Question

why atmospheric pressure decreases with altitude

Answer 1

Pressure with Height: pressure decreases with increasing altitude. The pressure at any level in the atmosphere may be interpreted as the total weight of the air above a unit area at any elevation. At higher elevations, there are fewer air molecules above a given surface than a similar surface at lower levels.

Answer 2

Atmospheric pressure decreases with altitude as when we go up or we increase our altitude the percentage of air in the Atmosphere decreases and now since the amount of air decreases the pressure also decreases