Air pressure and altitude

Air pressure is the weight of the whole column of air above a surface. At sea level the average pressure is about 1013 hectopascals. Because air can be compressed, it is densest at the bottom, where most weight rests on it. That is why pressure does not fall evenly with height but quickly at first and then more and more slowly – physicists say: roughly exponentially.

A good rule of thumb: at about 5,500 metres the air pressure is only around half of the sea-level value. On the highest summits on Earth it is roughly a third. The share of oxygen in the air stays the same everywhere, around 21 percent. What changes is the amount: every breath contains fewer air molecules and therefore less oxygen. Experts speak of the falling partial pressure of oxygen – the part of the total pressure that is due to oxygen.

Air pressure depends not only on height, however, but also on the weather. High-pressure areas often bring settled weather, low-pressure areas often bring cloud, wind and rain. These fluctuations are much smaller than the difference between valley and summit, but large enough to affect instruments.

Water boils when its vapour pushes as hard as the air around it. If the air pressure is lower, the water reaches that point at a lower temperature. As a rough rule, the boiling point drops by about 1 °C for every 300 metres of height. At 3,000 metres water is already bubbling at roughly 90 °C – it looks just as boiling, but it is less hot.

For cooking this means: pasta, rice and pulses take longer and you use more fuel. A lid on the pot saves energy. A pressure cooker works exactly the opposite way to altitude: it holds the steam in, raises the pressure inside the pot and lets the water get hotter before it boils.

For boiling drinking water, health authorities recommend longer boiling times at high altitude because the water there is less hot. Follow the official advice for your region and plan enough fuel for it.

A barometric altimeter is really a pressure gauge. It knows how pressure decreases with height and converts it into metres. Near sea level one hectopascal corresponds to roughly eight metres of height. If the pressure changes because of the weather, the device “thinks” you have changed height.

That is why you reset the altimeter at known points: at the car park, at a hut, or on a summit whose height is printed on the map. If you stay in a hut overnight and see in the morning that the altimeter has “risen” although you have not moved, you know: the air pressure has fallen. That can point to worsening weather.

A barometer alone is not a weather forecast, though. It shows a trend at one place. Whether and when the weather turns is told much more reliably by the official forecast – the barometer is a useful addition on site.

The body can get used to thinner air, but it takes time. Anyone who climbs too high too fast can get altitude sickness. Typical signs are headache, loss of appetite, nausea, dizziness and unusual exhaustion. With these complaints you do not climb any higher.

If it gets worse – for example with confusion, unsteady walking, or breathlessness even at rest – going down is the most important thing, and medical help is needed. Then call the local emergency number (112 in the EU) or mountain rescue. Detailed medical rules are in the high-altitude entry; this text does not replace medical advice.

You also notice smaller pressure differences: when driving downhill or on a plane your ears pop because the pressure in the middle ear has to equalise. Swallowing or yawning usually helps. Sealed bottles, tubes and bags can puff up at altitude and squirt when opened.

A popular misconception is that at altitude there is “less oxygen in the air”, as if the mixture changed. The mixture is the same – only the air is thinner. This also explains why extra oxygen from cylinders helps: it raises the oxygen share and so makes up for the lower pressure.

A simple experiment makes the pressure visible: seal an empty plastic bottle in the valley and take it up the mountain. At the top it is taut, because the denser valley air is still pushing inside. Back in the valley it is squashed. It works the same way in reverse with a bottle sealed at the top.