eli5, why is there less oxygen at higher altitudes?
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Oxygen is a gas. Alongside nitrogen gas and a bit of CO2 and argon, it makes up our atmosphere.
These gases have weight. The ones higher up are bearing down on the ones closer to the ground. The ones down here are being squished by all of that weight above them.
Because they’re being squished, they are denser. More of the gas fits into the same space – more of the gas fits into your lungs.
Air has mass (weight) and unlike liquids, air is compressible, meaning you can squeeze more of it into a smaller area.
So all of the air around the planet is just stacked on top of other air, and all that weight compresses the bottom layers to make it more dense.
There’s quite literally more air at sea level than there is at 10,000 feet because of this compression. If you filled a jar with sea level air, and another with 10,000 feet air, despite being the same size, and containing the same thing, the sea level jar would weight slightly more because there’s more air squeezed into that space.
So, following that same logic, there’s more oxygen at sea level because there’s more air.
Atmospheric air consists of the same proportion of oxygen at all altitudes. But because air density and pressure decrease with altitude, not only does every air contain less oxygen per unit of volume, but the gas pressure which forces oxygen to diffuse into your blood is also less.
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