How does air Exert Pressure

The molecules of several gases make up air. The molecules are constantly interacting with one another and with the walls of the container. When a molecule collides with a wall, it causes a little amount of force to be exerted on the wall. The gas exerts pressure as a result of the sum of these contact forces. The higher the pressure, the more particles that impact the walls.

While air pressure can refer to the pressure of air within a limited space (such as a car tyre or a football), atmospheric pressure refers to the pressure exerted by air molecules above a certain point in the Earth’s atmosphere. 

Due to the weight of air particles above, the atmospheric pressure rises as we travel closer to the Earth. This is why air pressure is lower at the top of a mountain than it is at sea level.