ELI5: Air weighs 14 pounds per square inch, yet we don't feel it crushing us. The notion that internal pressure somehow acts as a counterbalance just seems to mean that we're being crushed in both directions. Shouldn't we feel this massive weight on us?

ELI5: Air weighs 14 pounds per square inch, yet we don't feel it crushing us. The notion that internal pressure somehow acts as a counterbalance just seems to mean that we're being crushed in both directions. Shouldn't we feel this massive weight on us?




Okay, now ELI4.


You're under an ocean of air. The deeper you go the more that pushes down from directly above you.


That did it!


Username checks out


Does it though?


Okay, now ELI3.


Pressure: Pushing down on me, Pressing down on you, No man ask for.








People on streets




So why doesn't a 1"x1" scale measure 14 pounds of pressure?


When we use scales, we use something called zeroing or tare. Tare is: if you need to weigh something in a container, you 'zero' the scale with the empty container and then add the thing you want to weigh. So... unless you could zero the scale in the vacuum, your scale is already accounting for that 14 lbs of air, you can't zero the scale without any air touching it


the air is also pushing the scale up from the bottom


yeah you're right. to be fair, there are many reasons why you can't "weigh" the air with a regular scale like that


Because the air is pushing the scale down AND up with the same force (and all other directions too). If you sealed the scale so that the air only pushed it on one side (i.e. created a vacuum underneath), it would read 14 lbs.


To be over-detailed, you would also have to seal above the scale and Tare it first... Then open it to pressure and take a measurement...


The air is pressing on the scale from every possible angle, so the downward forces are quality countered by the upward


Put a hat on. At first you might notice the weight, after an hour not so much. Gravity is a hat you’ve been wearing your whole life.


If I take your arm, and press my finger into it really hard, it won't make you weigh more. Still, my finger is exerting a force on you. My finger can only weigh maybe 0.25lb, but when i curl it really hard, it can probably exert 20-30lb of force. Air is exerting that force on us at 14lb/sq inch at all times.


I want to add that psi is force divided by area, so all that air weight is nice a spread over you. Where as a bullet puts all of its force into a small area so it can penetrate you. Also injuries from pressure sprayers can get nasty too


With the bullet, people are often surprised that broadly speaking, both the shooter and shootee experience the same amount of force - one through the stock of the gun into their shoulder, the other through the bullet tearing through them. Simplified obviously (ignoring air resistance, overpenetration etc).


Not quite - the shooter experiences a much lower force because the momentum transfer takes time. This is because of any springs in the gun, the shooter's arm acting like a spring etc, whereas the bullet comes to a stop in less than 1 ms in the person being shot. But the total momentum transfer, F*t, is the same.


Eli4 - thanks for the laugh


Explaining this to people when trying to explain boiling point seems to be even more difficult hahaha I love physics, man.


If it helps, we're already "crushed." Our species evolved on this planet; we evolved "resistance to crushing of 14.7 psi_a" as part of the process, because otherwise our ancestors would've died. With regard to the physics, most of our body is water, which is relatively incompressible, so that's not a hazard. What the barometric/atmosphere pressure *does* push on is the pockets of air in our lungs. Coincidentally, those pockets of air are *also* at about 14.7 psi_a (except we're actively breathing in/out), so the net force of the atmosphere on our lungs is basically zero. This means that there's no crushing at all; both sides being pushed the same means no net force to collapse our lungs. And our bodily structures, as mentioned above, evolved to survive that pressure, so they won't be damaged. Hence, we are crushed into a ball by outside pressure because we have internal pressure to resist it, and our individual cells/parts don't get crushed because they are individually strong enough to resist this pressure.


> most of our body is water, which is relatively incompressible > What the pressure does push on is the pockets of air in our lungs. > those pockets of air are also at about 14.7 psi I think this is the key insight missing from many of the answers. The gasses in our body are also at 14.7 psi because our bodies formed at that pressure. That means we push out the same amount that we are pressed on by the air. When divers dive deep below the water, the pressure around them increases, but so does the pressure of the gasses in their body if they go down slowly enough (to an extent). That’s why it’s so dangerous to raise quickly as a diver; because the gasses in your lungs are under pressure, so when you remove the pressure from the water, the gasses in your lungs (and in your blood, yikes) expand, which puts pressure on the inside of your body. And can cause all sorts of issues Edit: A slight correction, gas pressure in the lungs is only an issue if you hold your breath which you’re trained not to do as a diver. Edit 2: There are several things somewhat wrong about what I said. Look at my replies for well explained corrections.


Go deep enough and jump up, and you too can be as sad as a blobfish


It’s like underwater bungee jumping! Why isn’t this a thing yet?


Because it kills the human.


*taps claw to forehead* Right, right. Of course


Not if you take these anti-pressure pills!


Good news, they're suppository!


That is good news😏


Look at me, I'm Dr. Zoidberg, homeowner!


[r/futurama](https://www.reddit.com/r/futurama/) is leaking...


