Air Pressure and Mount McKinley
The air pressure atop Mount McKinley is lower than is found at the same elevation on other high mountains in temperate latitudes.
The lower pressure is caused almost exclusively by the lower temperatures encountered at higher latitudes.
The lower pressure is not caused by the Earth's rotation "whipping outwards" the equatorial atmosphere.
Validation of these claims may be found in the author's original article. There one finds a detailed account of the formalism underlying these statements.
Here a brief explanation is provided.
As colder air is denser, at low temperatures the entire atmosphere is compressed downwards. Thus a specified air pressure will lie at a lower altitude in a cold environment compared to a warmer one.
Latitude effects air pressure two ways. Earth's rotation is not felt at the poles, accounting for a minor 0.34% increase in the gravity felt there relative to the equator. The Earth is slightly squashed toward the poles; and this increases the gravity felt at the poles by an additional 0.19%.
The atmosphere's vertical pressure profile is determined by these changes in temperature and gravity. Temperature can change pressure by up to 15% from standard values. The summed 0.53% change in gravity shifts pressure by the same tiny fraction - a minor amount that can be neglected for mountaineering purposes.
These statistics, and similar data for Aconcagua and Mount Everest, are used to compare pressures on Mount McKinley with these other continental highpoints.
In Table 1 a comparison is made of air pressure at Mount McKinley's summit and Aconcagua, South America's highest mountain at 22,841 feet.
Table 1. Mount McKinley / Aconcagua altitude differences in feet for matching air pressures at Mount McKinley's summit. Rows are calendar days during the Mount McKinley climbing season; and columns similarly for Aconcagua. Variabilities (after the "±" signs) are due to weather.a
|December 15||January 1||January 15||February 1||February 15|
|May 1||1291 ± 483||1371 ± 483||1389 ± 483||1353 ± 483||1274 ± 483|
|May 15||1109 ± 479||1189 ± 479||1207 ± 479||1170 ± 479||1093 ± 479|
|June 1||892 ± 475||971 ± 475||989 ± 475||953 ± 475||876 ± 475|
|June 15||717 ± 472||795 ± 472||813 ± 472||777 ± 472||700 ± 471|
|July 1||521 ± 468||598 ± 468||615 ± 468||579 ± 468||504 ± 468|
aTo account for latitude differences add 55 feet to all tabulated values.
In Table 2 a comparison is made of air pressure at Mount McKinley's summit and Mount Everest, Asia's highest mountain at 29,028 feet.
Table 2. Mount McKinley / Mount Everest altitude differences in feet for matching air pressure at Mount McKinley's summit. Rows are calendar days during the Mount McKinley climbing season; and columns similarly for Mount Everest. Variabilities (after the "±" signs) are due to weather.a
|April 1||April 15||May 1||May 15||June 1|
|May 1||1476 ± 489||1583 ± 489||1706 ± 489||1814 ± 489||1946 ± 489|
|May 15||1292 ± 485||1399 ± 485||1521 ± 485||1628 ± 486||1759 ± 486|
|June 1||1073 ± 481||1179 ± 481||1301 ± 481||1407 ± 481||1537 ± 481|
|June 15||896 ± 477||1001 ± 477||1122 ± 478||1228 ± 478||1357 ± 478|
|July 1||697 ± 473||802 ± 473||922 ± 474||1027 ± 474||1156 ± 474|
aTo account for latitude differences add 63 feet to all tabulated values.
During the May and June climbing season, Mount McKinley's summit air pressure rises from increasingly warmer temperatures as summer is approached. From May 1 to July 1 the change in pressure is, on average, equivalent to "descending" a full 743 feet. Deviations from this value occur due to changes in weather.
In Table 1 direct comparison is made of true elevations at which the same air pressure is observable on Mount McKinley and Aconcagua. The central entry of that table represents the middle of their respective climbing seasons. Using that entry, one must be an average of 989 feet higher on Aconcagua to observe the same pressure as at the summit of Mount McKinley - 21,309 feet. Earlier days in the Mount McKinley climbing season, such as May 1, result in larger altitude differences approaching 1,300 feet.
In Table 2 direct comparison is made of true elevations at which the same pressure altitudes are observable on Mount McKinley and Mount Everest. The central entry of that table represents the middle of their respective climbing seasons. Using that entry, one must be an average of 1,301 feet higher on Mount Everest to observe the same pressure as at the summit of Mount McKinley - 21,621 feet. Earlier days in the Mount McKinley climbing season, such as May 1; and/or later days in the Mount Everest climbing season, result in larger altitude differences approaching 1,950 feet.
Changes in weather cause deviations from these values. Thus from the bottom row of Table 1 it is possible in late June for the air pressure at Mount McKinley's summit to be observed on Aconcagua's slopes in December and February at the same 20,320 foot elevation.
Similarly, from the top row of Table 5 it is possible in early May for the air pressure at Mount McKinley's summit to be observed at 22,600 feet on Mount Everest's slopes in May - 2,300 feet higher than Mount McKinley's summit elevation.
These altitude differences for matching summit air pressure are not due to colder than standard temperatures at Mount McKinley. The differences arise from warmer than standard temperatures at the comparison mountains; in turn because most mountains, including Aconcagua and Everest, are climbed during their respective warmer months.
Account of latitude lowers the air pressure at Mount McKinley's summit by an amount equal to 55 feet of extra summit altitude relative to Aconcagua; and 63 feet relative to Mount Everest.
The claim has been made that on Mount McKinley 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.
However as we have seen, poleward pressure changes amount to only 0.53% - an insignificant amount compared to changes wrought by temperature.
Thus the claim that Earth's rotation is the cause of lower air pressure at Mount McKinley is false. Lower air temperature is the overwhelmingly important cause.