Hi, this is my first post.

I was told by a trekking group leader that air pressure (and hence available oxygen and required acclimatization time) is not always the same for a given altitude, and can vary from one mountain to the next. I was talking about El Plomo in Chile (which is 5,400m) and I was suggesting to him that a 3-day trek, which is what he is planning, is too fast for me given that trekkers start in Santiago Chile. That is about 500m, the actual walking starts at about 2000-3000m however for acclimatization purposes most trekkers wake up the first day at about 500m.

I've always thought of air pressure as fairly constant at a given altitude with just a 1-3% variation for temperature (this can be calculated) and perhaps another 1-3% variation for weather conditions on the day (that's more of a guess, but assuming that air pressue variations due to daily weather fronts are similar to sea level).

But he said that the air pressure and oxygen amount at the summit of El Plomo is usually actually higher than some mountains at a lower altitude (4.000-5000m peaks).

Can that be true? Perhaps due to air currents and air flows which is what he seemed to be hinting at?

If he's right, then it means that the amount of oxygen on some mountains might be at least 5-10% higher or lower than other mountains at the same altitude - and all the time, not because of weather from day to day or temperature changes.

Due to convection, the troposphere is much thicker at the equator than it is at the poles (I believe it's around 16-17 km thick in the case of the former and less than 10 in the latter).

At sea level this doesn't make a difference, although the poles are usually dominated by high pressure and the equator by low pressure, so you might get slightly more oxygen at sea level near the poles. However, as you can imagine, when you go up in elevation, you pass through more of the troposphere's blanket of air at the poles than you do near the equator. The higher you go, the greater the difference will be.

Having said all that, I wouldn't think that there would be a big difference between air at 2000-3000m in Chile vs the same elevation near the Canada-U.S. border. You only start to see a big difference at very high latitudes and elevation e.g. Denali's summit vs the same elevation on Everest.

Useful Info Wardenac I did not think of the equator vs poles difference.

K

Thanks for this. On pressure variation with latitude and temperature (related factors):

"Pressure altitude is effected by latitude-dependent variations in the acceleration due to gravity at sea level. The contribution is sufficiently minor to be neglected in the climbing arena." http://www.cohp.org/ak/notes/pressure_altitude_v6.html

I found this: "So the further north or south you go from the equator, the lower the air pressure will be at a given altitude." Source: https://8kpeak.com/pages/altitude-real-feel "The lower air pressure is caused almost exclusively by the lower temperatures encountered at more northern and southern latitudes."

"Because of its far northern latitude of 63 degrees, Denali has lower barometric pressure than the world’s other high mountains." http://www.livescience.com/40595-den...-mckinley.html

"The Mount McKinley summit pressure during the May and June climbing season is on average encountered 300 meters (1,000 feet) higher on Aconcagua at 6,500 meters (21,350 feet); and 400 meters (1,300 feet) higher on Mount Everest at 6,600 meters (21,650 feet), each mountain being considered during its respective climbing season." http://www.cohp.org/ak/notes/pressure_altitude_v6.html (This looks like the best source but will have to save it for a full read for later.)

From the above quote that's a 2% pressure difference at the same altitude between Denali and the others. Not a huge amount really - almost enough to be neglected in planning. If you're crazy enough to climb large peaks in winter though, differences would increase.

More data here showing bigger pressure variations with temperature than I originally said. https://8kpeak.com/pages/altitude-real-feel But only by looking at year round variations - it's not like I am ever going to climb in Antarctica during the winter so probably not something to worry about.

On weather variations:

Also thought this was noteworthy: "High pressure usually means stable air and good weather, while low pressure can indicate instability and stormy weather. Therefore, if you wake up to bad weather, not only will you have to deal with the adverse conditions but you will feel like you are climbing at a higher altitude than you physically are." Source: https://8kpeak.com/pages/altitude-real-feel

Kind of obvious really but never really thought about it.

On the question about local variations irrespective of weather, temperature or major latitude difference:

None of what I've found above gets to the question of whether it's realistic for air pressure to be different at a given altitude for two peaks close together on a global scale - say 10km or 200km apart, that kind of range. A plausible reason might be a prevailing wind, if the Puelche wind coming from the East of the Andes coming from 2000m high Argentina plains then blows across the Andes and blows down to the west side of the Andes, which drops more quickly to nearer sea level, could that effect of low pressure winds from the East drive down the pressure on the Western side of the Andes vs the East? It's logical, but I'm not convinced that it would make such a huge difference. The Andes are pretty wide you would think pressure differences would level out soon enough due to the inevitable movement of high pressure air into low pressure spaces that can easily be seen by opening a bottle or crisp packet closed at a different altitude. The air in this case equalizes very very fast.

Also, if what the guide says was true (and he was talking about mountains in the same region of the country), and if it was a big effect, you would think it would be more documented. THe fact that I'm struggle to google support for his comment may indicate it's a small effect.

So I'm not convinced, not yet anyway.

You'll see a larger and more pertinent delta in pressure (barometric) due to weather systems changing than due to (relatively small) distance. Wind and atmospheric pressure are related, but not the same thing; wind is just air trying to equalize differences in regional pressure, which itself changes due in large part to the tendency of heated fluids (like air) expanding and rising (normally due to convective solar heating). If wind and atmospheric pressure equated, no-one would bring oxygen to the Himalaya, just a fan!


Interestingly, the (average) partial pressure of oxygen in air declines at a steady rate as you ascend, but to compound your loss, as the ratio lessens, you actually get worse at extracting it from the air. This might be more pertinent to high-altitude climbers.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1114067/




From what I can tell, none of this supports what your guide friend is saying though. I would think there are too many important variables that would be difficult to predict in advance (personal efficiency and fitness, local weather, pack load, etc.) that a statement like that could easily get buried in "maybes" & "what-ifs". Add to the mix that he might have a vested interest in moving people (and their money) to mountaintops......

I looked up El Plomo on the map and it sits at 33 degrees of latitude. That is basically smack in the middle of the subtropical high, centered around 30 degrees of latitude. You can find more information by reading up on the Hadley cells of atmospheric circulation, if interested.

The main takeaway, though, is that air pressure tends to be slightly higher in these desert belts than near the equator or the mid-latitudes (maybe by a couple of percentage points).

I can't think of a reason for why two mountains that are in close proximity to one another would have markedly different air pressures at the same altitude, though... unless one of them was in the eye of a hurricane. Localized convection circuits, that vary based on small-scale weather and topography fluctuations could have an effect, but I wouldn't know how much and would guess that it isn't substantial.

Thanks for those comments, that's interesting.