Because there are fewer gas molecules at higher altitudes.
Atmospheric pressure is simply the force with which the molecules of gas that make up the air push down on a surface area.
Let's say that you start at sea level. If you draw a square on the ground of
The more molecules of gas you have above your
More specifically, the force I'm referring to is actually the weight of the molecules.
If you move this
That happens because there are fewer molecules of gas at higher altitudes. Once again, gravity is to "blame" here.
Most of the gas molecules are held close to the surface of the Earth by gravity. As you move away from the surface, you will find fewer and fewer molecules of gas, which means that the force with which they can press down on a surface decreases with increasing altitude.
Think of it like this - if fewer molecules are above your