What Is the Air Pressure (in Millibars) at the Top of Everest?

Air pressure at the summit of Mount Everest averages roughly 331 millibars over the course of a year, but the number climbers actually encounter during the popular May and October windows is a bit higher, typically 333 to 337 millibars. That is about a third of the standard sea-level pressure of 1,013 millibars. The figure is not fixed, though. It shifts with the seasons, swings day to day with passing weather systems, and has even been creeping upward over the decades as the climate warms.

How the Summit Pressure Was Actually Measured

For a long time, the scientific community had exactly one direct barometric reading from Everest’s summit: 253 Torr, recorded in October 1981 during the American Medical Research Expedition to Everest. That single data point, converted to millibars, works out to about 337 millibars. Despite being just one measurement, it became the benchmark that researchers used for years to model summit conditions and estimate how much oxygen a climber could extract from the air up there. In May 1997, a second direct measurement came within about 1 Torr of that original reading, and weather balloon data collected at the same time agreed closely. The conclusion from both measurement campaigns was that on the days when climbers typically reach the summit, during the pre-monsoon window in May and the post-monsoon window in October, pressure sits in a narrow band of 251 to 253 Torr, or about 335 to 337 millibars.¹

More recent work using longer weather-balloon records and reanalysis data puts the climbing-season average slightly lower, at about 333 millibars, with very little spread from day to day during good weather.² The small gap between 333 and 337 millibars reflects the difference between a broad seasonal average and the cherry-picked calm days when people actually stand on top. Both numbers are valid; which one you quote depends on whether you mean “the average May day at summit altitude” or “the day a climber is likely to summit.”

Why Pressure Swings With the Seasons

Everest’s summit pressure is not constant because the atmosphere itself is not a uniform blanket. The troposphere, the layer of air that contains most of our weather, is thicker over the equator and thinner toward the poles. Everest sits close enough to the tropics (about 28°N latitude) that it benefits from a relatively tall column of warm air overhead during the Northern Hemisphere summer. In winter, that column contracts and cools, compressing less air above the peak.

Weather balloon data show that summit-level pressure is roughly 15 millibars higher in midsummer than in midwinter, a swing of about 11.5 Torr.³ That sounds modest compared to sea-level weather variations, but at these extreme altitudes, where every millibar translates to meaningful differences in available oxygen, it matters enormously. Midwinter pressure on Everest drops to around 324 millibars, which is equivalent to being on a mountain roughly 200 to 300 meters taller in terms of the oxygen your lungs can actually use.²

On top of the seasonal cycle, day-to-day weather adds its own turbulence. The passage of large-scale weather systems through the Himalayan region brings significant swings in pressure, temperature, and wind speed, especially during the pre- and post-monsoon climbing periods.⁴ A deep low-pressure trough rolling through can temporarily knock several millibars off the summit pressure and push conditions from survivable to lethal for climbers already above 8,000 meters.

Why Textbook Atmosphere Models Get It Wrong

You might expect that you could just plug Everest’s altitude into an atmospheric model and get the summit pressure. The problem is that the most commonly used models, such as the International Standard Atmosphere, systematically underestimate the pressure at very high altitudes. They were designed for mid-latitude aviation and assume a temperature profile that does not account for the warmer tropical and subtropical air columns found near the equator. Because Everest sits at a relatively low latitude, the air above it is warmer than these models assume, and warmer air means a slower drop-off in pressure with altitude.⁵

The practical consequence is that standard-atmosphere calculations predict a summit pressure well below what is actually measured. If climbers and physiologists relied on those models, they would conclude that unassisted ascent is even more impossible than it already nearly is. The fact that the real summit pressure is several millibars higher than the textbook figure is one of the reasons human beings can reach the top of Everest without supplemental oxygen at all. A few extra millibars of pressure at this altitude translates directly into a few extra molecules of oxygen per breath, and at the edge of survivability, those molecules make the difference.

What a Third of Sea-Level Pressure Means for Your Body

At sea level, each breath delivers enough oxygen to keep your blood well saturated. At the summit of Everest, the air contains the same fraction of oxygen (about 21 percent), but the total air pressure pushing those molecules into your lungs is only a third as strong. That means each breath delivers roughly a third of the oxygen you would get at sea level. The body responds with dramatic compensations: the heart beats faster, breathing rate skyrockets, and over weeks of acclimatization, the blood produces extra red cells to carry whatever oxygen is available.

Even with full acclimatization, the numbers are startling. Arterial blood gas samples taken from climbers at 8,400 meters, where the barometric pressure was about 363 millibars, showed an average oxygen pressure in the blood of just 24.6 mmHg, with some individuals as low as 19.1 mmHg.⁶ For reference, in a hospital setting, blood oxygen that low would constitute a medical emergency. The oxygen content of arterial blood at that altitude was about a quarter less than at 7,100 meters, a camp just 1,300 meters lower. Data from the 1981 expedition suggested that at the summit itself, arterial oxygen pressure drops below 30 Torr and the body’s ability to use oxygen maxes out at about one liter per minute, far below what most people use during a brisk walk.⁷

This is why the summit of Everest sits right at the boundary of human survival. A few millibars less and the body could not extract enough oxygen to keep consciousness, let alone climb.