My favorite kind of leak compared to the other kinds I’ve been experiencing


Bachelor Chow will do that :/


*scratches mane with claw* I think I get it.


Ha, ha. "Claw" is an odd way to refer to your fingernails, *fellow humans*. Ha, ha.


Again, why isn’t it a thing?


I have a few people I would like to nominate to try this out 😏


They're only qualified to do it, as long as they (hypothetically) wouldn't nominate you back (they don't know about this rule). How many people you have left?


Tough qualification rule there. I think I would still make it with the one I have in mind over anyone else. I definitely had to stop to think about it.


Haha, I thought it up because I have some in mind myself, but we have beef so if we both had the chance to nominate people, I'd be screwed also!


And I’m done for the day, thank you. Time to get off the pot and go to work


By the magic of telecommuting, you can take a dump *while* doing work.


*Yes, ma'am. I can load that into your car for you.* *Brrrrt.*


This made my night


Or even better, _do_ pot while doing work...


>Time to get off the pot and go to work Glad to hear you're coming clean. 😂 /s




It is a thing, but more of a reverse skydive. That's how the deep dive record holders do it. Launch down a rail holding their breath as long as they can. Then pull a string that fills a balloon to make them bouyant enough to launch back up to the surface. They do it fast enough that gas compression/decompression inside them isn't as big an issue.


Decompression isnt an issue unless you take air in at depth, if you are doing it on a breath hold you dont get the bends. You will get nitrogen narcosis but not a decompression illness. Also sometimes allegedly your husband forgets to fill the tank on the balloon attached to your sled and you dont come back up. See Audrey Mestr.


> Audrey Mest From Wikipedia: On October 4, 2002, with a dive team under her husband's supervision, she made a practice dive off Bayahibe Beach in the Dominican Republic to a record depth of 166 metres (545 ft). After more deep dive practices, on October 12 she prepared to attempt a dive to 171 metres. On reaching 171 metres she opened the valve on the air tank to inflate the lift bag which would raise her rapidly to the surface but the cylinder had no air in it. A rescue diver arrived and inflated the lift bag with his air supply but the bag did not rise fast enough due to insufficient inflation, a strong current, and the riser rope being non-vertical.\[2\]\[3\]\[4\] A dive that should have been no more than three minutes resulted in her remaining underwater for more than eight and a half minutes. By the time her husband put on scuba gear and dove down to bring her unconscious body to the surface it was too late and she was pronounced dead at a hospital on shore. The dive was controversial and heavily criticised as the setup did not match common freediving safety standards. Much of the diving community critique targeted her husband Ferreras who had hurried an underfunded organisation for this record attempt, previously planned for a later date, notably with too few safety divers, lacking proper rescue equipment, with no doctors at sea and shore. Ferreras was in charge of Mestre's lift bag air tank and did not allow any of the team to check that the tank had been charged. Mestre was not recovered to the surface until nine minutes into her dive, she had a pulse at the surface, but there were no medical doctors available to treat her and minutes were wasted with Ferreras attempting to resuscitate her in the water. An ESPN documentary film written and directed by Alison Ellwood in 2013 had live video footage of the incident and interviews with crew members and staff. ​ \--this sounds like manslaughter straight up, he should be held accountable.


He put on a big show about trying to “save” her. Ive seen documentaries on it it is super messed up.


yeah the whole not letting anyone check her bag thing is all i needed to hear, that’s not what a caring person does


This doesn’t seem wrong to me. If I’ve checked a piece of critical gear that I’m responsible for and someone else touches it, I’ll have to recheck it. What blows my mind is that she didn’t check it herself as part of her own prep. As an example, when diving with Nitrox, this is something you learn to do twice (at the shop and as you put on your tank).


Murder. You mean 1st Degree Murder. This looks premeditated as *hell*


Thank you. Although I haven't dived in decades, I *totally* remember that it's *breathing compressed air at depth* that creates the problems. + Hold your breath coming up and you'll blow your lungs apart from the inside out. + Stay down too long, at any given depth, and a rapid ascent will cause any gases dissolved in your blood -- especially nitrogen -- to come out of solution and take up space in your joints. AKA [The Bends](https://www.emedicinehealth.com/decompression_syndromes_the_bends/symptom.htm). Fun Fact: Divers refer to their gas load in [Feet of Seawater](https://en.wikipedia.org/wiki/Metre_sea_water). 33' == 1 atmosphere.


I just watched a nice video summary of that incident: [https://youtu.be/YVMTWygVl5c?t=338](https://youtu.be/YVMTWygVl5c?t=338) The strange part was that a safety diver was at the bottom with her, and they both had to know that she was as good as dead under those circumstances, but there was no way they could help her, or at least they didn't. I would guess that in retrospect, if they had a do-over, the trade-offs related to her using that diver's air to breathe would seem different since they now know that she really wouldn't survive the problematic failed re-surfacing process. In the video the narrator, who is a former Navy Seal, so at least a somewhat experienced diver, said that it would be problematic for her to breathe that diver's air supply related to problems with it expanding as she returned to the surface. I don't get why breathing for a couple of minutes and then completely exhaling for the return would be a problem, but then I'm not a diver either. It seems possible that "the bends" would be a second factor, that her blood would absorb more dissolved nitrogen during that process, and she might die anyway, unless she could come back up slowly, which isn't a problem for a more typical "no limit free diving" case.


