Peak bagging: it’s all about the latitude

AUTHOR: DR LUCY OBOLENSKY

Pressure Altitude vs True Altitude

To understand why this is we first need to understand the difference between pressure altitude and true altitude:

• Pressure altitude is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure as that of the part of the atmosphere in question. 
• True altitude is the elevation above sea level.

The ISA is a theoretical set of conditions used by the aviation industry to provide a baseline to compare fluctuations in pressure, temperature, density and viscosity within the Earth’s atmosphere. At sea level, pressure is universally calculated at 760 mmHg and temperature is 15℃ and under these conditions, true altitude and pressure altitude would be the same. However, a true altitude of 3000m would mean a pressure altitude of 3070m on a low pressure day, and this effect will be magnified with factors considered below:

To calculate pressure altitude the equation is as follows: 

{(Sea level pressure – 760) X 304.8} + true altitude

Note: 304.8 is the elevation gain in metres corresponding to a loss of height of 1 inch of mercury

Oxygen at Altitude 

We acquire oxygen from the atmosphere. Air is compressible and has a weight, and the pressure effect of this weight at sea level we know to be 760 mmHg. 

Atmospheric pressure forces air into the lungs and enables gas transfer. Therefore, pressure altitude is of great importance when mountaineering. 

As we ascend in altitude, the air becomes less compressed and is therefore less dense. Although there is the same percentage of oxygen in the atmosphere, there are fewer oxygen molecules per given volume, thus reducing the partial pressure of oxygen.

Latitude  

So why is latitude so important to the choice of your high altitude acclaim? 

Using ISA, the height of a mountain can approximately determine the pressure of the air on the summit. However, factors other than true altitude will affect air pressure, in a similar manner to the effect of wind chill and precipitation on approximate temperatures on any summit. A major factor affecting pressure altitude on the summit is the latitude of the mountain, and therefore the corresponding temperatures. 

The atmospheric pressure around the globe is not actually uniform. The trophosphere (the innermost layer of the Earth’s atmosphere) is almost double the width at the equator as it is at the poles. The further you travel from the equator, the lower the air pressure. The difference in this air pressure is caused by the lower temperatures found at increasing latitudes.

Chimborazo – in the climbing season, with likely lower pressure altitude than true altitude.

Temperature and Latitude

We know that cold air is denser than warm air. Therefore at a lower temperature, air will be more compressed. At any given isosurface (a surface of equal pressure) will lie at a lower true altitude in a colder environment than a warm environment. This effect can be dramatic: temperature can change pressure by up to 15% from the ISA values. 

Data gathered for scientific research has extrapolated that in January and December the pressure altitude on K2 is approximately the same as that on Everest. If Everest is climbed in warmer months (as it usually is), its pressure altitude is nearly always lower than its true altitude – which is good for climbers!

A more extreme example can be seen on the Antarctic plateau. Vostok station records mean daily temperatures of -65 ℃ at an elevation of 3420m from April to September. Using the corresponding sea level temperature, this gives a pressure altitude of 4280m. That is 860 metres higher than the true altitude.

What does this mean for climbers? 

What this means for climbers is that:

1. Physics can be really interesting!
2. Pick your mountain carefully if meterage is your main objective and consider Chimborazo over Denali.
3. Understand the weather patterns before you go, including best seasons for climbing and low and high pressure variations whilst you are on the mountain.

Are you interested in learning more about this topic?

Why not take a look at our Altitude Medicine Course ? Whilst you’re there, why don’t you take a look at our other courses too?