The atmosphere is a thin layer of air on the surface of the Earth.

Air is a GAS and its molecules are always moving. When the molecules COLLIDE with the surface of the Earth, an atmospheric PRESSURE is created.

The atmosphere gets LESS DENSE as the HEIGHT above the ground (altitude) increases.

This means that the air MOLECULES are FURTHER APART which leads to FEWER COLLISIONS with an object.

This means that as height INCREASES, atmospheric PRESSURE DECREASES.