Tukuk-OSHalf the mass of the atmosphere sits below just 5.5 kilometres, and 90% below about 16 km: the blanket of air is, relative to Earth’s size, thinner than skin on an apple. The first floor, the troposphere, holds almost all weather and water vapour because almost all the mass and the heat engine (the surface) live there. Above it, temperature does something counter-intuitive: it stops falling and rises, because ozone is busy absorbing sunlight and warming the layer — this stable, warm floor is the stratosphere, home of jets’ smooth rides and of the protective ozone layer.
Higher still the air grows cold again (mesosphere, the coldest place on Earth, where meteors burn) and then astronomically hot in the thermosphere — thousands of degrees, yet a thermometer there would freeze, because there is almost no matter to touch it with: “temperature” is average energy per molecule, not a feeling, and the exosphere simply fades into space around the Kármán line, the conventional 100-km edge of space. Astronauts do not float because there is no gravity — Earth’s pull at the ISS is still about 90% as strong as on the ground — they float because they are perpetually falling around the planet.
Everything that keeps you alive sits in the bottom 16 km of that stack: weather, ozone shielding, jets. And that is exactly why what we put into the air matters so directly: the atmosphere is thin, and it mixes globally in a year or two, which turns a local emission into a planetary ingredient.