'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
availability).
* 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
_nastes_....
The "equivalency" calculations I made follow from which all conclusions were
drawn:
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
9.4%
0.250 6707 9,298 30,505 7400 10,259 33,657 3,152
9.4%
0.100 6707 15,443 50,667 7400 17,039 55,903 5,235
9.4%
0.267 6707 8,850 29,035 7400 9,764 32,036 3,000
9.4%
0.980 6707 134 440 7400 148 485 45
9.4%
0.982 6707 122 399 7400 134 440 41
9.4%
The fraction refers to the pressure as a fraction of the pressure at sea
level.
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
altitude.
> > 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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