Our atmosphere has a composition of 78% Nitrogen,
21% Oxygen, 1% others (H2O, Carbon Dioxide, Argon…)
regardless Altitude.
For civil aviation, we mostly concern about Troposphere, Tropopause and Stratosphere.
Troposphere - Maintain Environment Lapse Rate ELR 2⁰C/1000ft
Tropopause (a line to separate Troposphere and Stratosphere) -On ISA condition (average 36,050 ft @ -56.5⁰C)
Tropopause is cooler at the equator, and warmer at the pole.
Stratosphere
-is consider Isothermal (same Temperature as altitude increases), then temperature increases as Alt increase until Stratospause
-Water vapor condensation is insignificant. Therefore, only slim chance of weather or cloud formation at stratosphere.