Air particles create atmospheric pressure

1. Earth is surrounded by an atmosphere, which is a layer of gases held around the planet by gravity.
2. Air particles are in constant random motion and repeatedly collide with exposed surfaces.
3. Each collision changes a particle's momentum, so the particle exerts a small force on the surface.
4. The combined normal force from many collisions acting on each unit area is atmospheric pressure.
5. Atmospheric pressure acts in all directions because air particles move and collide in all directions.

Gravity creates a pressure gradient

1. Gravity pulls every air particle towards Earth, so the particles have weight.
2. Air near Earth's surface supports the weight of the air above it.
3. This weight compresses the lower layers of the atmosphere, pushing their particles closer together.
4. The lower atmosphere therefore contains more particles in each cubic metre and has a greater density.
5. A denser layer produces more particle collisions with each square metre of surface each second.
6. More frequent collisions produce a greater total force per unit area, so atmospheric pressure is greatest near Earth's surface.

Pressure decreases as height increases

1. At a greater height, there is a shorter column of atmosphere above a surface.
2. The shorter column contains less air, so the weight of air pressing on the layers below is smaller.
3. With less compression, the air particles are farther apart and the air density is lower.
4. Fewer particles are available to strike each unit area, so collisions occur less frequently.
5. The total force per unit area is therefore smaller, which means atmospheric pressure decreases with height.
6. The decrease is not uniform because air density also changes continuously with height.

The atmosphere thins gradually

1. The atmosphere has no sharp upper edge at which pressure suddenly becomes zero.
2. Air becomes progressively less dense with height because the weight and compression from overlying air progressively decrease.
3. Although gravity becomes slightly weaker with height, it still acts strongly enough over ordinary mountain and aircraft heights to keep pulling air towards Earth.

Pressure changes have visible effects

1. A sealed flexible packet taken to a high altitude can expand because the external atmospheric pressure decreases while its internal pressure initially remains higher.
2. A flexible container moved to lower altitude can be compressed because the external atmospheric pressure increases.
3. A barometer gives a lower reading at greater height because the atmosphere exerts a smaller pressure.
4. Aircraft cabins are pressurised so the pressure inside does not fall to the much lower atmospheric pressure outside at cruising height.