# Why Does Atmospheric Pressure Change As Debth Changes?

First off, you need to understand what atmospheric pressure is. In a nut shell, it's the ambient pressure (force) that surrounds an object. At sea level, that pressure is approximately 14 pounds per square inch ( one atmosphere). In other words, there's a square inch column of air above the object that has a mass of 14 pounds and that column extends from sea level and up to the point where the earth's atmosphere ends. That entire column weighs in at 14 pounds. That's your starting point.
Water has mass too. More per square inch than air which is why it's ambient pressure changes even more rapidly than air as related to depth below sea level. At sea level, we know that the atmospheric pressure is 1 atm. At a depth of 30 feet, there is now an atmospheric pressure surrounding an object of 2 atm ( 28 psi). One atm accounts for the mass of the actual (gas) atmosphere and one atm is the mass of one square inch of water from a depth of 30 feet to the surface. At 60 feet, atmospheric pressure is 3 atm. At 90 feet it's 4atm. At 120 it's 5atm and so on.
There's a subtle difference between salt versus fresh water because of their different densities, but to keep it simple you can use the above to give an approximate atm at a given depth.
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As you go deeper into the oceans the Atmospheric pressure increases with depth. At the sea level the atmospheric pressure is 760 Pounds per square inch, the atmospheric pressure experienced is in direct correlation with the distance from the center of the earth and the weight of atmospheric air which is equivalent to 10 cars stacked on top of each other at the sea level. When you go into the water you move through a dense environment which is denser than the air at sea level, so it's the weight of the water that imposes that pressure as you go deeper. That is why divers breath a mixture of 96% Nitrogen and 4 % Oxygen in their tanks. The deeper you go the more that pressure increases and require the hulls and glasses of submarines to designed specially so as to cope with increasing force exerted by the weight of the water.
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Anonymous commented
Sorry Ellie, but your answer is incorrect in many aspects. At sea level it's one atm or 14 PSI, not 760. Additionally, the breathing mix that you're quoting isn't what diver's normally breathe. That gas mix will actually kill you since it's what's called hypoxic ( has not enough O2) and it's also so high in Nitrogen that even if you survived it for a few minutes, you'd be so narc'd out of your head you wouldn't realize you were dying.