Why does air pressure decrease the higher you get in altitude?
Less air molecules in higher altitudes, exerting a smaller force on the layer below. The force is air pressure.
Each layer of atmosphere exerts a force on the layer below it. Due to the weight of the entire air column of air exerting a force on the layer below, the bottom layer becomes compressed.
As the altitude increases, the amount of air above that level becomes smaller and smaller. Therefore, exerting a smaller force of air just below it.
The force becomes air pressure.
Hope this helps :)