Scientists talk about the amount of air around
us in terms of something called "air
pressure". Air pressure refers to the
trillions and trillions of air molecules that
are pushing down on you right now. When you
are at sea level, there are more air molecules
pushing down on you than when you are at the top
of Mount Everest, which is 29,029 feet above sea
level. More air pressure at sea level means the
air is more dense, so the molecules are packed
closed together and the air feels thicker than
when you're on top of a mountain, where the air
will feel thin.
Think of it in terms of stacking a bunch of
pillows on top of each other. If you stack a
million pillows in a column, the pillow at the
base is going to feel a lot of weight on it so
it's going to flatten out and become denser, while
a pillow in the middle of the column isn't going
to have a much weight on it so it won't be as
dense. The feathers in the pillow at the base are
going to be all squished together, while the
feathers in the pillow in the middle are going to
have some space between them. Those feathers are
just like air molecules.
The higher up you go
in altitude, the less air pressure there is, so
the air molecules aren't as squished together and
the air feels thinner. This is why it's harder
to breathe at the top of a mountain - there is
simply less oxygen around for you to breath in!
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