The sky ends at the Karman line, which is located at about 67 miles above sea level. Above this line, space begins. However, technically, the sky does not end so much as the atmosphere, or sky, thins until there is no oxygen left.
According to The Guardian, one of the main factors that separate the sky from space is that a human being needs a spaceship in order to be up that high. Below that line, airplanes are necessary.
The sky, or atmosphere, is made up of layers of gases that sustain life on this planet. Not only is oxygen necessary for many life forms, but the atmosphere also protects the Earth from the full force of the sun. The atmosphere is made mostly out of oxygen, carbon dioxide, nitrogen, argon, trace amounts of other gases and water vapor. This water vapor is necessary in order for the continuance of the water cycle and life on Earth. Of these gases, nitrogen and oxygen make up the greatest percentage.
The atmosphere is divided into five layers: exosphere, thermosphere, mesosphere, stratosphere and troposphere. The troposphere, or first layer of atmosphere, is where most of life occurs. The exosphere is the thinnest and outermost layer of atmosphere.