They already said it, blobfish


Could be an interesting execution method. Probably not more humane than the current favorites.


Even imagining 'death by soft tissue expansion' for a couple seconds is too much for me. Poor fish...


Go Byford Dolphin, the only documented case of *actual* explosive decompression.




> That’s why it’s so dangerous to raise quickly as a diver; because the gasses in your lungs are under pressure, so when you remove the pressure from the water, the gasses in your lungs (and in your blood, yikes) expand, which puts pressure on the inside of your body. And can cause all sorts of issues. No, the gas in your lungs expanding isn't an issue at all unless you hold your breath while ascending, which is something they cover in training. What *is* a concern is that gases dissolve better in water (i.e. most of you) at higher pressures. So if you've been down long enough to let that happen, and then the pressure drops quickly (sudden ascent), those gases will form bubbles in your blood and tissue, which is what gives you the bends.


I’ve got a few hundred hours diving. Not an expert by any means. Short of a crazy emergency situation it would be hard to hold your breath enough to matter when ascending. You almost never have a full lung capacity due to keeping neutral buoyancy and your mouth is forced open partially due to the regulator so air tends to “leak” if you aren’t paying attention. You’d have to actively close your throat (right term?) and then still go quite a ways pretty damn fast. Anyway, yeah we are taught to breathe out slowly as you go up. Just figured I’d chime in it would be really hard to screw it up


Depends on the depth, holding your breath from 50m to 40m would still be really bad, the air in your lungs would expand by 20%( I think), but rising from 10m to sea level would make the air double in size and be catastrophic.


No. When it comes to hydrostatic pressure, 10 meters is 10 meters. It doesn't matter how deep you are. The air in your lungs will expand by the exact same amount ascending from 50 meters to 40 meters as it would from 10 meters to sea level. Water is incompressible, so the amount of pressure on your body increases linearly as you descend.


You’re not considering the fact that the pressure increases are cumulative as you descend. 5atm to 4atm would be a 20% increase in gas volume, but 2atm to 1atm would be 50%, even though you only ascended 10m in both cases.


That sounds so horribly painful. Which, I have read enough to know it is...but jeez it sounds horrible.


Sort of. The gases dissolve in your blood at higher pressure, and when you have lower pressure they come out of solution and form bubbles in your blood.


i call those blobbles


Blobbles of death


*Fun* blobbles of death... because it really just happens in situations only a scientist would call "fun".


The pressure at which they come out is equal to that higher pressure. Well, specifically, the partial pressure. So, in a sense, the original statement was not incorrect. This is why we often measure dissolved gases in units of pressure; that is the pressure they exert as they would be exiting!


This is why in scuba diving training they tell you to never hold your breath as you ascend. The gas in your lungs from a deeper pressure will expand as you rise and if it rises too much it can blow your lungs out, literally. This is not a problem for a free diver who holds their breath the entire time. Their lungs will only contract as they go down and when they come back up they will only expand to the original volume


Freediving is the craziest thing. I cannot imagine all the things that could go wrong and how dangerous it is


and we have a problem bringing up deep sea creatures for study, because they cant handle the lower pressures.


In other words, every thing that isnt squished or exploded already, has formed the correct counterbalance of inside and outside pressure, or, in yet another way, everything that became an organism did so because it was able to achieve this balance.


My max depth has only been 115 feet, but even then, I don't feel any difference in pressure. Just in how fast I breathe down my tank.


You felt the difference in your ears on the way down until you did a Valsalva to increase the pressure in your middle ear so that your eardrum wasn't being pushed in by the water pressure.


Very true! I clear my ears so often I wasn't even thinking about it. I was meaning, while at depth, I don't feel that pressure on my skin at all. The water column doesn't feel heavy.


Going up fast won't affect the air in your lungs unless you're also holding your breath, which is why doing that is a massive no-no when diving.


And in fact, deep-sea creatures evolved to be crushed even more. Bring them up to the surface, and they basically turn into a dead mushy mess.


Bring them up quickly enough*. If it is done slowly, they will be preserved for the most part, as they equalize too.


Always wondered if a fish would get hurt if it swam up a column of water held up in the air. I'm talking a column formed from a pipe that had been filled and inverted while under the surface, then tilted upright. I know there would be a vaccum at the top (well, full of water vapor), but I'd imagine near the "surface" of the water would be deadly to them since the pressure is so low it would boil.


This column of water is only 33 feet tall. Since fish are adapted to handle this much pressure change down, I imagine they’d be ok going up as well. Humans can handle the pressure change from 1 to 0 atmospheres, but we can’t breathe. Edit: After looking around a little more, it looks like a rapid pressure change can be an issue even when the pressure changes are less than an atmosphere. This is particularly bad for larvae. I think in the scenario described, where the fish willingly swims up, it's likely to only swim as much as it can comfortably, then adjust and repeat until it gets to the top (like swimmers who pop their ears when they dive).


