Thinner than WHICH air?

Photo courtesy of www.mounteverest.net

A week or so ago I attended a lecture by a man who had recently climbed Everest. We should be, of course, impressed by people who have climbed Everest. However, the feat is becoming more and more common. This climber told us that there were around 250 mountaineers on Everest during the period he was making his ascent. Not all of them made it to the top, of course. To set a record these days, it is necessary to make the ascent two or three times in the same year, preferably using different routes.

Putting aside logistics, the difficulty that the human body has to adapt to altitude and the presence of the very strong winds on Everest, the ascent isn't that difficult in a mountaineering sense. There are sherpas to carry your things (some have made more than a dozen ascents of the summit) and so, essentially, all you have to do is acclimatise yourself, wait for a weather window, then drag yourself up a fixed rope to the top. From the advanced base camp, you can do the return trip in a day.

Another interesting thing I learnt, was that helicopters have an altitude ceiling above which they cannot go. Only recently has a specially adapted helicopter been able to reach the summit of Everest. Why do helicopters have this ceiling? In fact it is called the 'thin air' ceiling. It results from the fact that the air around the summit of Everest (and everywhere else at this altitude) is not dense enough to provide resistance to the helicopter rotor blades, and hence the whirlybird does not rise. It is the same reason that aircraft are able to fly faster at higher altitude and why prototype very fast aircraft are tested at high altitude.

It's difficult to imagine that something which is completely invisible could be thinner. I suppose it helps to remember that there is such a thing as liquid CO2 and that it may be present somewhere close by to you, dear reader, in the form of a CO2 fire extinguisher.