* Atmospheric Pressure: Air pressure decreases with altitude. This is because the weight of the air above pushes down on the air below. As you go higher, there's less air above you, so the pressure is lower.

* Density and Pressure: Air density is directly related to pressure. Higher pressure means air molecules are packed closer together, resulting in higher density.

* Temperature: Temperature also plays a role, but it's more complex. Generally, temperature decreases with altitude, which would lead to lower density. However, the pressure effect is usually stronger, so the overall effect is lower density at higher altitudes.

In summary: The decrease in atmospheric pressure with altitude causes the air density to be greater at the bottom of a mountain compared to the top.