Question about atmosphere pressure

[bolbol2054]

we define the ATM. pressure as the weight of cylinder of air above the Earth with surface area for its base equal 1 m2 referring to this definition how could we illustrate the pressure at
1- a room with roof .
2- a room without roof .
3- vacuum room.

in case of 1 why will it be mistake to think that the roof will carry most of weight of the cylinder
is it a misconception of pressure ?

 

[vlado_skopsko]

Air occupies every whole. Pressure depends of the height of the of the fluid above the place we messure, so there are equipotential surfaces that have same pressure that affects all the liquid (air) on that height. Unless the room is isolated from the surroundings the pressure inside will be the same as the pressure outside. Air is simular with water so maybe that will help you to imagine the situation better.
By the way the pressure does not depend on the area, the force does.