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Feb27-12, 01:48 AM
P: 482
Quote Quote by DaveC426913 View Post
If it intersects the axis then that means at one point there is pressure, and at a point an arbitrarily-small distance away the pressure is zero - no air. That's a boundary.
Its not a hard boundary because for arbitrarily-small distances the corresponding pressure differences would be arbitrarily-small to.

Quote Quote by DaveC426913 View Post
Most us can intuitively grasp that this is not the way the atmo tapers off.
Trying to understand physics intuitively is sometimes not a good idea. An adiabatic atmosphere would taper off this way and it is more realistic than the isothermal case described by the exponential curve without boundary. Explaining why the real atmosphere has no upper limit is not as easy as you seem to believe. The pressure gradient depends on the temperature profile and that's a quite complex topic in the upper atmosphere.