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The density
of air decreases with altitude, i.e. with the diminution of atmospheric
pressure to which the air molecules are subjected.
At sea-level, each cubic metre of air contains a mass of around 1.25
Kg. At 5,000 metres (around the height of Mont Blanc), this mass goes
down to 0.75Kg/ m³ and at 9,000 metres (roughly the height of Everest),
it's down at 0.47Kg/ m³. At 18,000 metres (the cruising height of
Concorde), it is already 10 times less dense than at sea-level, at
0.123Kg/m³. At this altitude, it's impossible to breathe.
Atmospheric pressure also goes down with the speed at which air is
displaced. According to the principle discovered by the Swiss physicist
Daniel Bernoulli (1700-1782), the faster a gas moves, the lower its
pressure. This principle is fundamental for building flying-machines.
The form of an aeroplane wing makes the air go faster on the upper
surface - so the pressure underneath is higher than on the upper surface,
which generates a force directed upwards, holding up the aeroplane.
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