Depends on the fish, it's usually their swim bladder that causes issues first when coming up quickly since it's a small sack of air. If they can't "burp" their swim bladder they can have problems with it inflating like a balloon and doing internal damage. 30ft can kill some of them.


To expand on the breathing thing: our ability to breathe **relies** on the pressure of the atmosphere. When we breathe in, our lungs expands and create a very slight vacuum relative to the air, and it rushes in to fill the space. Without any pressure, no air would enter no matter how hard we breathed.


This is what it means by the air being "thinner" at higher altitudes. There is less pressure to force the oxygen into your lungs/bloodstream. One of the reasons jet liners have pressurized cabins and mountain climbers use supplemental oxygen.


Well, if there was zero pressure, there'd also be zero air.


... and zero fluids in general




Yeah you mentioned it, but it’s not a coincidence, it’s a necessary condition of survival.


>otherwise our ancestors would've died. Every time I see this I want to be a smartass and point out that our ancestors did, in fact, die. So this is me being a smartass. Sorry for interrupting. Have a wonderful day.




>If it helps, we're already "crushed." See, I knew it.


I'm shocked this is the top comment and still misses the most important component. Say you hold your hand flat out, the pressure below the hand and above the hand is the same which is why it basically cancels itself out. This is the reason a scale will always show you the same weight, no matter the surrounding air-pressure.


I think that the *most* important part that it misses is that feeling this pressure would be constant sensory "noise" coming into the brain, without being of any use. We've very literally evolved to not bother noticing regular air pressure.


It's pressure, not weight. It's not a directed force, it's force on every surface. The air pushing in from above has the same force as the one pushing from below, same for left and right, etc. And yes, we *are* being constantly squished together by air pressure. That pressure is even necessary for life. Water is only liquid at room temperature because the pressure squishes it together, preventing it from boiling.


So water would boil in space?


Yes. Violently boil first, then, giving off a lot of heat by boiling (and a little by radiating), it would freeze quickly. You can even replicate this in a laboratory vacuum.


You can see a demonstration of just that here: https://youtu.be/glLPMXq6yc0


Heat is lost in phase transformation to gas because there's no pressure in space. But the process of freezing isn't as rapid as you think, because radiation is the *only* method of heat transfer in space.


Heat is not only lost in radiation, but - as you say - in phase transition while the liquid water is boiling away. That has a fairly quick cooling effect on the remaining liquid water. You can quickly freeze water even in laboratory vacuum. In our everyday experience, we almost always boil things by increasing its temperature. But if you are boiling things by decreasing pressure, you can quickly cool and even freeze liquids this way.


So why can’t we create a device like a blast chiller that works by creating a vacuum?


That's basically how an air conditioner works. Increase the pressure of a gas on one side, turning it to liquid and creating heat. Remove that heat outside through a radiator with fans, bring it inside and reduce the pressure, turning it back to gas and absorbing heat ("creating" cold).


Right, but that’s within the coils and such. What about creating a vacuum within the food chamber so it cools quicker as well?


The vacuum chamber would boil off some of the water, cooling it, but would also insulate it. Additionally, the rate would be limited by the fact that it stops being a vacuum when the chamber is filled with water vapor, and achieving a very low pressure (near vacuum) is difficult. In short, it would be slower than you expect, dry out whatever you are trying to cool, and stop cooling it when the object runs out of water.


Ok. Thanks for the explanation.


You've sort of described freeze drying: https://science.howstuffworks.com/innovation/edible-innovations/freeze-drying2.htm


That's horrendously energy inefficient. Also, this cooling relies on boiling off material in order to cool. So eventually you will have vaporized your food, leaving only those materials behind which do not become gases at 0psi. And then you don't have cooling anymore. You have absolutely dry food, though.


Gotcha. Thanks.


Inefficient but very useful to be able to dry things without heat damaging the substance. Pharmaceuticals rely on this . Lyopholisation.


You have just described "freeze drying". Every eat something freeze dried (EG Astronaut Ice Cream)? That process is basically done in a partial vacuum where the moisture sublimates away.


Swamp coolers also work by taking advantage of phase changes


Which is why they work best in dry climates such as Arizona and not nearly as well in Florida.


Well, guess what. Blast chillers also work on this principle. They just don't use water, but other refrigerants. Those have better chemical properties to be practical. Read up on the refrigeration cycle, if you are interested. https://en.wikipedia.org/wiki/Heat\_pump\_and\_refrigeration\_cycle


Heat loss on phase change is the principle behind sweat after all


>radiation is the *only* method of heat transfer in space. A lot of people don't realize this. When physicists talk about things being very hot in space, that's an *absolute* term. Earth's thermosphere ranges anywhere from 200°C to **2000° C** yet humans can exist in it without overheating. In fact you're more likely to freeze to death in it. Air molecules are too few and far between to permit effective heat transfer. So you might run into a dozen 2000° molecules and never realize it happened.


