Air pressure, temperature & UV radition in relation to altitude

1
Initial height
metres a.s.l.
2
Ascending or descending?
How far?
metres

When you by metres you can expect:

in temperature of °C

in air pressure of hPa / mbar

in UV radiation of %

Note: The values given apply only in ideal conditions. In practice there are many different contributing factors, esp. the weather conditions.

Every mountaineer knows that the air changes as the altitude increases. It becomes “thinner”. But what does this actually mean? And how do we explain this phenomenon?

How does the air pressure change as altitude increases?

In order to understand why the air pressure decreases as the height increases, we can simply imagine a 10 m high glass cylinder with a diameter of 10 cm. We drop cotton wool balls into this cylinder one after another, until is full to the top. The cotton near the top is under no pressure and is therefore just as soft as before. The cotton in the middle is pressed down by the cotton which lies on top of it. It is put under a pressure which decreases its volume. This causes it to lose its bulk. The cotton right at the bottom is put under the pressure of all the cotton in the cylinder and is pressed completely flat. This cotton is put under the greatest pressure, and this means that it has the smallest volume.

Although it is the same cotton as that at the top, the properties of the individual cotton balls inside the cylinder change. If we were to cut the cylinder into 10 cm thick slices and count the cotton balls inside, we would discover that the number of balls decreases from the bottom to the top. This is because the cotton with the smallest volume has the greatest density. The density therefore gets smaller towards the top and so the pressure exerted on each layer decreases.

The same principle applies to the air which surrounds us, although we don’t have a 10m high cylinder, rather a 100km high atmosphere. In this atmosphere every air molecule presses on the molecules below it. Both the density as well as the air pressure become greater, the closer you get to the earth. Or alternatively lesser, the higher up you go.

The boiling point decreases at altitude
Cooking at higher altitude uses more fuel

The initial effects become obvious relatively quickly. Since the volume of the lungs remains constant, you take in more air molecules with every breath at ground level and therefore also more vital oxygen, than at 5000 metres. The air in the mountains becomes thinner and the higher you go the more often you need to breathe in order to take in the same amount of oxygen.

Another interesting effect is that the boiling point of water lowers as the pressure decreases. In other words, this means that water boils at below 100°C. But don’t be fooled: It also means that cooking food takes even longer than at lower altitude and you need more fuel for your outdoor stoves.

How does the UV radiation change as altitude increases?

Not only does the air pressure change, the intensity of the UV radiation also changes with increasing altitude. UV radiation is absorbed by the atmosphere. So when there is less atmosphere above us, it logically follows that less radiation is absorbed. The UV radiation increases by around 2% with every 100 metres of altitude.

So when you are in the mountains you should make sure that you bring enough UV protection including appropriate sun protection products and clothing with integrated UV protection. UV radiation can penetrate even when the sky is overcast, and since the intensity of the radiation is higher in the mountains than at lower altitudes, it is easy to underestimate the stress your skin might be under.

How does the temperature change as altitude increases?

This brings us to the final point, the temperature. Every mountaineer will know that it gets colder, the higher you get. In fact, it gets around 0.65° Celsius colder per 100 metres of altitude, assuming stable weather conditions without stratification. But why does this happen?

Our planet earth produces virtually no warmth itself. Nearly all the thermic energy on the earth’s surface comes from the sun. This energy, which reaches the earth in the form of rays, is absorbed by the earth and is stored as warmth. The earth then emits this warmth back into its surroundings (the air). The atmosphere heats up.

The atmosphere warms up from the bottom to the top. The closer one is to the earth, the more thermic energy, and therefore warmth, is retained, and so the warmer it is. The air pressure declines at the same rate up right up to the edge of the atmosphere, but the temperature variation in the atmosphere is not consistent. The outer layers, that is altitudes of around 100 kilometres, are heated directly by the sun and are very hot. Thus there is a non-linear temperature variation.

But in the altitudes relevant for mountain sports, that is between sea-level and just under 9000 metres, the change is relatively linear and our calculators deliver reliable results.

However, it is important to take note of those effects, which are not directly dependent on the physical properties of the atmosphere. The air is constantly in motion. There are high-pressure areas and low-pressure areas.

If a high-pressure area (high air density, high air pressure) meets a low-pressure area (low air density, low air pressure), this automatically results in a pressure equalisation. This means that air from the high-pressure area flows into the low-pressure area. If this happens close to the earth’s surface, we recognise it as wind.

The formation of these high and low pressure areas is dependant on many, sometimes very local factors. If large air masses move towards a very high mountain and can’t simply pass over, the air accumulates in front of the mountain. The air pressure increases.

The temperature of the atmosphere also determines its density. It is generally true that: Higher temperature means higher air pressure, lower temperature means lower air pressure.

So, what does this mean for us alpinists and outdoor athletes?

In principle we all prefer a cloudless, blue sky. So, stable high pressure areas are exactly what we need, since in these areas cold air masses from the higher altitudes flow down towards the earth and are warmed up in the process. This means that clouds disperse and the sky is clear.

In low pressure areas, air masses flow upwards away from the earth, causing the air to cool down and clouds to form, which then leads to rain.

High and low pressure areas don’t only develop next to one another, however. They can also form into stratification, which means that air masses with different pressure lie above one another. We mentioned earlier that the air pressure decreases with increasing altitude. However, when certain weather phenomena prevail, this is not the case.

The special case of thermal inversion

Thermal inversion in the Upper Rhine Valley taken from Kandel.
Thermal inversion: Fog and smoke remain close to the ground

For example, when a high pressure area is situated above a low pressure area. This phenomenon is usually highly localised and is known as thermal inversion. When this happens, the layer of warm air prevents the layer of cold air from rising.

This often results in a very thick cloud or fog build up close to the ground. There is often a clearly discernible border, at which the clouds appear to stop abruptly. This is the beginning of the high pressure area. Above this cloud layer you can see clear air with good visibility, while inside it there is only very restricted visibility and the temperature drops sharply. When this happens the temperature and pressure distribution of the atmosphere produce a localised reversal of physical expectations. Pressure and temperature drop, the lower one gets.

The limits of predictability

In principle, it is relatively easy to predict how the pressure, temperature and density will change with increasing altitude. But the principles only apply in stable weather conditions without weather changes, that is those with constant atmospheric humidity. However, since the atmosphere is constantly in motion, the atmospheric humidity can also change very quickly in some cases. In these cases, the temperature and air pressure do not act as predicted in this simplified model. But the issue of atmospheric humidity goes beyond the scope of this article.

Important: If you are out in the mountains for extended periods, especially at very high altitudes, it is very important that you get information beforehand about the current weather conditions and, if necessary, the weather forecast for the next few days!

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