The density of air decreases with altitude, meaning that there is less air per unit volume. This is because gravity pulls the air molecules closer to the ground, compressing them into a smaller space. As you climb higher, the weight of the air above decreases, allowing the air molecules to spread out and become less dense.

The air pressure also decreases with altitude. This is because the weight of the air above pushes down on the air below, creating pressure. As you climb higher, there is less air above to push down, so the pressure decreases.

The combination of lower air density and lower air pressure makes the air at the peak of a mountain thinner than the air at sea level. This can make it more difficult to breathe, and can cause altitude sickness.