Question

Why does atmospheric pressure decreases with the increase in altitude?

Answer 1

The pressure at any point on the earth's surface that is due to the weight of the column of air above it; it therefore decreases as altitude increases.

Answer 2

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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.