Atmospheric Pressure
Due to the earth’s gravity, this atmosphere is held to the earth. Due to the atmosphere above the surface, the weight of this atmosphere provides an atmospheric pressure of 1 atmosphere = 14.7 lb/in2 at the earth’s surface.
To account for this pressure, the atmosphere, if it were uniform, would have to be about 10 kilometers (6 miles) high. But the atmosphere is not uniform - it decreases in density with height.