atmosphere

The gaseous envelope surrounding the Earth's (solid and liquid) surface. The definition can be generalized to any celestial body, but in this entry we will refer only to the Earth's atmosphere, usually simply called "atmosphere." It constitutes one of the five subsystems making up the climate system. The air forming it is distributed in layers whose number varies depending on the criterion considered. Based on the vertical temperature profile, the atmosphere is divided into five well-defined layers (troposphere, stratosphere, mesosphere, thermosphere, and exosphere), in which, alternately, thermal rises and falls occur. This is the atmospheric division that prevails, although other complementary ones can equally validly be established, depending on chemical composition or electronic structure. Although some authors set the upper limit of the atmosphere at a certain altitude, such a limit has no physical meaning, since the boundary with space is diffuse. Air density decreases progressively as one ascends, without it being possible to affirm that from a certain level onward it equals zero. While for aeronautical purposes the boundary between the atmosphere and outer space is set at 100 kilometers of altitude - the so-called Kármán line -, in Meteorology, it is most common to place that boundary 300 kilometers above our heads. Most of the atmosphere's mass is concentrated in its lower part, where air density decreases very sharply with height. Approximately half of that mass is located in the first five kilometers of atmosphere and 99% in the first thirty. From there upward, the air is highly rarefied.