* Air pressure is caused by the weight of the air above. The higher you go, the less air is above you, and therefore the lower the pressure.

* Mountains are high. This means there is less air above the mountain peaks compared to sea level.

* The atmosphere gets thinner at higher altitudes. The air becomes less dense and less able to exert pressure.

This is why climbers need to be very careful about altitude sickness, which can occur due to the lower oxygen levels at high altitudes.