The equatorial bulge in barometric pressure
An issue related to temperature variation is the presence of an equatorial bulge in atmospheric pressure as one ascends to high altitude (Ward, et al. 2000: 26-28;West 1996). In low latitude areas, barometric pressure at a given altitude is correspondingly higher than is pressure at that altitude in mid and high latitude areas of the world.
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Figure4-7. Latitude against barometric pressure (West 1996). Lines show altitude in km. Annual temperature seasonality is minimal in the equatorial areas and thus seasonal effects at low latitudes are not shown in this graph. |
Models of variance in barometric pressure with latitude at a given altitude based on data derived primarily from weather balloons explain why there is greater available oxygen at 5000m in low latitude areas than in high latitude areas. The differences in pressure are particularly notable during high latitude winters.
|
Altitude |
15° latitude |
60° latitude, summer |
60° latitude, winter |
Equivalent Altitude |
|
4000 |
475 |
462 |
445 |
3479 |
|
5000 |
419 |
406 |
387 |
4400 |
|
6000 |
369 |
356 |
335 |
5310 |
Table 4-2. Equatorial bulge and effects on barometric pressure (in torr) at 15° and 60° latitude. From data in (West 1996: 1851). Equivalent altitude column shows altitude at 60° latitude with equivalent pressure to the value shown at 15° latitude.
As shown in Figure 4-7 and Table 4-2, the equatorial bulge in pressure means that the available oxygen at the Chivay source at 5000 masl is only close to the pressure that of a location nearly 1000m lower if the source were located at 60° latitude (in winter). In other words, during the winter at 60° latitude, at approximately the latitude of Anchorage Alaska, one would have to be at only 4400 masl altitude to find available oxygen at levels equivalent to that found at 5000 masl at the Chivay source. Barometric conditions similar to those found near the summit of Nevado Ampato at 6000 masl can be found in winter at 5310 masl near Anchorage, AK. The high altitude Chivay source is not as inhospitable as would be this zone at the equivalent altitude in higher latitude areas. Resource patches, such as rich bofedal grazing areas, are sufficient to draw seasonal or permanent residents to these altitude zones in the Andes. Raw material sources, particularly mining operations, are another significant draw to high altitude locations both in past and in modern times (Ward, et al. 2000: 336-344).