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The gaseous layer enveloping a body of substantial a mass either gaseous and /or solid positioned as an envelope due to the gravity of the mass.
Atmosphere is referred to as a layer surrounding a celestial body, mostly planets. Atmosphere meaning makes it clear that when say ‘Atmosphere’ it doesn’t necessarily mean Earth’s Atmosphere. There are other heavenly bodies which have their own Atmosphere.
Celestial bodies having Atmosphere
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The Atmosphere of Earth is a layer of gases surrounding it and held in place by the Earth’s gravitational force.
The atmosphere has a mass of about 5×1018 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface.
The atmosphere becomes thinner and thinner with increasing altitude.
The lower levels of the atmosphere are warmer than the upper levels. This is because in the upper layers there is hardly any molecular activity to generate heat. The lower layers receive a lot of electromagnetic radiation enabling it to be warmer.
The effects of Atmosphere become noticeable at an altitude of 120 km (75 mi).
Though it is considered that there is no definite boundary between the atmosphere and outer space, the Kármán Line is the scientific measurement. This line between atmosphere and outer space is located at 100 km (62 mi).
The stratosphere, mesosphere and lowest part of the thermosphere are collectively referred to as the Middle Atmosphere, which spans heights from approximately 10 to 100 km.
Atmosphere mostly contains dry air with about 1% of water vapours in them.
The air content and the atmospheric pressure in the atmosphere are not conducive for inhabitation. Troposphere is the only region of the atmosphere which is capable of supporting life.
The evolution of Earth’s prevalent atmosphere is not completely understood. It is considered that it resulted from a gradual release of gases both from the Earth’s interior and from the metabolic activities of life forms.
The Atmospheric composition of Earth is composed of various gases and water vapours. Apart from gases the composition may include in very meagre amounts - dust, pollen, spores, sea spray and volcanic ash. Industrial pollutants like chlorine, fluorine compounds, elemental mercury and sulphur dioxide [SO2] may be present. The content of various gases in the atmosphere isn’t uniform; it varies with distance from Earth. The atmosphere near the surface is composed primarily of Nitrogen and Oxygen both of which are the most abundant gases in the atmosphere. The atmosphere farthest from the Earth is composed of Helium and Hydrogen. The farthest part of the atmosphere is the thinnest and the nearest part is the thickest. This is because the gravitational pull of Earth is greatest near the surface.
Atmosphere Composition Details
|
Contents |
Percentage |
|
Nitrogen (N2) |
78.084% |
|
Oxygen (O2) |
20.946% |
|
Argon (Ar) |
0.9340% |
|
Carbon dioxide (CO2) |
0.039445% |
|
Neon (Ne) |
0.001818% |
|
Helium (He) |
0.000524% |
|
Methane (CH4) |
0.000179% |
|
Krypton (Kr) |
0.000114% |
|
Hydrogen (H2) |
0.000055% |
|
Nitrous oxide (N2O) |
0.0000325% |
|
Carbon monoxide (CO) |
0.00001% |
|
Xenon (Xe) |
0.000009% |
|
Ozone (O3) |
0.000007% |
|
Nitrogen dioxide (NO2) |
0.000002% |
|
Iodine (I2) |
0.000001% |
|
Ammonia (NH3) |
Traces |
|
Water vapour (H2O) |
~0.40% |
Atmospheric structure of Earth is defined by various layers. Earth's atmosphere is divided into five main layers.
From Nearest to Farthest, these layers are:
The troposphere is the nearest layer of atmosphere from the Earth. It is also known as the First, Lowest or Innermost layer of Earth’s atmosphere.
It extends to between 30,000 ft (9 km) at the poles and 56,000 ft (17 km) at the equator. Troposphere is also the largest part of the atmosphere; it comprises about 80% of the mass of the atmosphere. It also contains 99% of atmosphere’s water vapour and aerosols.
The distributions of troposphere’s gaseous components are largely uniform. Water vapours however do not have a uniform distribution as they are dependent on the rate of evaporation and transpiration. The amount of water vapours is normally greatest near the surface and decreases with height. This is because water is formed on the surface of Earth and there is a loss of water as we go further from the Earth.
The troposphere contains most of Earth’s clouds and weather occurs in this layer. The reason for this is that most of the water vapours and dust particles in the atmosphere are in found in this layer. The bottom of the troposphere, right next to the surface of Earth, is called the Boundary Layer.
The temperature of the troposphere generally decreases as altitude increases. The troposphere is mostly heated by transfer of energy from the surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude. The temperature there is around -55° C (-64° F). This promotes vertical mixing. The flow of the atmosphere generally moves in a west to east direction.
The border between the troposphere and stratosphere is called the Tropopause. It is a temperature inversion layer and there is little mixing between the two layers of the atmosphere. Tropopause does not have a uniform height; it varies depending on the latitude and season. Thus, the tropopause near the equator is about 20 km (12 miles or 65,000 feet) above sea level. Near the poles the tropopause is about 7 km (4 miles or 23,000 feet) high. The jet stream is just below the tropopause. These are fast flowing air currents having speeds of 400 km/hr (250 mph).
