How does atmospheric pressure change with altitude?
Atmospheric pressure decreases as altitude increases.
You can think of atmospheric pressure in the same way you would think of ocean pressure: the deeper you dive, the greater the pressure because more water is pushing down on you.
At sea level, you have the weight of all the air above pushing down on you. Thus as you increase your altitude, the air gets thinner and there is less air pushing down on you (resulting in lower atmospheric pressure).