Science, Tech, Math › Science How to Define Atmosphere Share Flipboard Email Print Martin Deja/Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated August 12, 2019 The term "atmosphere" has multiple meanings in science: Atmosphere Definition Atmosphere refers to the gases surrounding a star or planetary body held in place by gravity. A body is more likely to retain an atmosphere over time if gravity is high and the temperature of the atmosphere is low. The composition of the Earth's atmosphere is about 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, with water vapor, carbon dioxide, and other gases. The atmospheres of other planets have a different composition. The composition of the Sun's atmosphere consists of about 71.1 percent hydrogen, 27.4 percent helium, and 1.5 percent other elements. Atmosphere Unit An atmosphere is also a unit of pressure. One atmosphere (1 atm) is defined to be equal to 101,325 Pascals. A reference or standard pressure is commonly 1 atm. In other cases, "Standard Temperature and Pressure" or STP is used.