Question

Why is atmospheric pressure at sea level higher than on top of a very high mountain?

Answer 1

Pressure=force/area
Under the sea you have million tons of water crushing into you

On a high mountain the atmosphere is thin and therefore less pressure

Answer 2

gravity


air pressure is due to change in momentum of air particles as they bash into whatever surface is feeling the pressure. if there are fewer collisions (or the collisions involve slower moving particles), the pressure will be lower.

gravity pulls the atmosphere towards earth (and the pull is greater the closer you get to the earth though that, i think, is of second order importance). so there should be more particles in a unit volume of air at earth's surface and thus more collisions per unit time, hence more pressure.

in addition, when air rises from the earth's surface, it is typically displaced by more dense (colder) air, again due to the relative gravitational pull on both, so the less dense air will travel further up in the atmosphere.