If it is a delicious 25 degrees in the Dal of Chamonix (1,035 meters above sea level), the mercury sinks at the top of the nearby Mont Blanc (4,809 m) almost through the freezing point. How is that possible?
You might think that it would be closer to the sun at the top of the mountain. But the difference in height between Top and Dal, a few kilometers, is totally not in the distance to the sun: almost 150 million kilometers.
Various explanations for the cold mountain top are circulating on the internet. A frequently heard – also in the geography lesson – states in short that the earth absorbs and broadcasts the solar radiation – and that you experience less heat radiation further from the earth’s surface. Just like when you sit further away from a stove. But … that mountain top is also part of the earth’s surface? Doesn’t it shine? And why do we actually depend on that indirect ‘earth radiation’?
We ask Kike Blasband. She is a geologist and energy expert at consultancy company BCG. And mountaineer. “The sun broadcasts radiation with short wavelengths, mainly in the form of visible light and ultraviolet radiation,” she says. “That radiation passes the atmosphere fairly unhindered and is only absorbed by the earth’s surface.” This radiation heats the country and the oceans, lakes and rivers, forests and cities. In contrast to the atmosphere, the earth’s surface is a very efficient soles of solar energy.
As soon as the earth’s surface of the earth absorbs, it broadcasts that energy, Blasband continues. “But now in the form of radiation with long wavelengths: infrared radiation. That is heat energy, which is then transferred to the air. Many gases in the atmosphere, especially methane, water vapor and carbon dioxide, are very good at absorbing this radiation. These are the well -known greenhouse gases: they hold the heat and prevent the earth’s surface from cooling again immediately. ”
The radiant surface
However, this process is less effective on larger heights, she emphasizes. As you get higher, the atmosphere becomes thinner. In other words, there are fewer gas molecules to absorb the long -wave radiation. What makes the air thinner at height? “At sea level, the air is closer because it is compressed by the weight of the atmosphere above it,” Blasband replies. “The higher you get, the less atmosphere there is above you, and the lower so the air pressure. And therefore also the lower the heat capacity of that air: the amount of energy that that air can absorb. ”
The mountain top is therefore part of the radiant earth surface, but the air at that height heats up less. A larger part of the heat radiation can escape unhindered, into the space. In short: “The” blanket “of greenhouse gases is thinner there and is therefore less insulating there.” The result is that the temperature falls on average by around 6 to 7 degrees with every thousand meters that you rise. Exactly how much, that depends on the humidity. “It is mainly the amount of water vapor that determines the heat capacity of the air.”
And then something plays with it: “Mountain peaks are often mainly white and gray, due to the snow and bare rock. As a result, they reflect a larger part of the sunlight and therefore warm up less than dark soil, or a forest in the first instance. ”
And what is the coldest that she has experienced on a mountain top ..? “… Kilimanjaro …? My nose almost froze off but it was very cool … “