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My main query is why the air pressure acting upwards would be greater than the water pressure downwards. Doesn’t hydrostatics say the pressure of the topmost layer of water in the glass has to be atmospheric and subsequently the water at the bottom of the glass would be greater than atmospheric pressure?

Yaku gebiljebak
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A little decrease in the air pressure inside the cup is sufficient to make the external (atmospheric) pressure big enough to keep everything in place. Water is cohesive so when the cup is made upside-down just enough water gets out of the glass sticking to the card (due to gravity) which drops the internal pressure. This imbalance is sufficient to keep the water from getting on the floor.

Doesn’t hydrostatics say the pressure of the topmost layer of water in the glass has to be atmospheric and subsequently the water at the bottom of the glass would be greater than atmospheric pressure?

Water is more or less incompressible so let's consider it to be like that. Hence after the magic is performed, the pressure at the topmost layer of water will be less than atmospheric pressure because the air pressure at the surface of that layer will be lower.

user8718165
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  • Thanks. Is my initial assumption that the pressure of water at the top of the glass would be close to atmospheric? I don’t really understand the dynamic when there is a rigid container, isn’t this container ‘holding’ up the weight of air above the glass? – Yaku gebiljebak May 30 '19 at 03:26
  • @Yakugebiljebak I edited my answer...please have a look – user8718165 May 30 '19 at 03:42