I may be misunderstanding. But wouldnt it do the opposite, it would take in heat as it boiled not give off heat?


Boiling is a cooling process. That seems counterintuitive at first, but look at it this way.... What is the temperature of a boiling pot of water? 100 Celsius, right? You have the stove burner on, adding heat, and yet the temperature stays at 100 Celsius. If you turn the burner even higher, it doesn't get any hotter... it just boils faster. Because the act of boiling is where the highest energy particles fly off away, taking their heat energy with them. That is where the heat is lost. Now, pressure adds to how hard those high energy particles have to be pushing to leave. So if you increase pressure,it increases the temperature you need to start boiling. And if you decrease pressure, it lowers that threshold. This is why the boiling point for water is lower in Boulder, Colorado than it is in Fiji. In a vacuum, with no pressure, the minimum boiling temperature of water is so low that it can boil and freeze at the same time. https://youtu.be/VP9wF7ZwmXI


I love how I can learn cool physics-based stuff out of nowhere even nowadays. Thanks for the explanation!


No. When boiling, water molecules turning into vapor are breaking the liquid bond and gaining energy. That energy must come from somewhere. (Remember, there is no Bunsen burner in space underneath your flask, lol.) The energy is coming from the rest of the (still liquid) water. Evaporation always cools the liquid that is evaporating. That's why you feel cold when you get out of the shower.


Gotcha, I was thinking from the steams perspective, not from the water.


This is why when smoking a pork shoulder them temperature will increase steadily before getting stuck (or "stalling") in the 140 degree ballpark for an hour or more. The pork reaches a temperature where the added heat is being cancelled out by water evaporating out of it.




Ah yes, the dreaded “stall”.


Whoa.. I had no idea why that happened


The way it was described to me is that every water molecule has a random amount of energy in that molecule (they constantly bump into each other and trade energy), and of course, the ones with the most energy are the ones that jump off the water, therefore the water that's left behind has less average energy per molecule.


In order for water to boil, it needs to hit a certain temperature at a certain pressure. To achieve this, it will steal temperature from surrounding molecules, so some will evaporate and others will get cooler. This is actually how sweating cools down animals. As those evaporated molecules leave the bubble of water, the bubble is effectively giving off heat


Mind boggling


There's a story I remember hearing in which a guy was in a vacuum chamber testing a spacesuit when his oxygen tube detached. This compromised suit pressure and he lost consciousness very quickly. He claims that the last sensation he remembers before passing out was his own saliva boiling in his mouth.


I legit thought this was gonna be a “suit squeeze” story, SO glad it isn’t, those are horrifying.


What is that? Space suit/dive suit crushing someone?


Yeah, it's a pressure differential issue... when deep diving in a dry suit, they keep the pressure equalized to the outside pressure, which can get VERY high. Blow a hose, and the outside pressure basically crushes you into your helmet. NOT a good day. Also had someone have the same kind of thing happen in a hydroelectric dam, when a diver got his hand a little too close to an intake pipe, and the suction basically took him out of his suit through his sleeve. Again, better days were had.


If anyone wants to learn more about the dangers of… [DELTA P](https://youtu.be/AEtbFm_CjE0)!!


When it’s got you, it’s got you.


I'm familiar with the story but that detail is... a new one


Fun fact that, water only boils at 100 C at sea level. If you live somewhere at higher altitude like in the mountains, when you boil a pot of water it will start boiling at a much lower temperature because of the lower air pressure. If you live in Denver, for instance, your water is actually boiling at 94 C.


It's funny how I've known this fact for a while, but I never fully understood it until it combined with other information from this thread. I feel like that's how a lot of school was too. They tell you things, you say okay and memorize them for the test, but you don't really understand them until a conversation/read years later.


This is a great video of almost exactly that - [water boiling in a vacuum chamber](https://www.youtube.com/watch?v=8LNXoIhH7Eo).


Yes, but you don’t even need to go to space to see evidence of this; even going to high altitudes will change the boiling point of water; that’s why foods have different cooking times or even cooking methods for high altitude cooking.


Apparently you cannot make hot coffee on the Everest, the water boils before it gets really hot.


A quick search says it's 68 °C, which is a lot lower than 100 °C at sea level. Liquid at ~60 °C will burn you if you try to drink it fast, so it is still "hot enough" but there will surely be something different about the taste because some substances will not be soluble at that lower temperature.


One key fact about pressure being exerted throughout the entire surface is what makes the difference in part of the OPs question. So it's not like "being crushed from both sides" the way you might think, since *objects* pushing on you from two directions are still separated from each other (i.e. not pushing directly against each other), but rather both pushing against a third thing in between them. By contrast, your interior pressure is in direct contact with the air pressure, so they just push against each other and nothing moves. With no net force, there's nothing for you to feel.


