The formula is only valid for dry air. For humid air the molecular weight changes.
For the plotted graph the following reasonable assumptions have been made:
Temperature at the ground: 20°C
Temperature decrease rate: - 1°C / 200 m
Temperature at 10000 m: - 30°C
Pressure at the ground: 1000 hPa
It can be seen from the calculated pressures in higher altitudes, that the pressure in 5,500 m is about 500 hPa in 11,000 m about 250 hPa.