Anyone who has climbed a major mountain is very aware of two climatic phenomena. 1) It’s harder to breathe the higher you go. 2) It gets colder the higher you go. If you are camping on the mountain add 3) it takes a lot longer to cook most food the higher you go. Though readily accessible, scientific values regarding these phenomena are often better understood by scientists, engineers, and meteorologists and less well known in the mountaineering community. I thought it would be worthwhile to post a table and few graphs with some basic explanation for future reference. Data referenced for this brief article are taken from the “CRC Handbook for Chemistry and Physics”, 76th edition. With apologies to the rest of the world, I’ve presented most of the data in American engineering units, i.e. – psi, degrees Fahrenheit.
Air pressure at sea level is 14.7 pounds of force per square inch. This is basically because you have very tall column of air above and around you that exerts pressure (force per unit area) in all directions. As you ascend higher in elevation, there is less mass of air between you and space. Pressure and mass concentration of gases correlate linearly. The pressure and concentration of oxygen, nitrogen, and everything else found in air decreases as you ascend. At 2500 m or 8200 ft, it’s about three-quarters that found at sea level. At 5500 m or 18040 ft, it’s about half. Combine this with the fact that air is only about 21% oxygen. There’s a good reason why it’s hard to breathe up there! Not wishing to ignite a debate on global warming, but we would have no oxygen to breathe if it weren’t for plants. So, if you love your oxygen, plant a tree! The curve shown in the figure below illustrates how pressure drops with altitude. Of course weather fronts, hurricanes, and other climatic disturbances can also influence atmospheric pressure. In the grand scheme of things the curve below is pretty accurate, and is exactly how your barometric altimeter provides you with an elevation value.
Thanks to "Coldfoot", who has provided the following information about latitude dependence on air pressure at altitude:
"Just to add a subtlety about the lapse rate - it applies in the troposphere where the atmosphere transports energy convectively, and more or less for that reason, the pressure variation with altitude depends on the overall temperature. The standard atmosphere applies around mid-latitudes. Closer to the poles, the atmosphere is colder and the pressure drops off more quickly with altitude. This underlies the idea that the air is thinner at some altitude on Denali than at the same altitude in the mid-latitudes. The sea level pressure is the same in Alaska, but the pressure falls off faster with height.
Water Boiling Point
As air pressure decreases, the temperature required to boil water decreases. A liquid being heated cannot exceed its boiling or phase change temperature. Much of our cooking relies upon moisture and temperature to convert raw food into something we can both enjoy and digest. As the boiling temperature drops, it takes much longer to cook many items. This is the guiding principle behind the pressure cooker. Increase the pressure, increase the boiling point, and you reduce the cooking time. Unfortunately those of us without Sherpas or llamas can’t readily haul a heavy kitchen appliance up the mountain with us. This is the reason why it becomes practically impossible to cook meals like rice and pasta at very high altitude. The graph below illustrates how the boiling point of water drops with altitude.
Temperature drop with altitude is much less precise than either air pressure or boiling point, but some rules of thumb can still be applied. There is a thermodynamic term called the adiabatic lapse rate. The basic concept is that if a gas loses pressure or expands without heat transfer, it will cool. This strictly doesn’t apply to climbing a mountain, as the same air doesn’t accompany you up the hill. The more accurate factor for an increase in altitude is the environmental lapse rate. This predicts a 3.6-degree F drop in temperature for every 1000 feet of elevation gain for dry air. There are a several caveats to be applied to the temperature drop rule of thumb: (1) It’s for dry air. Humid air behaves differently. (2) It often works reasonably well if you are in an airplane. If you are standing on a large land mass, i.e – a mountain, the thermal mass of the mountain can greatly affect the local temperature. (3) Air often does not behave ideally. The best example that many of us have observed in the colder months is a temperature inversion. Cold, still air can settle into valleys overnight. In these conditions, the mountain top may be warmer than the valley floor. All things considered, the following graph can still be a useful rule of thumb when planning gear for your next outing.