So imagine you are in a body of water: e.g a bath. You place a clear container in the water, like a jug, and fill it up. When you then invert the container and slowly rise it above the surface, why does the water stay inside the container until the container breaks the surface?
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Because all the air is pressing down on the water in the bathtub in a way that it isn’t pressing on the air inside the jug. Until the pressure gravity pulls on the water in the jug is stronger than that atmospheric pressure, it’ll stay put inside the jug.
The athmosphere(the air around the world) has weight and it is constantly a pressure on us. We don’t notice it because we got used to it. When you put a container in water the pressure that pushes the water down will make the water go up in the container until the air in the container gets squeezed enough and pushes the water back and they find a balance.
If the water lowered, it would dramatically lower the pressure inside the container, effectively sucking the water up. The water outside the container is under one atmosphere of pressure, and that pressure applies to the entire volume of water, so the water in the container is is under one atmosphere of pressure too, and so is the air in the container. If you pull the container up faster than the water can rise, you’d have the same amount of air but a bigger volume – a lower pressure, which the water will shove up into until it equalizes
Notwithstanding the fact that water would boil away, if our bathtub was in a vacuum and we raised the up turned vessel, the water it contains would not rise, and space (more vacuum) would form above it as the vessel rises.
Why?
Because no force exists to move the water.
Water in a regular bathtub is being acted upon by the atmosphere. If you weigh a 1 inch by 1 inch column of atmosphere that extends from Earth’s surface to space, you would find it weighs about 15 pounds.
We don’t feel this weight because our bodies are flexible and this pressure acts on all sides of (and within) our bodies.
But in a rigid vessel with an orifice, if you try to draw a liquid out leaving nothing but vacuum behind, the liquid must act against atmospheric pressure that tries to fill this vacuum.
When you turn a bottle upside down to drain it, the weight of the liquid pulls it down while air pressure pushes in. Water falls around the air as it finds a path down, creating a bubble which move up (basically water falling around a pocket of air).
If the air pressure cannot break the surface tension of the water to enter the vessel, then water stays in the vessel. You can see this if you seal a glass of water with a sheet of paper, then invert it. The paper stays in place and the water stays in the glass. Air presses in on the paper, sealing it against the glass. Bubbles only form when a gap between the paper and glass becomes big enough to allow air pressure to break water’s surface tension and enter the glass.
In the bath tub example, if the weight of the water is less than the air pressure exerted on an area equivalent to the size of the orifice, then it stays in the vessel. If the water weighs more, it falls and a vacuum forms above it.
A great example is a really long drinking straw. If you apply a vacuum to the end of a drinking straw, then extend it up from the water surface in the bath tub, at a length of about 30 feet water in the straw would stop rising and the space above would be a vacuum. At this point, water in the straw weights the same as a column of air of the same width as the straw. Air pressure is insufficient to press more water up into the straw.
A good way to intuitively feel this is to take a syringe (like is used to administer children’s medicine) and seal your finger over the opening. You find you need some force to draw the plunger, which creates a (near) vacuum inside the syringe.
The force you feel is the weight of the air pressing back against the plunger. The wider the plunger, the greater the force. This would be the same amount of force trying to push water into the syringe had we inverted it in a bath tub and raised it, as in your original question.
If the water were to start falling out of the jug the moment the water level in the jug met the water level outside of the jug there would be nothing to fill the void left behind the water. Nature abhors a vacuum and won’t allow a low-pressure bubble to form behind the water when it can keep that area filled with water instead. The weight of the water in the jug is supported by the water in the bath and won’t fall out until you break the surface and atmospheric pressure isn’t enough to hold that much water in place against vacuum potential.
That’s also the reason pouring water out of certain containers with small mouths glug. As the water falls out air has to push its way in to fill the vacuum forming behind what you’re pouring out.
In some ways it’s a form of hydraulics, applying a small force over a large surface area to get a large amount of force over a small surface area. The bath has a lot of surface area with small atmospheric force pushing on all out and that provides enough force when the container’s opening is smaller relative to that surface area that it can resist the pull of gravity on the amount of water above it in the container.
There is air inside the cup. Where would the air go?
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