The Pressure Window for Climbing Without Bottled Oxygen

Between 1979 and 2019, more than 10,000 successful ascents of Everest were recorded, but only about 208, or roughly two percent, were made without supplemental oxygen. The vast majority of those oxygenless summits, over 80 percent, were achieved in May, with October accounting for another 11 percent. Both months have summit pressures above the annual mean about three-quarters of the time.⁸

Climbers going without bottled oxygen do not just pick the right month; they pick the right day within that month. The average summit pressure across all oxygenless ascents was about 335 millibars, roughly 4 millibars above the long-term annual mean of 331 millibars. Within May and October, successful oxygenless summit days tended to fall around the 70th to 80th percentile of pressure for their respective months, meaning these climbers were reaching the top on days with better-than-average air pressure for the time of year.⁸

The consequence of getting unlucky with weather is quantifiable. Reconstructions of summit pressure over four decades show that aerobic capacity at the summit can swing dramatically depending on the barometric conditions. The worst reconstructed low-pressure event, during a deep trough in February 1993, would have cut a climber’s maximum oxygen uptake by about 24 percent compared with the best high-pressure event recorded in August 2010. In practical terms, that translates to a climbing speed reduction of roughly 40 percent, an enormous handicap at an altitude where even healthy, acclimatized people move at a crawl.⁸

Climate Change Is Slowly Raising the Pressure

An unexpected side effect of global warming is that Everest’s summit pressure has been gradually rising. As the troposphere warms, it expands vertically, lifting the pressure surfaces upward. For a fixed point like a mountain summit, this means slightly more air overhead and therefore slightly higher barometric pressure. Research has argued that this warming is of sufficient magnitude to make the mountain, over time, incrementally easier to climb from a physiological standpoint, though the effect is slow enough that it would not be noticeable to any individual climber on any given day.⁹

Climate model projections estimate that the effect is strongest in winter, when summit pressure is at its lowest. Minimum annual summit pressure is expected to increase by roughly 2.5 millibars per degree Celsius of global warming, while mean and maximum pressures rise by about 2.0 to 2.2 millibars per degree.⁸ The practical upshot is that the most extreme low-pressure events on the summit are becoming less extreme. This does not mean Everest is becoming easy; 333 millibars is still brutally thin air. But the lowest troughs, the days when the mountain is most unforgiving, are being lifted ever so slightly, potentially widening the physiological margins for future climbers.

How Everest Compares to K2

K2, the world’s second-highest peak at 8,611 meters, is often called the more dangerous mountain, but its summit sits about 237 meters lower than Everest’s. That altitude gap translates to a meaningful pressure difference. During climbing season, K2’s summit averages about 347 millibars compared to Everest’s 333, a difference of 14 millibars.² That extra pressure gives climbers on K2 a substantially better oxygen supply. Estimated maximum oxygen uptake on K2’s summit during climbing season is roughly 10 percent higher than on Everest’s, and climbing speed is estimated at about 223 meters per hour versus 190.

In midwinter, the gap narrows. K2’s summit pressure drops to about 326 millibars, while Everest’s falls to 324. The two mountains become much more similar in terms of barometric severity, even though their altitudes remain the same. What changes is the wind: midwinter winds on Everest average about 41 meters per second, roughly 50 percent stronger than K2’s 27 meters per second.² K2’s winter danger comes more from its technical terrain and avalanche-prone slopes, while Everest’s winter challenge is overwhelmingly about the combination of low pressure and ferocious wind.

Animals That Fly Through Everest-Level Air

Humans are essentially at their physiological limit at 337 millibars, yet bar-headed geese routinely fly at altitudes near and above the summit of Everest during their migration over the Himalayas. These birds manage this feat through several biological advantages working together. Their hemoglobin binds oxygen more tightly than that of lowland birds, which means their blood picks up more oxygen from each breath even when the pressure is extremely low. They can also hyperventilate far more aggressively than mammals without suffering the brain blood-flow problems that hyperventilation causes in humans. When mammals breathe too fast at altitude, the resulting drop in blood carbon dioxide causes blood vessels in the brain to constrict, reducing oxygen delivery to the organ that needs it most. In bar-headed geese, this constriction does not happen, so they maintain robust oxygen flow to the brain even in severely thin air.¹⁰

The contrast is striking. Humans at 335 millibars can barely walk and sometimes cannot think clearly. A bar-headed goose at the same pressure is sustaining powered flight, one of the most oxygen-demanding activities in the animal kingdom. The same barometric reading that represents an absolute frontier for our species is, for these birds, just another piece of the commute.