* Air Pressure and Gravity: Air pressure is caused by the weight of the air above a certain point. Gravity pulls air molecules down towards the Earth's surface.
* Less Air Above: At higher altitudes, there's less air above you. Imagine a column of air extending from the top of the mountain to the top of the atmosphere. The column at the top of the mountain is much shorter than the column at the base, meaning there's less air pressing down on the top of the mountain.
* Density Decreases: As you go higher in altitude, the air becomes less dense. This is because the air molecules spread out more due to the reduced pressure.
In summary: The greater the weight of the air column above a point, the higher the atmospheric pressure. Since there's less air above the top of a mountain, the pressure is lower.
