As altitude increases, how does air pressure change? Why?

The air pressure decreases as altitude increases because. The pressure of air at any level can be interpreted as the total weight of the air above a unit area of any elevation. At higher altitudes, there are fewer air molecules above a given surface than a similar surface at lower altitudes. Also, dense gases in the atmosphere are present in a higher quantity at lower altitudes, as the are denser and gravity pulls them down, thereby decreasing the air pressure at higher altitudes further.