Mt. Whitney Portal Store & Hostel Forums General Mt. Whitney Portal Store Message Board barometric pressure at White Mountain summit

I hiked Mount Whitney last week (6/10) and checked the White Mountain weather summary for that day when I got back to see what the weather was. Turns out the temp was lower than I thought (14 degrees F at 10 am -- I am told that White Mountain and Whitney summits are fairly comparable for weather data). But what I can't figure out is the barometric pressure changes during the day. The pressure was 13 inches of mercury at 9 am and it doubled to 27 inches mercury by 5 pm (almost the same as at sea level). I had always thought that the pressure correlated with the oxygen availability in that reducing the pressure by half decreased the oxygen available by half. Am I correct to think that the oxygen availability at 10 am at the summit was half of what it was later in the day? Any mountaineers out there to explain this?? Thanks.

Joeski

The sometimes flaky readings reported on the White Mountain Web site have been discussed here before.

Barometric pressure averages 101 kPA (29.5 in Hg) at sea level and varies around that figure by a few percent. It falls off exponentially with altitude, but you have to go up to 16,800 feet for it to fall by half. A typical pressure at the summit of White Mountain might be 16 in Hg. It is safe to assume that it is never 27 inches up there!

The available oxygen at altitude is related to the partial pressure of oxygen in the air. Partial pressure of oxygen in the atmosphere regardless of the altitude is related to the ratio of the other gases that exist. At altitude it is the drop in oxygen’s partial pressure (or ratio of oxygen to other gases) that affects the efficiency that our lungs can draw the oxygen out of air. My understanding is that barometric pressure has a negligible affect on this relationship.

Oxygen makes up 21% of the atmosphere, both at sea level and at high altitude. The partial pressure of oxygen is therefore 21% of the barometric pressure. If you ascend to roughly 17000 feet, the barometric pressure is cut in half, and so is the partial pressure of oxygen.

The effect of altitude on human performance is due to the decrease in oxygen partial pressure, not to any change in the composition of the atmoshpere -- the ratio of oxygen to other gases (almost all being nitrogen) stays the same as you change altitude.

I'm reaching way back in the memory banks and may have to do some research here, but my recollection is that Dalton's Law states that the partial pressure of any single gas in a mixture of gasses is directly related to the proportion of those gases. Total P = P1 + P2 + P3...etc.

You're right -- I think we are saying the same thing. Oxygen makes up 21% of air. Therefore, its partial pressure is 21% of the total pressure. At sea level, the partial pressure of oxygen is 21 kPa (21% of 101 kPA). At around 17000', the total pressure is only 50 kPa and the partial pressure of oxygen is 11 kPa (still 21% of the total).

Joeski,

I checked the weather data for that day (a doubling in pressure seemed very strange for me, if not impossible), and the historical data of the white mountain station gives data indicating the pressure ranged from 596 to 601 mBar (I'm used to the metric system, although the Bar is not a metric unit either.... 1 bar (1000mBar) is about 760 mmHg or about 30 inches Hg. I think you misread the graph: there is an offset in it.

On a similar topic: has anybody got a backcountry estimate for the relation between altitude (pressure) and boiling temperature that doesn't require using Clapeyron's equation (I can't do logarhythms without a calculator). Also, at standard pressure, you can boil water to kill most parasites/bacteria. But at high altitude/low pressure the boiling temperature drops (at the Whitney summit to somewhere around 70 celsius (158F). Does this still kill the bacteria as efficiently? I am wondering whether the main mode of action of sterilising by boiling is denaturing of proteins (which would be less efficient then at altitude) or by vaporising water inside the organism and thereby causing it to expand and get damaged (which would be equally efficient then at altitude, but would happen at a lower temperature.

any thoughts?

Here is an article from USA today with related links to additional information.

http://www.usatoday.com/weather/wbarocx.htm

Thanks everyone for the input. The historical data site for 6/10 is now corrected so that throughout the day there was a roughly 17 in Hg pressure, so it was just bad data that I had first read. The old data did not make sense to me.

Joeski

At 15000 feet, the boiling point of water is 184 degrees F. See, for example,
http://www.fetco.com/boilingpoint.htm

A reasonable approximation is:
Boiling point = 212 - 1.84*H
where H is height above sea level in thousands of feet and temperature is in degrees F.

Thanks, AlanK.

The data in the table on the site you linked are linear (which is roughly ok at pressures we can encounter in the mountains. I found a site where you can enter the pressure and the site calculates the corresponding boiling point using the Trouton-Hildebrand-Everett approximation.

http://www.ch.cam.ac.uk/magnus/boil.html

Tobias -- I like that Cambridge site. Thanks.

I thought about that linear relationship a bit more. I found a note (but have not tracked down the reference yet) that gives the boiling point (deg. F) versus pressure (in Hg) as:
T = 49.161*ln(P) + 44.932

Since atmospheric pressure falls off exponentially with altitude, the relationship of boiling point to height above sea level must be linear. So, all one needs to remember is 1.84 deg. F/1000 ft.