terry writes:

You see the fact that the volume of the atmophere or of space is
infinite is quite irrelvant because nobody wants to know what the
average density of the whole atmophere is ( which of course will
approach zero depending on your definition of where the atmsophere
actually ends).

It is very highly relevant. If you increase the temperature of the
atmosphere, for example, the pressure does not rise, because nothing
constrains the atmosphere--it simply expands. Atmospheric pressure comes from
gravity, which is a constant, and not from any constraints applied to the
volume of air, of which there are none. In the highest portions of the
atmosphere, the temperature rises to several thousand degrees, but the
pressure remains extremely low. At the surface, you might see variations in
absolute temperature of 1/3, but you won't see variations in pressure anywhere
near that magnitude.