As we go to higher altitudes such as top mountains the atmospheric pressure is found to decrease. The following are the reasons due to which atmospheric pressure decreases at higher altitudes:
(i) Density of air decreases at higher altitudes.
(ii) Height of the air column above us also decreases.
A variation between the altitude and pressure/density is shown in the graph given below.
The graph above shows how both air pressure and air density decrease with increasing altitude.