On 09/07/2014 12:43, Micaela Addison Kemp wrote:
Noctilucent clouds form at 80/85 km (50 miles) high, which is close to the edge of space (there is very little air at that height).

I was wondering about this statement (80 km is not very high) but found that it was correct: it is the same height that the Lockheed SR-71 “Blackbird” flies at:

http://www.bbc.com/future/story/20130701-flying-the-worlds-fastest-plane

There is so little air at that height that most aircraft cannot function. Also, the curvature of the earth can be seen, as can the stars. It is a magical place where the Earth begins to become space (there is a marvellous sequence in a movie called “The Right Stuff”--1983, also a book--that allowed me to spot that).

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Alex Kemp