaches zero — which is why the edge of space is a matter of agreement.

4. 4. Ozone: two different problems

   How sunlight makes and unmakes ozone, how chlorine unmakes it faster, and why the hole opens over Antarctica each spring. Then the ozone problem and the greenhouse problem, side by side, which differ in every way that matters.

5. 5. Where the air came from

   Earth has had three atmospheres, and the third was made by life. Then the question of why a planet keeps any air at all: the fastest molecules against the escape velocity, on Earth, Mars and the Moon.