Due to increase in the  altitude of the atmosphere , atmospheric pressure decreases as we go higher up above the earth's surface.Atmospheric pressure is the pressure applied at a point due to the weight of the air above that point. As we go up, the altitude increases and the amount and the density of air above it decreases. So the pressure due to it decreases
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Air has weight and due to gravity most of the air remains close to the ground. As a result air exerts pressure on us due to its weight. This we call atmospheric pressure. The atmospheric pressure at the sea level is 9.8N per square inch. As we go higher and higher, the air becomes less denser and so the pressure on high altitudes is also less.