How does hydrostatic equilibrium explain the variation of atmospheric pressure with altitude?

In hydrostatic equilibrium the upward pressure force from the air below balances the downward weight of the air above it. Because each layer of air must support the weight of the layers above it, the pressure at a given altitude is the sum of the weights of all the air above that point. As you go higher, there is less air above you, so the weight—and therefore the pressure—drops. For example, at sea level the weight of the whole atmosphere above gives about 1013 hPa, but at 10 km altitude the weight of the remaining 10 km of air is only about 250 hPa, showing the pressure decreases with height. Thus hydrostatic equilibrium explains the systematic decline of atmospheric pressure as altitude increases.