Earth is "wrapped" with gases that we call "air". This air is a mixture of different gases: It contains approximately 79% of nitrogen (N2), about 21% oxygen (O2), circa 0.03% carbon dioxide (CO2) and many other gases in minuscule amounts. The layer of air wrapping earth is called the "atmosphere". Earth's atmosphere is affected by its gravity, which is why air has a weight of its own. This weight affects other layers of air and causes them to compress. Therefore, the air is more dense on the lower layers of the atmosphere (nearer to the surface of earth), and its density lessens as you go higher. Air is thinner at mountain tops and denser at sea level. The denser the air is, the more pressure it creates upon earth.