Atmosphere
Composition
The Earth's atmosphere is a layer of gases, commonly known as air, that surrounds the planet and is retained by Earth's gravity. The atmosphere of Earth protects life on Earth by creating pressure allowing for liquid water to exist on the Earth's surface, absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).
Structure
The atmosphere has a complex structure, divided into five main layers. These are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Each layer has distinct characteristics and serves different roles in the functioning of the Earth's climate and weather patterns.
Troposphere
The troposphere is the lowest layer of Earth's atmosphere. It extends from Earth's surface to an average height of about 12 km, although this altitude varies from about 9 km at the poles to 17 km at the equator. The troposphere is bounded above by the tropopause, a boundary marked by stable temperatures. The troposphere is denser than the layers above it (because of the weight of the gases pressing down on it from above), and it contains approximately 75% of the atmosphere's mass and 99% of its water vapor and aerosols.
Stratosphere
Above the troposphere lies the stratosphere, which extends from the tropopause to about 51 km. The stratosphere is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is a result of the ozone layer that absorbs solar radiation and heats the stratosphere. The stratosphere contains the ozone layer, an important component that absorbs and scatters the solar ultraviolet radiation.
Mesosphere
The mesosphere extends from the stratopause to 85–90 km. Temperatures drop with increasing altitude to the mesopause that marks the top of this middle layer of the atmosphere. It is the layer where most meteors burn up upon entering the atmosphere.
Thermosphere
The thermosphere extends from the mesopause to the exobase. Its height varies with solar activity and ranges from about 350–800 km (220–500 mi; 1,150,000–2,620,000 ft). The temperature of the thermosphere gradually increases with height. Unlike the stratosphere beneath it, wherein a temperature inversion is due to the absorption of radiation by ozone, the inversion in the thermosphere occurs due to the extremely low density of its molecules.
Exosphere
The exosphere is the outermost layer of Earth's atmosphere. It extends from the exobase upward, mainly composed of hydrogen and helium. The particles of this layer are so far apart that they can travel hundreds of kilometers without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the magnetosphere or the solar wind.
Physical properties
The physical properties of the Earth's atmosphere include temperature, pressure, density, and the composition of the various gases that make up the air. The atmosphere is primarily composed of nitrogen (78%) and oxygen (21%) with trace concentrations of water vapor, carbon dioxide, and other gaseous molecules.
Role in climate and weather
The Earth's atmosphere plays a key role in the planet's climate and weather. It absorbs energy from the Sun, redistributes it around the planet, and prevents much of it from escaping back into space. This process, known as the greenhouse effect, is critical to Earth's ability to sustain life.