The Earth's atmosphere consists of a thin layer of gases surrounding the Earth. Most gases surrounding the Earth are held in place by gravity. The most abundant gas in the Earth's atmosphere is nitrogen, accounting for 78%, followed by oxygen with 21%, argon with 0.9 %, carbon dioxide with 0.03% and traces of other gases. Even though, the atmosphere extends for hundreds of kilometers above the Earth's surface, it is compressed to about 6 km (4 miles) above the Earth's surface, with no clear boundary between the atmosphere and outer space.
The Earth's atmosphere is divided into five distinct layers. These layers show variations in temperature according to height. Other variations include chemical composition, movement and air density, which serve to differentiate each layer. Most variations occur when one layer transits into another upper layer at regions known as pauses.
The first 11 km (9 miles) of the atmosphere are known as the troposphere and is where breathable air exist. The troposphere width varies from the equator to the poles, with its deeper portion at the equator and the shallower portion at the poles. In this layer all known weather phenomena on Earth occurs. In the troposphere temperature decreases with altitude at about 6.4 °C per km (7 °F per mile). The troposphere contains almost 80% of the atmosphere's mass. At the top of this layer, there is a thin layer, known as the tropopause, dividing the troposphere and the next upper layer.
Above the troposphere is the stratosphere, extending to about 50 km (31 miles) high in the atmosphere. Stream-like currents of air flowing at high speeds are contained within this layer. In this layer, the temperature increases with height due to the absorption of ultraviolet radiation by the ozone layer, which lies in the upper stratosphere at about 15-30 km (). At about 50-55 km lies the boundary layer, called the stratopause, above which the temperature decreases with altitude. The stratopause divides the stratosphere and the next layer called the mesosphere.
Following the stratosphere is the mesosphere, which extends to about 80-85 km (50-53 miles). In this layer, most meteors suffer desintegration upon entering the Earth's atmosphere. The temperature decreases with height in the mesosphere and reaches its minimum at the mesopause, which is termed as the coldest place on earth, with freezing temperatures of -85 °C (-120 °F). The Boundaries in the mesosphere may vary based on latitude and season, with the lower boundary situated at 50 km above the surface of the Earth and the mesopause at altitudes of 100 km above the Earth's surface.
Above the mesosphere is the thermosphere. In this layer, temperature increases with altitude with the highest temperatures rising to about 2,000 °C (3,630 °F). In this layer, ultraviolet radiation causes ionization, producing ions with a positive or negatively electric charges. In the thermosphere, there is extreme low density with molecules so far apart that it may take up to one kilometer for collisions among molecules to occur. The layer dividing the thermosphere and the next layer, the exosphere, is the exobase and is located at about 350-800 km (220-500 miles) from the surface of the Earth.
The layer at the outer upper atmosphere is known as the exosphere. This layer is composed of the lighter gases, hydrogen and helium, with atomic oxygen and carbon dioxide in the lower portion of the exosphere. The composition of this layer is the less dense of all the atmospheric layers, with particles displacing for hundreds of kilometers and rarely colliding with one another. In the upper par of the exosphere, the influence of solar radiation on atomic hydrogen is superior to the gravitational pull of the Earth and is considered as the transitional zone between the Earth's atmosphere and outer space.
The atmosphere may be divided based on its chemical properties into two distinct regions; the homosphere and the heterosphere. The homposphere comprises the lower 75-100 km (46-62 miles) and is the region where the gases are uniformly mixed from the base to the top. The heterosphere is the layer above the homosphere and is where the lighter gases, mostly hydrogen and helium, have gained greater heights and are able to scape into intergalactic space. According to phys.utk.edu, the Earth's present atmosphere is not the original Earth's atmosphere. The current atmosphere is considered as an oxidizing atmosphere, while the Earth's original atmosphere was a reducing atmosphere.It is thought that this atmosphere did not contain oxygen within.