Your inner ear is a (semi-) closed chamber filled with air so there can be a net force due to imbalance between air pressures in the ear and externally, especially when you've recently changed elevations. But that's the exception to the rule I guess. I'm sure there are plenty of other enclosed spaces in your body with less-flexible walls with nerve endings such that the deformation caused by pressure equalization as the contents of the enclosed space compressed could cause pain, but [it takes over 200 atmospheres of pressure to compress water by 1%](https://www.reddit.com/r/askscience/comments/4xeaiv/how_much_pressure_is_needed_to_compress_water/) and our bodies' enclosed spaces have a high water content, so the deformation is tiny. That's why your eyeballs don't pop out of your head when you get on an airplane I guess. You might feel that.


OK, pressure vs weight...I'd like to know some more. Let's say (just for discussion's sake) that when I stand up, my body weight puts 14 psi pressure on my feet. I definitely feel that weight/pressure, even though I've had all of evolutionary history to get used to it. Why is air weight/pressure of a column of air so different from a column of my body mass? Is it a fluid vs solid thing? (and not to be rude, thanks so much for responding to my very naïve questions ;-) )




What are some reasons that cause your body's psi exerting outward and the air's psi to become imbalanced?


A few random examples I can think of off the top of my head: Change in elevation, you’ll notice a pressure on the inside of your eardrum, until you get lucky and it finally “pops”, then equalizing the pressure. Sinus issues or inflammation in general, creates “pressure” in the body. Uh, if you build up gas in your digestive track, the difference in pressure between your intestinal track and the rest of your body may trigger some bloating / abdominal pain. If you swim to the bottom of a pool, you may notice the increase in pressure (as the water exerts more psi than air does). If I’m having sinus issues, and I do this, it may feel uncomfortable.


while "track" makes sense the word is actually "tract"!


You ever have a head cold or sinus pressure? Not the exact same but you get the idea. Or when you fly and your ears pop, that internal pressure adjusting to external.


Couple of nitpicks. 1) Your feet get *less* than 14psi. My foot is ballpark 9" x 4". That's 36in\^2, times two feet is 72in\^2. I weigh 220lbs so 220lbs divided by 72in\^2 is only 3psi. Now obviously running/jumping exert far more pressure on your feet but the point I wanted to make was that just standing there, your feet see quite a bit less than 14psi. 2) Gravity is an acceleration, not a force. The equilibrium force required to balance that acceleration is called weight, and it scales with the mass of the object. To stop a tennis ball accelerating at 9.8m/s\^2 it takes far less force than stopping a hippopotamus from accelerating at 9.8m/s\^2. Gravity supplies the acceleration, we measure the equilibrium force required to hold that object stationary and call it weight.




> 1) Your feet get less than 14psi. My foot is ballpark 9" x 4". That's 36in^2, times two feet is 72in^2. I weigh 220lbs so 220lbs divided by 72in^2 is only 3psi. Now obviously running/jumping exert far more pressure on your feet but the point I wanted to make was that just standing there, your feet see quite a bit less than 14ps your whole foot generally isnt in contact with the ground(or at least not under pressure), all the pressure is on the balls and heels of your feet


OK, that was an ELI20...but thanks for clarifying things. And my, what big feet you have... ;-)


Gravity is one of the fundamental forces. Weight is the force exerted by gravity, and since it scales linearly with mass in a constant gravitational field all objects in free fall experience the same acceleration. We generally measure weight by holding things in static equilibrium and measuring the normal force, but strictly speaking weight is independent of acceleration in a reference frame stationary relative to the gravitational body. It's true that acceleration and gravitational field strength are impossible to tell apart without outside information, but gravity is as much a force as anything else


>Is it a fluid vs solid thing? Exactly. Solids stay in shape under force, while gases don't. You can push on a pole without the pole getting wider, but you can't do that with a gas. Just imagine what would happen if the ground was liquid: you would sink, and not feel force on your feet. You can actually try that out. It's called scuba diving.


Think of how you feel in a strong breeze.. that is caused a tiny unbalanced difference in air pressure. When you’re in a room without a breeze, the air pressure on you is largely balanced and you can’t feel it.


There are two things here to think about: 1) our insides are at the same pressure as our outsides (and are mostly incompressible for that matter). In other words we are at equilibrium with the air around us. 2) what makes us _feel_ the sensation of crushing is our bodies getting twisted around or deformed. This is also how we perceive something has been crushed....its physical form has changed. Because we are at equilibrium (point 1) we don't perceive any crushing (point 2). But why are we at equilibrium? Well it's important that pressure comes from all available directions. Imagine a water balloon. If you poke it with your finger, it would go inward where you poked it. If you put it between two flat plates and squished, it would compress outward through the gap between the plates. If you surrounded it completely except for one hole and squeezed, it would push outward through the hole. All that is deformation and it's "crushing"...if the balloon had nerves it would feel it, and you can see it happen. But if you surrounded the balloon completely with no gaps and pressed. It's made of water, so it wouldn't go inward really because water isn't very compressible. It'd remain the same shape. If it had nerves it wouldn't really feel it because there would be no change in the shape of the balloon to detect. The balloon doesn't have any gaps to be pushed into, so it doesn't change shape, so it doesn't get "squished"




