Atmosphere
Air temperature decreases by about 6.5ºC (43.7ºF) per kilometre. The upper limit, or tropopause, of the atmosphere or troposphere is reached at 10–12 kilometres (33,000–40,000 ft.), where the temperature is –56ºC (–70ºF).
Ozone is detected in the upper atmosphere at 10–40 km (33,000–130,000 ft.). It protects life on the ground from excessive ultraviolet radiation from the sun. Ozone present at the cruising altitude of a passenger aircraft does not pose a health hazard, although it may slightly irritate mucous membranes. Ozone filters in new aircraft remove the gas effectively.
Air pressure
The mean air pressure at sea level is 1,013 millibars (mb) equal to 760 mm of mercury (mmHg) or 14.7 lb./sq. in. The pressure decreases to one half at 5.5 km (18,000 ft.) and to one quarter at 10 km (33,000 ft.). The proportion of gases in the atmosphere remain the same as the total pressure decreases; at 10 km (33,000 ft.), the air still contains 21% oxygen, but the air pressure has decreased to one-quarter of the pressure on the ground.
Air resistance decreases as the barometric pressure diminishes; therefore, jet aircraft fly at a high altitude, since fuel consumption is also reduced as the air resistance decreases.
Keywords
Atmosphere
