Everything in this subject happens in the lowest few dozen kilometres of air, and almost all of it in the lowest ten. This first lesson maps that territory: the layers of the atmosphere up to FL 650 (flight level 650, the syllabus ceiling), the tropopause that divides the weather from the calm, and the temperature — its definition, its units, and the steady fall with height that organises the whole structure.
The layered atmosphere
The atmosphere is divided vertically by what the temperature does: each layer is defined by its temperature behaviour, and the boundaries are where that behaviour changes. Up to FL 650 an aircraft can meet two layers and the boundary between them.
Troposphere — the bottom layer, where temperature falls with height. It holds roughly three quarters of the atmosphere's mass, nearly all its water vapour, and therefore practically all its weather: clouds, precipitation, thunderstorms and most turbulence live here.
Tropopause — not a layer but the boundary above it: the level where the temperature stops falling. It acts as a lid on the weather below.
Stratosphere — the layer above, reaching well beyond FL 650: dry, stable air in which the temperature at first stays constant with height and then, higher up, slowly rises, because ozone there absorbs the sun's ultraviolet radiation and warms the air around it.
The dry air of the troposphere is a remarkably constant mixture: about 78 per cent nitrogen, 21 per cent oxygen, and roughly 1 per cent argon with traces of carbon dioxide and other gases: the same proportions at the surface as at the tropopause, because the constant stirring of the layer keeps it mixed. The one ingredient that varies is water vapour, anywhere from almost none to a few per cent — and that variable few per cent is what the rest of this subject is about.
The stratosphere holds one hazard worth naming early: ozone. Its concentration peaks well above airline traffic, but ozone can occur at jet cruise altitudes, especially at high latitudes in late winter and spring where the tropopause is low, and it is an irritant gas, harmful to lungs and airways, which is why aircraft that cruise high carry catalytic converters in their air-conditioning supply to destroy it.