**Your submission has been removed for the following reason(s):** Top level comments (i.e. comments that are direct replies to the main thread) are reserved for explanations to the OP or follow up on topic questions. Joke only comments, while allowed elsewhere in the thread, may not exist at the top level. If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message={url}%0A%0AThe%20concept%20I%20want%20explained:%0A%0ALink%20to%20your%20search%20for%20past%20posts%20on%20the%20ELI5%20subreddit:%0A%0AHow%20is%20this%20post%20unique:) and we will review your submission.


you know what fair enough


No, because it’s normal. When you feel pressure, it’s *abnormal* pressure, like someone sitting on your leg or something. If we evolved with more or less pressure, we would notice the difference on earth. Think of it like diving deep in a pool or lake; the deeper you go, the more it hurts because of the pressure. On the contrary, look at the blobfish. They’re so “blobby” because we see them in lower pressure environments than they’re used to. Their bodies can’t hold their shape because the pressure normally holds them together. As a result, they just sag everywhere.


Bonus question: a naked man on the moon would appear fatter because of the lack of air pressure?


Not sure about you, but I feel the consistent weight of unmet childhood expectations crushing me.


I was gonna say lol… i constantly feel crushed.


You ever put a cup over your lips and started to suck out the air? You know that tingling, pulling, vacuum feeling? That's what it feels like when their isn't pressure there to "crush" you. Air pressure is like a nice comfy heavy blanket we keep wrapped around us at all times. Take it away and you are exposed, and it... sucks (air guns).


One point I haven't seen yet is bouyancy. The 14 pounds of pressure on us isn't weighing us down, we're actually *slightly* lighter than we would be in a vacuum. Think about floating in water, the dense water pushes us up instead of crushing is down to the bottom. Air is also a fluid, but it's less dense than us so we sink to the bottom of this huge air ocean we're in. It still actually makes us lighter As far as not noticing it/feeling comfortable with it, our bodies feel a difference in relative pressure. We notice this in our ears during a pressure change (air travel, swimming) because trapped gasses push against our eardrums. However, once we "clear" our ears we no longer feel it until the pressure changes again. The rest of us, for the most part, is made up of water which is incompressible. If we're in a 50psi environment (scuba diving), our body is also around 50psi so we don't feel any pressure difference. There's no force pushing on our body to feel. Atmospheric pressure sensors are the same way, they're measuring the difference between vacuum and the pressure around it. If those sensors didn't contain a vacuum, they would be unable to measure or "feel" the pressure, you can only measure relative fluid pressure. Typical atmospheric air can only be consumed between 10-40 psi. However, if special air mixtures are used the body can actually handle a much wider range without any issues or discomfort, provided pressure is changed slowly enough. Space EVA suits are kept as low as 4.9 psi and the deepest scuba dive record is currently 1090 feet which is 472 psi of water pressure. Our bodies can handle even more than that. We can't survive at pressures where liquids don't stay liquid anymore and gasses don't stay gasses.


Thanks...I like the buoyancy angle...!


14 lb of pressure is being applied to every square inch of your body from every direction 24/7. Your body evolved in that environment. If you were to add 14 lb to a square inch of your arm, you would feel it. But also, if you were to apply a complete vacuum to a square inch of your arm, you would feel a similar difference in pressure (though structurally different by anatomy). Thing is, there's no such thing as "vacuum force", and the most vacuum you can apply is 0, or 14 psi below normal. It's the lack of normal air pressure that feels weird, or damaging. One of the reasons being exposed to outer space is dangerous is because air in your system is supposed to be pushed in from all directions at 14 psi. Without it, that air expands... and kills you. Air is always trying to expand. Air is weighed down by gravity which keeps it on our planet. Air at the Earths surface is compressed by the 14 lb of air per square inch above it which gives us our standard, life-supporting pressure.


No, an entire cubic meter of air only weighs about 3 pounds (1.3kg) on earth’s surface. You are just feeling the pressure from all the air above you going up to space. That doesn’t push down, it pushes in all directions equally. So it squishes you.


You *do* feel that pressure crushing you - you feel it 24/7, and you're accustomed to it, so you don't even notice it. Since all life on the planet (or on the surface, anyway) has evolved under that pressure, all life on the planet is adapted to live under that pressure.


So you're saying you feel it but you don't feel it. Got it.


Not true, you can't physically feel pressure, only a difference in pressure. If you're in a plane at 10ish psi, so is your body and you can't tell once your ears are equalized. Deep sea diving with 100 psi of water around you? You still don't feel anything.


Then how do you explain Paramore's song *I can feel the pressure*?




