The Pressure Effect

Posted by Jeremy Windsor on Jun 11, 2021

The summit of Mt Everest currently stands at 8850m above sea level. But sometimes it doesn't feel like that height. Why? The bottom line is that the human body isn't really interested in the altitude it's standing at. Instead what it really focuses upon is the pressure of the surrounding air. As we climb, the number of gas molecules in the air starts to decline. As their numbers shrink, so does the number of times they collide. The force that these collisions create generates an atmospheric pressure. Molecules such as oxygen only move into the body if there's a difference in pressure between the atmosphere and the body's cells. When there's a large difference, like that seen at sea level, oxygen flies into the lungs, binds rapidly to haemoglobin and crashes into the cells. However on the summit of Everest, where the atmospheric pressure is approximately one third of that at sea level, movement is agonisingly slow and cells are often starved of oxygen. 

New research published in iScience reveals that the atmospheric pressure on the slopes of Mt Everest varies significantly. The data supporting these findings was drawn from a series of measurements taken at automatic weather stations installed on the slopes of Mt Everest in 2019. 

In 2019 a series of 5 automatic weather stations (AWS's) were installed in Nepal's Sagarmartha National Park - Phortse (3816m), Mt Everest Base Camp (5315m), Camp 2 (6464m), South Col (7975m) and The Balcony (8434m). Further details can be found here

The fluctuations in atmospheric pressure were the result of changes to the ambient temperature. When the air is cold, molecules slow down and make fewer collisions. Meanwhile in warmer conditions, they move more rapidly, collide more frequently and generate a greater atmospheric pressure. Therefore it's not surprising to hear that the lowest measurement was recorded at the South Col of Mt Everest in January 2020 (35.8KPa) and the highest in June 2019 (38.7KPa).

Barometric pressure measurements taken at the South Col (7975m) on Mt Everest between May 22nd 2019 and August 1st 2020. The units on the y axis are hPa which can be converted to KPa by dividing by a factor of 10. Sea level barometric pressure is approximately 101.1KPa

What impact does this have upon the mountaineer? The answer is that the altitude "felt" by the body can vary enormously. Extrapolating the results from the AWS measurements at the South Col and The Balcony, the researchers were able to show that the extremes of atmospheric pressure measurements on the summit (8850m) were comparable to mountains that ranged from 8649m to 9387m in height - a difference of 738m!

To illustrate how different barometric pressure measurements effect mountaineers on the summit of Mt Everest it's worth spending a minute thinking about the impact this has on an individual's VO2 max. VO2 max is commonly used to help us understand the aerobic fitness of an individual. The fitter we are, the more oxygen we are able to utilise during physical activity. Typically, most active men and women have a VO2 max of between 30 and 50 mls of oxygen per kilogram per minute (mls/kg/min). As we climb, our VO2 max falls. The reasons for this aren't clear - but are likely to centre around a series of barriers that limit the passage of oxygen into the cells at altitude. On the summit of Mt Everest, a VO2 max is believed to be less than half of it's sea level value. To climb up a flight of stairs you need to be capable of utilising approximately 14 mls/kg/min. That's pretty close to the VO2 max of the mountaineer on the summit. Therefore, a small fall in barometric pressure will push up the height of the mountain and cause the VO2 max to fall further still. If this drops to a value that's close to what is needed to exercise, movement will slow to a snail's pace and any chance of reaching the summit will disappear.

An analysis of data taken between 1979 and 2019 showed that changes in barometric pressure produce significant fluctuations in VO2 max from -18.8% (February 1993) to +6.9% (August 2010). What do these changes in VO2 max mean in practise? In February 1993, a mountaineer would walk 41.2% slower than in August 2010!

But what of the future? The study shows that the effect of the climate emergency is now starting to be felt on the summit of Mt Everest. With an increase in global temperature, a rise in barometric pressure has been clearly observed. The study data shows that this equates to a rise of approximately 0.04KPa for each decade between 1979 and 2019. The authors argue, that even if we meet the Paris Climate Agreement and restrict global warming to 2 degrees C, barometric pressure looks set to rise on the summit by 0.4 to 0.5KPa compared to pre-industrial times. At the summit, this would result in a 3.3% increase in VO2 max. Further rises would be inevitable if global warming was to continue. As the authors state in their conclusion, "this interesting consequence may be a powerful means to engage the wider public in climate change...".

The barometric pressure measurements in 1979-1988 (blue) compared to those in 2010-2019 (red).The difference is easiest seen in the monsoon months (June to September) when there is much less variability in the recorded dataThe units on the x axis are hPa which can be converted into KPa by dividing by a factor of 10. Sea level barometric pressure is approximately 101.1KPa

We spoke to lead author Dr Tom Matthews from Loughborough University about the study and it's implications. Here's what he told us...