Stratosphere
Stratosphere is the second nearest layer of atmosphere from the Earth. It is the second major layer of atmosphere from lowest to highest layer. It is placed above the troposphere and below the mesosphere.
The stratosphere extends from the tropopause to about 51 km (32 mi; 170,000 ft). The stratosphere is situated between about 10 km (6 mi) and 50 km (30 mi) altitude above the surface at moderate latitudes, while at the poles it starts at about 8 km (5 mi) altitude.
The altitude of the bottom of the stratosphere varies with latitude and with the seasons. It position is slightly lower in winter at mid and high latitudes and slightly higher in the summer.
It occurs between about 8 and 16 km (5 and 10 miles, or 26,000 to 53,000 feet) above Earth's surface. Stratosphere near the equator is placed around 10 km (6 miles) from Earth. It is placed around 8 km (5 miles) near the poles.
Stratosphere is characterized by rise in temperature with increase in height. This is caused by ozone layer which increasingly absorbs ultraviolet radiation from the sun and restricts turbulence and mixing of air. The upper part of the stratosphere is warmer as compared to other parts which can be −60 °C (−76 °F; 210 K) at the tropopause.
It is region of intense interactions among radiative, dynamical, and chemical processes. The horizontal mixing of gases is much more rapid than in vertical mixing.
The stratosphere is dynamically stable because of the presence of warmer layers above and cooler layers below. Lack of regular convection and restriction of turbulence makes the layers of air quite stable here. That is why commercial aircrafts fly in the lower stratosphere. Most of the commercial airlines fly at an altitude of 9–12 km (30,000–39,000 ft).
Ozone gas is found in abundance in the stratosphere. It is most abundant in mid stratosphere. Here the conditions are favourable for O and O2 to combine to produce the ozone as there is less amount of ultra violet passing through at this level.
In the upper part of the stratosphere atomic oxygen is found in large amount. The large amount is result of enormous amount of ultra violet light and the destruction of both ozone and diatomic oxygen.
The stratosphere has a very dry environment and the water vapour content is very less here. As a result very few clouds are found in this layer. Amongst the few clouds are PSCs - Polar Stratospheric Clouds or Nacreous Clouds. These clouds occur in the lower stratosphere near the poles in winter. They are found at altitudes of 15 to 25 km (9.3 to 15.5 miles) and form only when temperatures at those heights dip below -78° C. They are also considered to be responsible for the formation of ozone holes by enhancing certain chemical reactions responsible for destroying ozone.
The region between the Stratosphere and Mesosphere is known as the Stratopause. It is located at 50 to 55 km (31 to 34 mi; 160,000 to 180,000 ft). The pressure here is 1/1000 sea level.
Stratosphere is able to support bacterial life, the reason why it is a part of the biosphere. Some bird species have been sighted at the lower levels of the layer, which includes Rüppell's Vulture and Bar-headed geese.
Mesosphere is the third layer of atmosphere from the Earth. It is located directly above the Stratosphere and directly below the Thermosphere.
The mesosphere starts at 50 km (31 miles) above Earth's surface and goes up to 85 km (53 miles) high. The mesosphere extends from the Stratopause to 80–85 km (50–53 mi; 260,000–280,000 ft).
The altitude (upper and lower) of the mesosphere depends upon the latitude and season. However, the lower portion of the mesosphere is usually located at heights of about 50 km above the Earth's surface. The uppermost part of the mesosphere is usually at heights near 100 km. It descends to heights of about 85 km at middle and high latitudes during summers.
Mesosphere is characterised by temperature decrease with increasing height. Mesopause, the upper boundary of the mesosphere is the coldest part of this layer. The mesopause is usually located at heights of 100 km from the Earth. In fact it is often called as the coldest place on Earth with average temperature around −85 °C (−120 °F; 190 K). The mesopause is also the region between the mesosphere and the thermosphere. The temperature varies according to latitude and season and can fall as low −100 °C (173 K; −148 °F). Such freezing environs are caused due to decrease in solar heating and increase in cooling by Carbon dioxide radiative emission.
The cold temperature of the mesosphere forces the water vapours to freeze. The frozen water vapours form ice clouds known as Noctilucent clouds, near the North and South Poles. These are unique amongst clouds as they form much higher up than any other type of cloud.
Mesosphere is also the layer where red lightning with downward bluish tinge, also known as elve and sprites, occurs. The luminous glow may extend upwards to ionosphere (around 55-60 miles) and sometimes downwards into the stratosphere below (to as low as 15-20 miles).
Mesosphere is also a Meteor burning layer. When meteors from space enter the atmosphere they start burning in the layer because of the friction between the gas and the mass of the meteorite.
