Pressure standing wave nodes at the end of the open side of

[crick]

I do not understand why standing sound waves can be formed in a one-side or two-side open tube. Consider a one-side open tube.

In particular how does the reflection of the wave at the open end occur? I found the following explanation.

The sound wave is a pressure wave and at the open end of the tube the pressure must be fixed to the atmosferic pressure and cannot vary, thus there is a node of pressure there.

I do not get why the pressure at the open end cannot vary from the atmospheric pressure.

Furthermore, if the outside pressure does not change, the sound wave (a pressure wave) is somehow trapped in the tube and do not exit from it. Nevertheless for example in a clarinet the sound wave do exit and propagate from the tube, I guess. Otherwise how could the sound be heard?

What is the reason why there must be pressure waves nodes at the open end of a tube? (As in picture, in (b))

 

[Simon Bridge]

Have you seen:
https://www.physicsforums.com/threads/how-does-stationary-sound-waves-in-open-tubes-happen.564184/
http://www.exo.net/~pauld/summer_institute/summer_day11sound/Reflections_on_an_open_end.html