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Q & A: Air Pressure vs. Water Pressure

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Most recent answer: 06/08/2009
Q:
Why don't you feel the pressure exerted by surrounding air like you do in water?
- Nicole Crochet
Chauvin, LA, USA
A:
Nicole -

Well, I'm not sure, but I think the reason that you don't feel air pressing in on you is basically because you're used to it. That sounds sort of strange at first, but if you think about it, it actually makes sense. For as long as people have been around, we've had air around us. The way that our bodies work is that our bodies push out just as much as the air pushes in. If our bodies didn't push out as much as the air pushes in, then we would get squished. That feeling of getting squished is what makes your body think that something is pushing on it.

Water, on the other hand, weighs more than air does. (If this doesn't make sense, try lifting a gallon jug of water compared with a gallon jug of air - the water is a lot heavier.) That means that water pushes harder in general. Well, our bodies can only push out as much as they're used to. And since they're used to air, not water, they don't push out as much as the water pushes in. So it feels like you're being squished. This is why it feels like the water pushes on you.

-Tamara

(published on 10/22/2007)

Follow-Up #1: feeling the pressure

Q:
Hold up a minute. I've been scuba diving and I didn't feel the pressure of the water, now I was only down 90 feet. I would say our bodies are not very sensitive to pressure change that is gradual, but if the air or water change was significant then you would notice it. I'm thinking it's like a frog in slowly warming water. To me what's even more amazing is that in space it's only a vacuum of 12psi (is that correct?) I wish someone could give examples of objects in space versus objects under water. For example: A Tire, underwater starting at 100 feet below water would explode at (x) feet, but a tire (would/not) explode in space?
- John (age 36)
FL, USA
A:
John-  Thanks for the first-hand info. I'm not quite sure what Nicole meant about 'feeling the water pressure'. One of the things you feel in water is just resistance to motion, due to the viscous drag of the water. That doesn't change much with modest depth changes, unlike pressure, so that would fit her experiences and your report that at greater depths you don't feel the pressure change, so long as it's slow.

The vacuum in space is pretty good, so the pressure is about 15 psi below what we're used to here. That means that the pressure change in going up from about 10 m under water to the surface is equivalent to the pressure change of going from the surface to space. The pressure drop of going up a mile (say to Denver) is a bit less than 20%.

Mike W.

(published on 06/08/2009)

Follow-up on this answer.