In the mesosphere all kinds of gases are mixed together in the air. This movement of air is enabled by waves of air in the lower atmosphere. The waves carry energy to the mesosphere.
The mesosphere is the least understood atmospheric layer. Very less is known about the mesosphere as the conditions here are not conducive for research. Its high altitude proves to be a major hurdle for weather balloons and jet planes to reach. The orbits of satellites above the mesosphere are also a concern. There aren’t enough methods to enable the placement of scientific instruments. Sounding rockets are however able to provide limited data for study.
The fourth atmospheric layer from the Earth is thermosphere. It lies directly above the mesosphere and directly below the exosphere.
It extends from about 80 km (50 miles) to between 500 and 1,000 km (311 to 621 miles) above Earth.
Thermosphere is characterised with increase in temperature with increase in height from the mesopause up to the thermopause. From here onwards, the temperature remains constant with height. The rise in thermospheric temperature is caused by high absorption of highly energetic solar radiation enabled by the small amount of residual oxygen which is composed of extremely low density of molecules.
Thermosphere temperature is highly dependent on sun’s heat and can rise to 1,500 °C (2,730 °F). Sun’s radiation contains X-ray and UV rays which is readily absorbed by the layer. When the sun is very active the temperature can reach up to 500° C (900° F).
There is a difference of about 200° C (360° F) in daytime and night time temperatures. Temperature rises up very easily in the lower thermosphere i.e. below 200 to 300 km altitude. The uppermost parts of the thermosphere have temperatures ranging from about 500° C (932° F) to 2,000° C (3,632° F) or higher.
This rise in temperature is so high that it inflates the thermosphere raising the height of the uppermost portion of the layer. Such inflations are responsible for causing variance in the height of the uppermost region which is known as Thermopause. The expansion of air in thermopause generates a drag force on satellites. The reason for which satellites are made to withstand this drag force. The thermopause is found at an altitude between 500 km and 1,000 km or higher. Thermopause is also the boundary which separates thermosphere and the exosphere.
Ultraviolet radiation (UV) in thermosphere causes ionization. High-energy solar photons break electrons away from gas particles producing electrically charged ions of atoms and molecules. This particular region of ionized particles is termed as Ionosphere. Ionosphere overlaps certain portions of the mesosphere, thermosphere and exosphere. It occurs at an altitude of about 85 km to 600 km.
Radiation affected particles and electrically charged particles enable radio waves to bounce off and be received beyond the horizon. The International Space Station orbits within this layer, between 320 and 380 km (200 and 240 mi).
In the thermosphere infrequency of molecular collisions causes the air to be poorly mixed. This also results in making the air very thin. Thin air prevents atoms and molecules of gases to rarely mix. The air here becomes so rarefied that an individual molecule of oxygen travels an average of 1 kilometer between collisions with other molecules. Depending upon their chemical properties, gases tend to separate.
Energetic ultraviolet and X-ray photons from the Sun also break apart molecules in the thermosphere making atomic oxygen (O), atomic nitrogen (N), and helium (He) the main components of air in the upper regions of the layer.
The intersection of the mesosphere and the thermosphere is known as the Turbopause. It lies at an altitude of roughly 100 km
Thermosphere is also known for the occurrence of the Aurora also referred as the Southern and Northern Lights. Auroras are caused by collision of charged particles (electrons, protons, and other ions) from space with atoms and molecules in the thermosphere at high latitudes. When these particles are highly charged they shed the excessive energy by emitting photons of light which have a colourful display. Such electrical currents are caused by atmospheric waves and tides enabling the rare collision of elements.
The status of Thermosphere as a layer of atmosphere is considered contestable. The reason for this is that it is almost space like because of the low density of air and technically space begins at an altitude of 100 km (62 miles).
The fifth atmospheric layer from the Earth is Exosphere. The exosphere lies above the thermosphere. It is the outermost layer of Earth's atmosphere and starts from about 500 kilometres (310mi) from the Earth’s surface.
It is mainly composed of hydrogen and helium.
The lower boundary of the exosphere is known as Exobase or Thermopause.
Temperatures in this layer remain constant, they do not rise or fall with decrease or increase in altitude.
Exosphere is characterised rare collision of molecules as the air is very thin. At least half of the upward-moving molecules do not collide with one another. The particles here are so far apart that they can travel hundreds of kilometers in a ballistic trajectory without colliding with one another. With rare chances of collision the particles ie. atoms and molecules often escape into space.
The loss rate of molecules through the exosphere is critical in determining whether Earth or any other planetary body retains an atmosphere.
Some opine that the outermost limit of the exosphere to be around 190,000 km (120,000 miles), about halfway to the Moon. However there is no clear consensus on this one and the boundary beyond exosphere is generally regarded as Space; there is no clear demarcation between outer space and the exosphere. Therefore, the exosphere is sometimes considered a part of outer space.
