Table 1.
Monthly Air Pressure Statistics and Successful Mt. Everest Ascents Made without Supplemental Oxygen (1979–2019)
| Month | Summits | Summits (% of Total) | Mean Air Pressure (hPa) | Air Pressure Range (hPa) | Exceeding Annual Mean (%)a | Exceeding Climbing Mean (%)b | Mean Climbing Air Pressurec | Mean Climbing Air Pressure Percentiled |
|---|---|---|---|---|---|---|---|---|
| Jan | 0 | 0 | 323 | 26 | 2.3 | 0.1 | – | – |
| Feb | 0 | 0 | 323 | 27 | 1.5 | 0.1 | – | – |
| Mar | 0 | 0 | 325 | 23 | 3.8 | 0.2 | – | – |
| Apr | 4 | 1.9 | 329 | 21 | 19.6 | 1.5 | 330 | 64.5 |
| May | 170 | 81.7 | 333 | 17 | 78.6 | 27.1 | 335 | 70.8 |
| Jun | 2 | 1.0 | 337 | 14 | 99.8 | 92.3 | 334 | 4.9 |
| Jul | 0 | 0.0 | 339 | 8 | 100.0 | 100.0 | – | – |
| Aug | 3 | 1.4 | 339 | 8 | 100.0 | 100.0 | 339 | 54.0 |
| Sep | 5 | 2.4 | 338 | 13 | 99.9 | 96.2 | 338 | 33.5 |
| Oct | 23 | 11.1 | 333 | 17 | 73.8 | 32.5 | 336 | 79.3 |
| Nov | 0 | 0.0 | 329 | 20 | 25.4 | 2.7 | – | – |
| Dec | 1 | 0.5 | 326 | 27 | 9.0 | 0.8 | 330.3 | 86.1 |
a
The percentage of hours within the month when air pressure is above the long-term mean (331 hPa).
b
The percentage of hours in the month when summit air pressure is above the mean across all oxygenless ascents (335 hPa).
c
The mean summit air pressure during all successful oxygenless ascents made in the respective month.
d
The corresponding percentile of the mean in c with respect to all summit air pressures in that month.