Why does the air get thinner as the altitude increase?​

As altitude rises, air pressure drops. ... As altitude increases, the amount of gas molecules in the air decreases—the air becomes less dense than air nearer to sea level. This is what meteorologists and mountaineers mean by "thin air." Thin air exerts less pressure than air at a lower altitude.

Explanation:

as altitude rises, the air pressure drops.

the amount of gas molecules in the air decreases- the air becomes less dense then air next to the sea.