A fluid can be either a liquid or gas. The pressure in fluids causes a force normal to any surface.
Pressure is the amount of force per unit area, measured in pascals (Pa).
In an atmosphere, pressure decreases as altitude (height) increases.
Air molecules colliding with a surface create atmospheric pressure.
The number of air molecules above a surface decreases as the height of the surface above ground level increases. So as height increases there is always less air above a surface than there is at a lower height.