Thanks Tom for speaking to us. Could we start by talking a little about the history of barometric pressure measurement on Mt Everest. Where did it start and what was known before your research was published?

I think the earliest continuous measurements of air pressure in the Everest region were made by the 1924 British Mt Everest Expedition. Intriguingly, those observations (made at the Base Camp on the north side) showed that air pressure dropped dramatically during Mallory and Irvine's ill-fated summit attempt, with the weather at high altitude perhaps not dissimilar to that endured by the infamous 1996 ("Into Thin Air") storm. 

The first spot measurement of air pressure at the summit was, I believe, taken on the 1981 American Research Expedition to Everest. Weather stations deployed at (or very near to) the South Col have measured air pressure fluctuations during short-lived and infrequent deployments since 1996. 

The installation of the AWS's in 2019 were an incredible achievement. Where do you think the data from them fits into what we already know about the mountain?

Thank you. It was only possible with the help of a fantastic Sherpa team from Phortse. Panuru Sherpa, our very experienced Sidar, was particularly important in making that effort a success. He and his team worked with Baker Perry and I to understand the unusual requirements and demands of building weather stations so high on Mt. Everest. 

Those weather stations add detail to the previous air pressure measurements, enabling much greater insight into the fine-scale fluctuations of air pressure that are so important for human physiology. But the weather stations provide much more than that. They monitor many other variables and that help expand our understanding of high-altitude Everest meteorology - including providing insight into how well weather forecasts perform, and how sensitive the region's glaciers may be to climate warming.


The first winter ascent of Mt Everest was by Krzysztof Wielicki (left) and Leszek Cichy (right) on the 17th February 1980. The mean atmospheric pressure in February is 1KPa lower than that found in the popular pre-monsoon climbing season (May-June). This, together with the challenges of low ambient temperature (-40 degrees C) and high winds (140km/hour) make Krzysztof and Leszek's climb of the mountain all the more remarkable. A fascinating account of their ground breaking efforts can be found here

The data highlights some dramatic fluctuations in barometric pressure. They clearly make a good case for the benefits of accurate weather forecasting. Did these results surprise you?

The scale of the pressure fluctuations in winter did surprise me. When you look in detail it's evident that, if you get lucky, you could pick a winter day that feels (from an oxygen availability point of view) very much like a typical spring day when most summit climbs take place. On the other hand, if you get unlucky, reaching the summit could be near impossible. I think this is an important point for those trying to claim a winter ascent without supplemental oxygen. That extremely difficult challenge can be made considerably easier (perhaps making it "possible") - by picking the right day to attempt the final summit push.

Fluctuations in the summit atmospheric pressure were found to be much greatest in the winter months (December to February - a 2KPa fluctuation) compared to the summer (July to September - a 0.6KPa fluctuation) Tom and his team noted a dramatic fluctuation of 2 KPa in just 5 days between the 8th and 13th February 2020. This could have enormous implications for a winter ascent without supplemental oxygen!


As you write, seeing the impact of the climate emergency upon Mt Everest has the potential to capture the public's attention. Have you seen the impact of climate change upon other extreme environments?

Yes. Unfortunately I have. I spent much of my PhD on and around glaciers in the Arctic, including in Iceland where the glaciers are retreating faster than almost anywhere else on Earth. Glacier retreat is an extremely vivid illustration of climate change - you can't fail to notice it. Everest doesn't escape. For example, the Khumbu Glacier has thinned by approximately 100m since the 1960's and you can see this very clearly from the glacier's trim lines on the valley sides (like a water mark marking the former level of a draining bathtub). You can also see it in the elevation of Base Camp, with hand-held GPS units showing heights well below the 5300 m or more often quoted (and spray-painted on boulders). Base Camp has lowered as the ice below has melted. Those departing for the summit therefore now have a slightly longer journey than they did several decades ago - maybe that offsets the slight increase in oxygen availability with climate change.

Thanks Tom for contributing to this blog post!

Further information about the impact of the global emergency upon the mountain environment can be found here.

If you would like to find out more about mountain medicine why not join the British Mountain Medicine Society? See this link for details.


The Birmingham Medical Research Expeditionary Society (BMRES) and the British Mountain Medicine Society (BMMS) have joined forces to organise the 2021 Altitude Research Conference. The face-to-face event will take place in Birmingham on the 11th September. Speakers will include Peter Bartsch, Jo Bradwell and Chris Imray. There will also be presentations from members of the UK's leading research groups as well as ample opportunity for researchers, young and old, to present posters and short talks about their work.

Further details can be found here.

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