Altitude as a function of latitude

[num1mc]

This is for Mtnguide on this CC thread who claimed to understand that an elevation of 17,000' in the Alaska Range is equivalent to 24,000' in the greater Himalayan, due to the thicker air column at latitudes closer to the equator.

 

Sorry dude, you're wrong as wrong can be.

In the climbing literature on Mount McKinley (and apart from Mr. Moore's work), the claim is made that breathing is more arduous for a given altitude because centrifugal force due to Earth's rotation is less than for more temperately located mountains. The atmosphere is "whipped outwards" at the equator, expanding the air column with a resulting higher pressure for a given true altitude.

 

A centrifugal effect indeed manifests itself by a change in the effective value of g, with g diminished slightly at the equator where the speed of rotation is greatest. g rises only 0.53% between equator and either pole; with 0.34% of this increase due to diminished centrifugal force with latitude. From Equation (1b) all pressure altitudes shift this small fraction as a result of Earth's rotation - roughly one part in 300.

 

For the summits of two 6,000 meter (19,700 foot) mountains, one at the equator and one at either pole, this fraction corresponds to a 20 meter (66 foot) pressure altitude difference. The pressure altitude correction is normally even smaller still since most mountains are not located exactly on the equator or in the polar regions.

Thus the claim that Earth's rotation is the cause of lower air pressure near the poles is false. Lower air temperature is the overwhelmingly important cause.

source: http://www.cohp.org/ak/notes/pressure_altitude_simplified.html

 

The math is rather ponderous http://en.wikipedia.org/wiki/Barometric_formula but when you plug in variations due to gravitational differences with latitude http://en.wikipedia.org/wiki/Earth%27s_gravity#Latitude you see that the result is a very small difference

 

There are regional differences in altitude effects, especially when comparing the tropics with near arctic environments, but it is not nearly as great as you claim

[chris]

Nice! That matches my experience, but its nice to finally have a resource that backs it up.

 

On a less harsh note for MtGuide, I'd be interested to see if Dr Hacket has changed his opinion since that lecture in 1996. I would be surprised if he hadn't.

[tbyrnestl]

During the climbing season Denali's summit has the same pressure as being at about 22k on Everest during its climbing season, about 1,300' of elevation between isobaric levels on the tow mountains.

Gravity has little to do with it. The colder polar atmosphere is denser and thus thinner and fixed points protruding into it like summits exist further up in the atmosphere and thus at lower pressures. But since Denali is typically climbed during its summer when the polar atmosphere is warmer, the effect is at it's minimum.

 

[genepires]

so that makes the badasses who climb denali in the winter even more Bad Asses.

[Coldfinger]

Thought you meant altitude in terms of actual height--which makes Chimborazo the highest point on earth.

 

The thread should be "barometric pressure as a function of season and latitude."

 

Neat topic.

[G-spotter]

During the climbing season Denali's summit has the same pressure as being at about 22k on Everest during its climbing season, about 1,300' of elevation between isobaric levels on the tow mountains.

 

 

Got some citation for that factoid?