1013.25 mBars rather than 101325 Pascals is the normal UK/American version of the International Standard Atmosphere. It is this figure that is used to calibrate pressure altimeters and calculate speeds (TAS) and altitudes at height. Airspeed is a function of height and pressure which is why the speed of sound is set at Mach 1; the actual airspeed varies with height. The air pressure and temperature varies widely over the surface of the planet so much of the inhabited surface is divided up into pressure setting regions both for flight purposes and also for other functions of weather monitoring. If you ever listen to the shipping forecasts you will recognise the names such as Forties, Dogger etc. The Barnsley region is entirely over land so only those who fly will be familiar with it.
The percentages of different gases also vary over the surface of the planet so there is a standardised internationally recognised version of air. This is used, for instance, to define clean air - that is air free from other impurities that might be found under localised conditions. The figures given are accepted as good everywhere, but where there is a lot of pollution or at height where the gases are at different densities because of temperature change (colder than -55 up at the tropopause which is where airliners fly on long journeys for fuel efficiency) the figures are different. When I flew on air-to-air refuelling flights we had a setting which allowed us to breathe 100% oxygen, good for the hangover, but if done for a sustained period could cause quite painful oxygen lung and oxygen ear.
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