Answer:
Atmospheric pressure is defined as the total weight of air over a unit area at a given height. As altitude increases, the amount of air decreases which decreases the weight of air over a unit area. Hence, atmospheric pressure decreases.
In other words, atmospheric pressure is the amount of force exerted on an area by the air above it due to the gravitational force of the earth. The gravitational force decreases on the increment of height because gravitational force is inversely proportional to the height which results in atmospheric pressure to decrease.
