Fun fact: it does fall down ! Everything falls down due to gravity. Think of the atmosphere as another ocean, the fluid dynamics you can see in water apply to all the gases around us too.
What we consider air and oxygen are floating in the gas soup at ground level and they have specific densities which causes them to separate… i.e. when you climb a mountain oxygen levels go DOWN!
Thanks for the explanation, would love me some gas soup
Open wide. You got some right now.
It also has waves of current and densities and temperatures, we call this the weather.
A current can be just a current, you don’t have to call it a wave. Oceans have currents too, they are not waves either.
eh, not really. mountains aren’t high up enough for this to matter, there’s less oxygen at mountaintops because there’s less air there in general (less air pressure). you need to go all the way to the top of atmosphere for this effect to matter
for example, if you release helium or hydrogen, it will eventually float up to the top of atmosphere and because it’s so light, it moves fast and at some point it will reach escape velocity and drift away into space
When they said oxygen levels go down, they presumably meant that the partial pressure goes down. That’s what people probably think of anyway since that necessitates supplemental oxygen (if you are high enough).
The supplemental oxygen is needed because the total pressure is down, not because of gas separation. Humans need some number of moles of oxygen per minute, and at the top of Everest they just cannot ingest that number fast enough because the total pressure is so low. The percentage of oxygen in the total gas mixture is also lower on Everest than the 21% at sea level due to the aforementioned specific density separation, but that effect is much smaller and insignificant compared to the total pressure shortfall. This is why the grandparent comment objected to mentioning it - this is an interesting fact on its own, but irrelevant to the question of supplemental oxygen on Everest.
Yeah, on rereading I see that I originally missed the specific mention of densities causing them to separate (instead of just “less oxygen”), so I now agree with the objection.
I can’t help but think the effects are related. Isn’t low air presure in the mountains caused by gravity making air fall down?
yeah but the composition of air at the mountaintop is still pretty much the same as at sea level
Yeah, sure.
It does, but as more air drops down, the pressure increases. This pressure then starts to push back against the air above it. Which is why we have atmospheric pressure at the surface, but that goes down to pretty much 0 in space.
Even in low earth orbit there are still some particles, which causes satellites and such to slow down, requiring them to fire some thrusters every once in a while.
Cool, thanks!
Follow up question: are there different densities in space?
Yes. Space includes planets, so the gravity wells of the planets, create different densities.
You said space, and not a perfect vacuum. So a vacuum in theory has no density. But there can be some elements so far apart they act as they effectively have no density.
According to this physics.stackexchange.com answer:
"I suppose the surprising thing is why the atmosphere doesn’t all fall immediately to the Earth’s surface to form a thin dense layer of air molecules.
The reason this doesn’t happen is that air molecules are all whizzing around at surprisingly high speeds - typically hundreds of metres per second depending on the temperature.
The air molecules bash into each other and knock each other around, and the air molecules near the ground bash into the air molecules above them and stop them falling down."
Detailed explanation from another answer:
"The key ingredient is temperature.
If it were zero then all the air would indeed just fall down to the ground (actually, this is a simplification I’ll address later).
As you increase the temperature the atoms of the ground will start to wiggle more and they’ll start to kick the air molecules giving them non-zero average height.
So the atmosphere would move a little off the ground. The bigger the temperature is the higher the atmosphere will reach.
Note: there are number of assumptions above that simplify the picture. They are not that important but I want to provide a complete picture:
1, Even at the zero temperature the molecules would wiggle a little because of quantum mechanics
2, The atmosphere would freeze at some point (like 50K) so under that temperature it would just lie on the ground
3, I assumed that the ground and the atmosphere have the same temperature because they are in the thermal equilibrium; in reality their temperatures can differ a little because of additional slow heat-transfer processes."
This is the way! It helps me to imagine what would it look like if the atmosphere consisted of a single nitrogen molecule. You place it on the ground but the ground has temperature (is warm) so your one molecule gets launched up into the vacuum on a parabolic trajectory at 500 m/s on average. If it launched at 45° it would reach 6km up and fall down, at 90° - 12km up - and that’s on average. Some would get launched faster and higher (following the long tail of the Boltzmann distribution), and hydrogen and helium even faster still because they are lighter. A few hydrogen molecules would be launched at speed above 11km/s, which is above Earth’s escape velocity, so they would escape and never fall down.
When you have many air molecules, they hit each other on the way up (and down), but because their collisions must be perfectly elastic, mathematically it works out that the overall velocities are preserved. So when your one nitrogen molecule gets launched up but on its way hits another identical molecule, you can think of them equivalently as passing through each other without colliding at all. (Yes, mathematically they can also scatter in some other random directions, but the important part is that your original molecule is equally likely to be boosted further upwards as opposed to impeded.)
The end result is that majority of the atmosphere stays below 12km, density goes down as you go up though never quite reaching zero, and hydrogen and helium continuously escape to space to the point none are left.
That is absolutely correct. The atmosphere is held in place, so to speak, by gravity. It is also spinning along with the earth. Some of the atmosphere is lost to outer space at a regular rate and again replenished by natural processes that are beyond the scope of your question.
What’s more interesting is the atmosphere is pressurized at a decreasing gradient the closer it gets to outer space thereby relinquishing the idea that a container is necessary to house all of our atmosphere. (I watch a lot of flat earth videos for funzies.)
Uhhh, the earth isn’t flat, just in case.
I genuinely laughed at the last sentence hahaha you silly silly person, of course it’s flat
I love that the answer is basically “Yes, it does, but the other air molecules get in the way.”
