It’s known to many people that it gets colder in the atmosphere if you climb to higher elevations. First however, let’s get some definitions for this blogpost. This post is a translation from the Dutch article I wrote on this, however in Dutch we just use 1 word. In English, there are 3 words: height, altitude and elevation.
First height: this is used to describe the height of an object relative to your position. If you are standing in front of your house, it might be 10 meter high. The house might be located at an elevation of 500 meter. This is the vertical distance between the Mean Sea Level (MSL) and the base of the house. Altitude on the other hand is an avionics term and describes the vertical distance between the position of an object and the earth surface. So if an airplane is flying at an altitude of 500 meter over the previously mentioned house, it is at an elevation of 1000 meters (above the sea level, as the house was already at 500 meter elevation).
However in common day language, altitude and elevation are used interchangeable. In this blogpost, or in fact all across the website, altitude and elevation are refering to the height above sea level.
So, back to the topic: Temperature decreases as you climb. But why? And how much is that difference actually?
The latter question is the easiest: the common rule is that temperature decreases with 6°C per 1000 meter. Or 0.6°C per 100m – in moist air. Dry air has a rate of change of about 1°C per 100m. Under exceptional circumstances, the difference could be more than 1.5°C per 100m. However there are also conditions where the temperature rises when altitude increases: inversion.
But why does it get colder with increasing altitude?
A slightly more difficult question, as there are more variables that influence this. Being the following:
– Expansion of air
– Dry air
– Snow cover
The first variable has the largest impact: the air pressure at sea level is about 1000 hpa. Air has a weight, which is influenced by all the air above it. This is causing the air pressure. With each meter you gain, the air pressure decreases as there is less air above you. As the air becomes lighter, it expands. And when a gas expands, it cools down: the same number of molecules get more room to move around, which causes less collissions that create the heat.
The second variable is shadow. What we often see in the mountains, most notably in autumn and winter, is that it takes a long time for the sun to rise above the mountains. This in turn means that it takes longer for the sun rays to reach the bottom of the valley: the valley is shrouded in the shadows for much longer. In the evening, the sun sets behind the mountains again, and the temperature starts to drop – sooner than in non-mountaineous regions or on a high plain.
Third: dry air. Air in the higher layers of the atmosphere is typically very dry. Dry air cools down much more rapid than moist air. This causes temperatures to plummet in dry air. The dry air also causes sweat to evaporate quickly, and lowers the perceived temperature.
These variables can cause microclimates. Narrow valleys with high mountains blocking the sun from the south might be much colder than what you expect based on elevation alone. In a wide, sunny valley though, the effects might be smaller than expected.
The conditions of dry air and a lot of shadow, combined with a large elevation and sufficient precipitation leads to snow. Snow is obviously white and has a very large albedo: 90% of all solar energy is reflected back into space. Under ideal conditions, the temperature can take a nose dive in high-altitude locations above a large snow cover. For example, the lowest temperatures ever recorded in Switserland (-41.8°C), France (-41.2°C) and Austria (-47.1°C) is in range or colder than continental Poland, Ukraine, Belarus or any of the Baltic states.
As snow falls due to the lower temperatures, they start to provide a feedback-loop as described above: temperatures will be colder than without a snow cover. New precipitation therefore has a higher likelihood to be solid too: either freezing rain or snow. This mass of frozen water is also an important part of the refrigerator later in the season: the melting snow extracts a lot of heat from the air to convert the snow to water. With this process, if it is strong enough, glaciers get formed and create their own climate.
Many years ago (September 4, 2009)…I was sleeping outside at an altitude of 2510 in the Val d´Hérens in Switzerland. According to the weather forecast, the freezing level would be at about 2300m and about 1 cm of snow was forecasted. Early in the evening, the snow hit. It wasn’t much, indeed about a centimeter. Then the skies cleared – for the rest of the night. The temperature plummeted to -5.9°C. Clearly, the snow had a large impact on this nightly chilling. At 10am, the sun climbed high enough to get over the ridge of the mountain, pushing the temperature towards the freezing level for the first time. Until then, it didn’t get warmer than -4.4°C.
The next day I was at a camping in Saas Grund, with clear skies. It was about 20°C in the valley, but as soon as the sun got behind the mountain, the temperature dropped. The next morning, the tent was frozen solid – just like all the cars around us. When hiking to Hohsaas (3300m), we ran into various frozen waterfalls. In September.
Experience taught me that it must be really cold to let a waterfall freeze. So what happened? The low temperatures in the evening stopped the melting of the glaciers and snowfields early in the evening. Of course a lot of run-off water is still present but considerably less. As there is less flow, its easier to freeze. The next morning, temperature rises and the water starts to flow again.
This knowledge is something which I still use in winter time. I live in the Netherlands, world-famous for being as flat as a pancake. Which isn’t true, we just lack mountains and high elevation, but it isn’t flat everywhere. When I look out of my window, I can see hills on the horizon with an altitude of slightly over 100 meters. As I live on 15 meters, the height is 85 meters from my position. In our drouzy winters, where we don’t get a lot of snow, this is often enough: sleet in the village can be (literally) 10 cm of snow just 60-80 meters higher. Although it is more common to be just 1 cm…