'Losing breath' is quicker at Alaska elevations

MexicoDoug at aol.com MexicoDoug at aol.com
Wed Aug 7 15:48:32 EDT 2002

Ken, thanks again for the comments.  I want to reply specifically to your 
comments that the difference of air pressure is trivial in Alaska vs. say, 
the US Rockies.  Indeed, the "scale height" of the atmosphere comes into play 
and is significant for Lepsters.  I will reply to some interesting points you 
make separately which are relevant to my questions, but as we went into an 
aside on "Whether Alaskan mountains are or aren't really more challenging" 
from the air pressure point of view, I am separating this part of the post.

You chose to comment related to sea level, whereas my original post was 
looking at elevations for mountain lepping leading to tired Lepsters and 
supposedly tired Leps.  I could find no information on the web comparing 
Alaskan pressures vs., say the Rocky Mountains at similar altitudes as in 
Mark's original post.

So I used the atmospheric pressure equation at Mt. McKinley-Denali assuming 
there is a 3000 foot perceived difference vs. Everest at peak altitude as 
presented commented on PBS's NOVA.  The results are:

* A Lepidopterist certainly would tire more quickly at an equivalent altitude 
and temperature in Alaska, especially when you start getting above 5,438 feet 
(the equivalent at Tampa's latitude of 6,000 feet re: pressure and Oxygen 

* The pressure does, in fact, drop off nearly 10% more quickly.

* Assuming the "atmospheric scale height" (where 36.8% [1/e] of the mass of 
the atmosphere lies below) of 7,400 meters at Everest (latitude of Florida 
and S. Texas) , that gives a scale height of only 6,707 meters at 
McKinley-Denali - and Fairbanks latitude would be even lesser as it is 
somewhat further north.

* What this means practically, for example, is that a Lepidopterist at 6330 
feet in Alaska gets the same air pressure and thus I assume, oxygen level, as 
one at 6,984 feet at Tampa, Florida's latitude.

* Alternately, the pressure _difference_ at Fairbank's elevation of only 440 
feet, is still 9.4%, or effectively 40 to 45 feet different (higher) from the 
'correct' equivalent pressure, which would be experienced by someone at 
Tampa's latitude, but 40 - 45 feet lower.  Due to the small elevation of 
Fairbanks, it doesn't impact human physiology much - until he goes into the 
mountains.  This is 1.85 millibars, and not incredibly different from the 
weather service 1 millibar you mentioned, which makes me believe I haven't 
obviously erred in the assumptions and calculations.

* Can we agree, then, that at the 6,000 - 12,000 feet range the influence is 
effectively the same at 5,438 - 10,876 at Tampa's latitude, and that the 
effect is significant on the Lepster predator, at least?  (All assuming the 
same temperature, if that is possible at the upper end of the range.)  These 
are basically the best Lepping days, right?

* And when you say 10,000 feet in Alaska, that has the pressure & oxygen of 
11,033 feet at Tampa's latitude, not a fun altitude to be chasing that 

The "equivalency" calculations I made follow from which all conclusions were 

H = Scale Height of Atmosphere                                      

 fraction H(AK)  meters feet        H (EV)  meters  feet          delta %

 0.750   6707   1,929   6,330       7400   2,129   6,984          654   9.4%
 0.500   6707   4,649   15,252     7400   5,129   16,828          1,576   
 0.250   6707   9,298   30,505     7400   10,259   33,657          3,152   
 0.100   6707   15,443   50,667   7400   17,039   55,903          5,235   
 0.267   6707   8,850   29,035     7400   9,764   32,036          3,000   
 0.980   6707   134   440            7400   148   485                 45   
 0.982   6707   122   399            7400   134   440                 41   

The fraction refers to the pressure as a fraction of the pressure at sea 
AK is Alaska; EV is Everest.
The meters and feet is the elevation above sea level giving the same pressure 
at either location.
delta the additional perceived subtracted elevation in feet which is 
equavalent in Alaska.       

28 degrees N        Tampa FL USA        
28 degrees N        Corpus Christi TX USA       
28 degrees N        Mt. Everest, Nepal

25 degrees N           Monterrey, Mexico
25 degrees N           Miami Zoo, FL, USA   

64.5 degrees N      Fairbanks, AK USA   
64.2 degrees N      Nuuk, Greenland 
63 degrees N        Mt.McKinley-Denali, AK, USA

In addition, I would like to point out that the incredibly high pressures in 
Alaska are not really applicable to this discussion as far as warm day 
mountain Lepping, for the following reasons:

1- They are not corrected for temperature, and I have presupposed that all is 
at constant temperature - i.e., a warm Lepping day in the mountains (if such 
a thing happens in Alaska).

2.- They are corrected for altitude.

Thus, the famous cold snap you described in an earlier post apparently led to 
Northway, AK, purportedly recording the second or third highest pressures 
ever measured by human instruments actually recorded a pressure much lower 
than one standard atmospheric pressure (98.5%), or 29.46 inches of mercury, 
at probably -75 F degrees at a weather station altitude of 1,713 feet, not 
the 31.85 inches reported for the 'record'.

Finally, regarding the water adding pressure to the air, I would say indeed 
it does, but along the lines you have mentioned  I have to agree it will be 
insignificant (unless giving the extra push to make another Alaskan 
"record").  I convinced myself of that by supposing the waters add a maximum 
of one inch deep of water, a heavy rainfall.  Sounds reasonable to me, 
anyway.  One inch high of water weighs only 16.4 grams over a square inch of 
area, or .036 pounds.  Considering one standard atmospheric pressure unit is 
14.7 pounds per square inch, the water only adds 2.5 millibars!  A one 
hundred mile column of air is a lot heavier than I assumed...

Best...Doug Dawn
Monterrey, Mexico

PS per Newton's Universal Gravitational Law, the gravitational force doesn't 
depend on latitude, I don't think, I guess you mean the force counteracting 
gravity due to earth's spin is smallest, which is why Alaskan's may not 
really be as massive as the bathroom scale implies.

fnkwp at aurora.alaska.edu escribe:

You presumbly mean 'at the same latitude' rather than 'at the same height'.
I got my figure of 1 millibar from the local Weather Bureau office--but
the difference at sea level is certainly not 10%! However, note that the
gravitational force at a given elevation is greater as you move towards
the poles--and that will decrease the scale height of the atmosphere.
A smaller scale height means the pressure falls off more rapidly with

> > how can I reconcile that with the commonly stated "fact" that if Everest
> > were at the same height as McKinley-Denali it would have an oxygen/atmos-
> > pheric? pressure equivalent of 3,000 feet higher than present and be
> > beyond the human ability to climb without supplemental oxygen

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