Layers of the Earth's Atmosphere
The atmosphere is divided into five layers. It is thickest near the surface and thins out with height until it eventually merges with space.
The exosphere is a thin, atmosphere-like volume surrounding a planetary body where molecules are gravitationally bound to that body, but where the density is too low for them to behave as a gas by colliding with each other. In the case of bodies with substantial atmospheres, such as the Earth's atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with interplanetary space. It is located directly above the thermosphere.The main gases within the Earth's are the lightest atmospheric gases, mainly hydrogen, with some helium, carbon dioxide, and atomic oxygen near the base of the exosphere. Since there is no clear boundary between outer space and the exosphere, the exosphere is sometimes considered a part of outer space.
The troposphere is the lowest layer of Earth's atmosphere. The troposphere starts at Earth's surface and goes up to a height of 7 to 20 km (4 to 12 miles, or 23,000 to 65,000 feet) above sea level. Most of the mass (about 75-80%) of the atmosphere is in the troposphere. Almost all weather occurs within this layer. Air is warmest at the bottom of the troposphere near ground level. Higher up it gets colder. Air pressure and the density of the air are also less at high altitudes. The layer above the troposphere is called the stratosphere.
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler higher up and warmer farther down. The border of the troposphere and stratosphere, the tropopause, is marked by where this inversion begins, which in terms of atmospheric thermodynamics is the equilibrium level. At moderate latitudes the stratosphere is situated between about 10–13 km (33,000–43,000 ft; 6.2–8.1 mi) and 50 km (160,000 ft; 31 mi) altitude above the surface, while at the poles it starts at about 8 km (26,000 ft; 5.0 mi) altitude, and near the equator it may start at altitudes as high as 18 km (59,000 ft; 11 mi).
The mesosphere from Greek mesos "middle" and sphaira "ball") is the layer of the Earth's atmosphere that is directly above the stratopause and directly below the mesopause. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring place on Earth with temperatures below 130 K (−226 °F; −143 °C). The exact upper and lower boundaries of the mesosphere vary with latitude and with season, but the lower boundary of the mesosphere is usually located at heights of about 50 kilometres (160,000 ft; 31 mi) above the Earth's surface and the mesopause is usually at heights near 100 kilometres (62 mi), except at middle and high latitudes in summer where it descends to heights of about 85 kilometres (53 mi).
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of theatmosphere's mass and 99% of its water vapour and aerosols. The average depth of the troposphere is approximately 17 km (11 mi) in the middle latitudes. It is deeper in the tropics, up to 20 km (12 mi), and shallower near the polar regions, approximately 7 km (4.3 mi) in winter. The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred metres to 2 km (1.2 mi) deep depending on the landform and time of day. The border between the troposphere and stratosphere, called the tropopause, is a temperature inversion.