Transport aircraft cruise where the air is thin, cold, and unforgiving. At FL350 (35,000 ft) the outside air is roughly −54 °C and the barometric pressure is only about 3.5 inHg — about one quarter of the sea-level value. Understanding this environment is the foundation of every other high-altitude topic.
Air is roughly 21% oxygen at every altitude, but what the lungs care about is the partial pressure of oxygen, not the percentage. As ambient pressure falls, the partial pressure of oxygen in the alveoli falls with it, even though the percentage is unchanged. By FL250 the partial pressure has dropped enough that an unpressurized, unsupplemented human is significantly impaired; above FL400 even 100% oxygen at ambient pressure no longer maintains a sea-level-equivalent alveolar oxygen pressure, which is why pressure-demand oxygen and pressurization are mandatory.
The International Standard Atmosphere (ISA) assumes 15 °C and 29.92 inHg at sea level, a lapse rate of about 2 °C per 1,000 ft, and a tropopause near 36,000 ft where temperature stops decreasing and stabilizes around −56.5 °C. The tropopause height varies with latitude — higher over the equator, lower over the poles — and its location matters because the strongest jet streams and much clear air turbulence live near it.
Jets climb high because thin air means low drag, and turbine engines are most efficient at altitude and at high true airspeed. A given indicated airspeed produces a much higher true airspeed (TAS) as you climb: at FL350 a 280-knot indicated airspeed may be 480 knots true. The trade-off is that the speed envelope narrows with altitude until the low-speed buffet and high-speed (Mach) buffet boundaries converge — the coffin corner — covered later in the aerodynamics section.
Thin air also means longer takeoff and climb distances at high-elevation airports, reduced engine thrust, and the need to manage cabin altitude through pressurization. Every high-altitude lesson that follows — oxygen physiology, pressurization, time of useful consciousness, and high-speed aerodynamics — flows from these basic facts about the atmosphere.