All mountain walkers will know that it gets colder as you go higher, and Yr Wyddfa (Snowdon) is no exception; it’s often the best part of 10°C colder at the summit, and that’s excluding windchill. (We’re assuming here that there is no sunshine warming us or the air around us.) But why exactly is this? Well, it’s due to several interrelated physical principles regarding the way our atmosphere works and how air behaves as it ascends.
In a nutshell, it’s primarily due to the combined effects of reduced air pressure and the expansion of air, both a consequence of gravity; also playing a part are the decreased proximity to the heat source (the Earth’s surface), lower water vapour content, and radiative heat loss.
So now you know! But if you want to know about these factors in more detail, then read on (though it gets a little bit nerdy) ….
A decrease in air pressure
Lower air pressure is the primary reason for colder temperatures at altitude. Essentially, air pressure decreases with altitude, so as you ascend, the air becomes less dense (i.e. the air molecules are more spread out), and this expansion causes the air to cool.
To put this another way, higher altitudes have less dense air with fewer air molecules; and since there are fewer air molecules to absorb and retain heat, the temperature drops.
So does gravity play any part in this process? Absolutely! Indeed, gravity is the primary reason why air pressure decreases with altitude in the first place, even though differences in gravity are relatively minimal at lower levels. It has been stated above that as you ascend, the air becomes less dense. Gravity pulls air molecules toward the surface, creating a denser and higher-pressure atmosphere at lower altitudes (where the weight of the atmosphere above it compresses the air) and consequently at higher altitudes, where there are fewer air molecules above and thus the weight of the atmosphere above it is lower, it results in a thinner, lower-pressure atmosphere.
This drop in air pressure with altitude in not linear, but exponential: at sea level, air is denser because gravity is pulling most of the atmosphere close to the surface, but with every increase in altitude, the reduction in air density and pressure is more pronounced because the gravitational pull on the remaining air becomes weaker. Also playing a part is the compressibility of air: at lower altitudes, the weight of the atmosphere compresses the air more tightly; at higher altitudes, there’s less weight above, so the compression is much less, leading to a more rapid drop in pressure.
Finally, on the subject of gravity, the Earth’s atmosphere is finite. Thus, as gravity weakens with distance from the Earth’s centre, so the atmosphere becomes thinner with altitude, contributing to lower pressure.
Snow and ice at the summit where it’s colder.
To delve a little deeper now into the initial statement about the decrease in air pressure, as has been stated, temperature drops as air expands (without gaining heat). This process is known as adiabatic cooling.
Adiabatic cooling is the process whereby when a parcel of air rises, it uses its energy to expand, which decreases its temperature. This rate of cooling is approximately 6.5°C per 1,000 metres (about 2°C per 1,000 feet), known as the environmental lapse rate. However, this rate of cooling is not a fixed figure as it is also affected by the moisture in the air. (When air is saturated with water vapour, condensation occurs as it cools, releasing latent heat, which slows the cooling rate.) Therefore, when the air is dry the temperature drop is about 3°C per 1,000 ft / 300m ascended; when the air is saturated the temperature drop is somewhat lower. It is therefore practical to assume an average of at least 2°C temperature drop per 1,000ft (300 m).
Solar radiation
The Earth’s surface is a primary heat source for our atmosphere: sunlight warms the Earth’s surface, which then radiates it back to warm the surrounding air. Therefore, the distance from this heat source will play a part: at higher altitudes, where there is less surface area to absorb this heat, and you are further from most of the surface, the less direct heat you will experience, contributing to cooler temperatures.
Water vapour content
Higher altitudes typically have less water vapour in the air (primarily because cold air holds less water vapour than warm air, and because lower pressure reduces the capacity for moisture). Water vapour is a greenhouse gas (it absorbs and emits infrared radiation, serving to trap heat in the atmosphere) so with less water vapour at higher elevations, there is less heat retention, leading to cooler conditions.
Radiative cooling
Heat escapes more easily at higher altitudes, where the thinner atmosphere is less able to retain heat, thus allowing more heat to radiate out into space.
Wind and weather patterns
The above factors all affect the actual ambient temperature. As warm-blooded humans, windchill is an additional factor which we have to contend with due to convective heat loss from our bodies. The wind is often stronger at altitude, exposure to which can cause the air to feel colder.
So as you can see, whilst it’s easy to say that it’s colder at altitude, the reason behind it is quite complicated!