Jump into Jupiter, and indeed you won't feel the pressure because the pressure will have crushed you into a tiny little flesh ball


Sounds like a dream


So one part of this question that the other answers don't really explain is where the weight from the air goes. Gravity pulls down the air, so surely you'd have a big column of heavy air. Right? What actually happens is that the gravity tries to squish down the air, but the air would rather spread out than squish, so it tries to go sideways, but sideways is more air also trying to squish down. So in a sense, the downwards force of gravity gets converted into an omnidirectional (every direction) force felt as pressure - instead of pushing in any direction, the air gets squished down at high pressure and pushes on every direction. This is what creates the pressure that other answers have discussed.


> Air weighs 14 pounds per square inch This is just a non-sense statement with no meaning. Work out the units. It is like saying "gasoline costs $5 per degree Fahrenheit". Not exactly "incorrect" but wildly off from the core idea you are trying to explore here. This all comes down to the physics concept of work and energy, but that is right on the edge of a one paragraph ELI5 treatment. Imagine placing a book on a tabletop. The book weighs one pound and has an area of one square foot. Gravity causes the book to push down on the table with one pound per square foot (force per area is known as pressure). The book is pushing down on the table, but nothing really happens. The table does not collapse, nor even move. From Newton's laws, since nothing is moving (accelerating) that means the whole system is in balance. We know the book is pushing down with 1 PSI, so that means the table is pushing upwards with 1PSI also to keep things balanced. Everything is steady and the table does not deed to "work" to keep the book up. Similarly, our meat-bag bodies have about 14 pounds per square inch always pushing down on us, because of the miles and miles of atmosphere above our heads. The atmosphere is the book and our face is the table... It is all in balance so nothing really happens, BUT, if you were to remove the external pressure by say exposing your body to the vacuum of space, that balance would no longer be in place and the internal pressure of your body would balloon outward without the confining force of the air pushing it back in. You would not be happy to find yourself in that situation.




Air doesn't weigh 14 pounds per square inch. I've been reading comments and haven't once found that clarification. In fact, air weighs about 2,6 pounds per cubic meter. So that's quite a different number. What you are referring to is pressure. I.e. Force per area. The problem here is the god damn Imperial system using pounds as if it were a force, when it's really a mass. No wonder you guys get things confused. 14 pounds per square inch is equivalent to the pressure that you get when you stack 14 pounds of weights onto a surface as small as one square inch. Pressure is what you feel on your ear drums when you dive or even when you drive through a tunnel. I think a good way to visualize it is this: Imagine a pillar of air one inch wide from the ground reaching all the way up to the edge of the atmosphere. This pillar holds around 14 pounds of air. That's how much this pillar weighs. Now stack those next to one another until you fill in the entire surface of the globe. That's why the air pressure down here is 14psi. Why is the pressure also pressing sideways when gravity only pulls air down? Air - much like water - wants to flow and level out. The air at the button of the pillar is being squeezed out from underneath and wants to move sideways. That's where the sideways forces come from.


We do feel it. It'd feel different if you found yourself in a vaccuum, don't you think? That being said, your body is mostly sensitive to changes in pressure. If the amount of pressure on you stays exactly the same for a long time you'll naturally stop noticing it. It becomes the "default" state and not worth any amount of your attention. This reminds me of people saying water has no taste. Are you sure about that? Is pouring water on your hand the same as pouring it on your tongue?


We are mostly liquid, including our nerve cells, with which we feel stuff. Liquid attains the same pressure as the gas in contact with it. If the pressure is the same everywhere - inside the nerve cells, outside them etc. - then there is no way for us to feel anything.




The key is very much that the air inside us is also at 14.7 psi. Hold your breath and go to the deep end of the pool or go in an airplane and don't pop your ears and you'll very quickly experience discomfort. Water pressure demonstrates this even faster because every 33 ft of water is another 14.7 psi. Scuba divers can comfortably go down well over 100ft though because the air they are breathing is equalized with the pressure of the water. The incompressible parts of our body don't much care and the airspaces are filled with air that is at the same pressure as the surrounding pressure so it isn't crushed. You can also think about it like how a baloon isn't crushed. The air in the balloon pushes out as much as the air pushing in and that makes everything at equilibrium.


In all honesty, we do feel it. But we are "built" to withstand it. For the feeling lost though, our minds just ignore it. Like the sounds of clock ticking is only loud when it's quiet, our brain calibrated itself to ignore it. If not, we would be overwhelmed and always "feel" the pressure 24/7, which I think it's rather morbid. Look at muscular dystrophy, when muscles are too weak, you cant breath normally and requires medical equipment, just because the muscles to breath is incapable of exerting the correct force to overcome the standard pressure.


Do you not feel the weight?


Your nerves are calibrated to be useful. Your body would not evolve to send useless signals constantly about the state of air pressure. That is a massive waste of resources. The same way if you make a scale to measure the weight of something you place on top of it, you would want to calibrate it to ignore its own weight. Otherwise you do not have a useful sensor.


Which gets me thinking. If you took a regular bathroom scale, put it in a vacuum chamber, and pumped out the air, would the scale read a negative weight, since the 1r pounds over every inch is no longer there?