At what altitude is the air density approximately 50% of its value at sea level under International Standard Atmosphere (ISA) conditions?
Air density is approximately half of the mean sea level value at 22 000 ft and approximately a quarter of the mean sea level value at 40 000 ft.
The ISA atmospheric pressure levels, flight levels and the corresponding altitudes and air densities from highest altitude to ground level:
| Pressure Level (hPa) | Flight Level | Altitude (ft) | Altitude (m) | Air density (kg/m3) |
| 50 hPa | FL600 | 60 000 ft | 18 288 m | 0.003 kg/m3 |
| 150 hPa | FL450 | 45 000 ft | 13 720 m | 0.238 kg/m3 |
| 200 hPa | FL390 | 39 000 ft | 11 890 m | 0.317 kg/m3 |
| 250 hPa | FL340 | 34 000 ft | 10 360 m | 0.395 kg/m3 |
| 300 hPa | FL300 | 30 000 ft | 9150 m | 0.459 kg/m3 |
| 400 hPa | FL240 | 24 000 ft | 7315 m | 0.569 kg/m3 |
| 500 hPa | FL180 | 18 000 ft | 5500 m | 0.698 kg/m3 |
| 600 hPa | FL140 | 14 000 ft | 4270 m | 0.796 kg/m3 |
| 700 hPa | FL100 | 10 000 ft | 3050 m | 0.904 kg/m3 |
| 850 hPa | FL050 | 5000 ft | 1525 m | 1.055 kg/m3 |
| 1013 hPa | - | MSL (0 ft) | MSL (0 m) | 1.225 kg/m3 |
Note: Do not confuse density and air pressure. Air pressure is approximately half of the MSL value at 18 000 ft.
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