Question

why is one not able to open windows of a airplane while flying in it

Answer 1

because of the air pressure outside doesn't allow to open the window

Answer 2

Gravity tends to keep air molecules concentrated near the ground, so the atmosphere thins out as you go up. The air becomes so thin at 10,000 feet (3,000 meters) or so that airplane cabins must be pressurized above that altitude to prevent occupants from suffering from hypoxia, or lack of oxygen. Because temperature and pressure go hand-in-hand (i.e. low-pressure air feels cold), pressurization is also necessary to keep cabins sufficiently warm.

At 35,000 ft. (11,000 m), the typical altitude of a commercial jet, the air pressure drops to less than a quarter of its value at sea level, and the outside temperature drops below negative 60 degrees Fahrenheit (negative 51 degrees Celsius), according to The Engineering Toolbox. Exposed to such conditions, you would quickly die.

Pressurization is normally achieved by pumping the cabin with "bleed air," or compressed air sucked in and heated up by the plane's turbine engines. Pressurization only works in an airtight fuselage. Were you to open a plane window, the compressed air inside would rapidly rush out, atmospheric conditions inside and outside the plane would equalize, and everybody would die.

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