Question

Why does oxygen level decreases as we move up the mountain

Answer 1

So we have two rules we put together:

• air is compressible.
• Air has weight.

Putting these two rules together, we see that the weight of air above can squish the air below into a smaller space. The more air there is above you, the more squished the air is around you. This is why the air is literally thicker at sea level than it is on a mountain. There is simply more air above you at sea level.

The air at a certain height is compressed by the air above it.

The pressure of air come from the other air on top of it, weighing it down. The higher you go, the less air is above you, and so there is less weight to keep the air compresssed.