You can neither see it nor feel it, but the air around you is both crucial to your survival and actually weighs something (which is why scuba and oxygen tanks are lighter when they’re empty). Air is thickest and heaviest at sea level, where humans have evolved to live. Planes, on the other hand, fly faster and more smoothly at high altitudes, around 35,000 feet (11 km). Way up there, the air is thin and frigid. Fly this high without your own air supply and you’d pass out from lack of oxygen. So, using a complex mechanical system that takes air from the engines, airplane cabins are “pressurized” to maintain the air pressure of an altitude that the passengers and pilots find comfortable.
Picture Credit : Google