Air 2 topics · 2 sources ScienceEnergyThe mixture of gases that surrounds the earth and forms its atmosphere, composed of, by volume, 21 percent oxygen, 78 percent nitrogen.US DOE Office of Energy Efficiency & Renewable Energy — Glossary of Energy-Related Terms retrieved 2014-05-26 Cite ScienceWeatherThe mixture of gases comprising the earth's atmosphere.NOAA National Weather Service — National Weather Service Glossary Cite