Atmospheric pressure reduce with altitude for two reasons-both related to gravity.

• The gravitation attraction (g) between the earth and air molecule is greater for those molecules nearer to earth than those further away – they have more weight – dragging them close together and increasing the pressure (force per unit area) between them.
• Molecules further away from the earth have less weight (because gravitational attraction is less) but they are also ‘standing’ on the molecules below them, causing compression. The lower down have to support more molecules above them and are further compressed (pressurized) in the process.

Note:  It is gravitational force minus the effect of the Earth’s spin (an effect that is greatest at the equator)

DO YOU KNOW?

When air is sucked through a straw with its other end dipped in a liquid, the air pressure in the straw decreases. This causes the atmospheric pressure to push the liquid up the straw.