atmosphere

1. One of the four components of the Earth's ecosystem (the other three are biosphere, hydrosphere, and lithosphere), it is a band of gases enveloping the Earth's surface. Ninety-nine percent of its mass is concentrated within 20 miles of the earth's surface, and its two largest constituents (in the lowest part) are nitrogen (about 78 percent) and oxygen (about 21 percent). The remaining one percent includes mostly argon and minute amounts of carbon dioxide, helium, krypton, methane, neon, sulfur dioxide, water vapor, and xenon. In meteorology, atmosphere is divided into seven layers called regions (from the lowest to the highest): troposphere, stratosphere, mesosphere, chemosphere, thermosphere, ionosphere, and exosphere.

2. Unit of pressure based on the average atmospheric pressure at sea level.