The layers of air above the earth form what is known as the atmosphere. Directly above the surface of the earth, the first layer (troposphere) extends upwards for 9 miles. Above this is the stratosphere which extends from 9 to 31 miles, and then the mesosphere which extends from 31 miles to 53 miles.
About 99 percent of the air is nitrogen and oxygen. All living creatures depend on these gases for their survival. It is held close to the earth by the pull of gravity. The most dense air is closest to the earth, with the least dense (or 'thin' air) furthest away.
