Sorry if this is considered a duplicate, but I have further questions that are based on an old post. I was told that air pressure is strong enough to hold up water. Is this true? The vapour pressure of water at room temperature is 0.0313 atm which is far smaller than air pressure. But air pressure is 101325 N/m$^2$ and a tall column of liquid water with a surface area of 0.001m$^2$ and height of 1000m is 997 kg. This translates into a net gravitational force of 9771 N or 9771000 N/m$^2$ which is greater than that of air pressure in this contrived example.
Also, in the experiment of inverting a cup with water where there is a flat sheet cover on the opening of the cup, does the flat layer simply suppress amplitude oscillations of the water at the interface between the water and layer and thus stop Rayleigh-Taylor instabilities from growing? Does the sheet have to be perfectly flat? What is stopping the sheet from falling down? Are there important water-surface interactions that keep the surface from falling? Is this valid for any liquid (e.g. oil) and any surface (e.g. denser than air) that is not porous? Is it still consider the Rayleigh-Taylor instability once the evolution become nonlinear?