Composition and Layers of Atmosphere
Earth’s atmosphere is mainly consisted of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases are often referred to as trace gases. The below table shows the composition of Dry atmosphere.
Composition of Earth’s Atmosphere
The upper boundary of the atmosphere is not clearly defined. For differentiation of aeronautics and astronautics, the Kármán line at 100 kilometers from sea level is used. Below around 100 kilometers or so, the atmosphere behaves like a fluid. The outermost layer of Earth’s atmosphere is mainly composed of hydrogen and helium. The particles 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 of the Earth. The atmosphere has been divided into several layers on the basis of change in height and some other factors such as change in climate etc.
These include the Troposphere (the lowermost), Stratosphere (stratified), Mesosphere, Thermosphere, Exosphere (outer space). Between individual spheres there are usually distinguished transitory layers, called ‘PAUSES’ where temperature varies but little with height The character and composition of the atmosphere changes as we go higher and higher.
Thus, there are 4 important spheres, with 3 pauses as follows:
- Troposphere with tropopause
- Stratosphere with stratopause
- Mesosphere with mesopause
- Ionosphere or thermosphere
Troposphere is the lowest portion of Earth’s atmosphere and contains approximately 80% of the atmosphere’s mass and 99% of its water vapour and aerosols. The average depth of the troposphere is approximately 17 km in the middle latitudes. The characteristic features of the Troposphere are its great density. In addition to nitrogen and oxygen, carbon dioxide, and water vapour (nearly all of the water vapour contained in the atmosphere is concentrated in the troposphere) and of numerous particles of various origin.
Thickness of Troposphere
It thickness of the Troposphere is maximum at equator, deeper in the tropics, up to 20 km , and shallower near the polar regions, at 7 km in summer, and indistinct in winter. In India, it is taken to be around 16 Kilometers. The thickness of the troposphere and consequently the atmosphere is maximum at the equator due to the reasons discussed below:
- High insolation and strong convection currents occur over the Equator
- One of the laws of Ideal gases called Charles’ law says that in an ideal gas, density decreases with increasing temperature, when pressure is constant. The hot air rises and the Earth is not equally heated everywhere. The troposphere is thicker over the equator than the poles because the equator is warmer. Heat differential on the planet’s surface causes convection currents to flow from the equator to the poles. This implies that the warmer the weather, the thicker is the troposphere. Thus the simple reason is thermal expansion of the atmosphere at the equator and thermal contraction near the poles.
- Air is less dense at Equator
- Over equatorial regions, where the surface is being heated strongly throughout the year and air warmed by contact with it is expanding and rising, the air all the way up to the tropopause is less dense than air to the north and south. Thus, density of the air is maximum at the equator. But here, you must note that almost same amount of atmospheric mass exists at both equator and poles but only the density of the air is less at equator and greater at poles
- Poles Exert more gravitational pull on atmospheric gases
- Gravity increases from equator to poles as the earth is not a perfect sphere. That means the gravitational force is more over poles. Hence the atmosphere is pulled with more force near the poles and leads to contraction of the atmosphere.
- The centrifugal force due to Earth’s rotation is maximum at Equator
- Because the speed of the rotating earth is greatest at the equator the atmosphere tends to bulge out due to friction and Coriolis force.
Chemical Composition of Troposphere
The chemical composition of the troposphere is essentially uniform, with the notable exception of water vapour. The amount of water vapour decreases strongly with altitude. Thus the proportion of water vapour is normally greatest near the surface and decreases with height.
Temperature of Troposphere
Temperature of the troposphere decreases with height. The rate at which the temperature decreases is called the Environmental Lapse Rate (ELR). The environmental lapse-rate (ELR) is about 0.6°C per every 100 meters. Temperature decreases at a nearly uniform rate with increased altitude.
The reason for lapse is that maximum absorption of the sun’s energy occurs at the ground which heats the lower levels of the atmosphere, and the radiation of heat occurs at the top of the atmosphere cooling the earth, this process maintaining the overall heat balance of the earth.
The boundary between troposphere and stratosphere, called the tropopause, is a temperature inversion. Tropopause refers to the altitude at which the fall in the temperature is stalled. This layer separates the troposphere from the stratosphere (the second layer of the atmosphere). This layer is usually quiet and no major movement of air takes place in it. Its height at Tropic of Cancer and Tropic of Capricorn is roughly 10 to 15 km, highest at the equator 18 km and at the poles it is about 8 km above the earth. In India, the tropopause is generally at a height of around 16 km. The altitude of the tropopause varies with the variations of sea — surface temperature, season, latitude, and weather systems, such as the passage of cyclones and anti-cyclones. So, Tropopause is not a hard lined boundary. The higher is the temperature of the lower layers, the higher is the height of this layer, the layer is lower where there is a cyclone below it. Also note that the tops of cumulus-nimbus clouds often float in his region.
The stratosphere is the second major layer of Earth’s atmosphere, just above the troposphere, and below the mesosphere. It is called stratosphere because it is stratified in temperature, with warmer layers higher up and cooler layers farther down. Top of the stratosphere has a temperature of about −3°C, just slightly below the freezing point of water. This is in contrast to the troposphere near the Earth’s surface, which is cooler higher up and warmer farther down. This inversion begins in tropopause.
The stratosphere is situated between about 10 km and 50 km altitude above the surface at moderate latitudes, while at the poles it starts at about 8 km (5 mi) altitude. Thus, stratosphere is nearest to poles altitudinally.
Why there are no Vertical Winds in Stratosphere?
The increase in the temperature with height in the stratosphere makes this region very stable place where the air tends not to overturn vertically. Thus vertical winds are almost absent in Stratosphere. In contrast with the atmosphere, where the vertical wind speeds are often several meters per second, in the stratosphere, they are seldom more than a few centimetres per second. The result is that it takes air a very long time to be transferred from the bottom of the stratosphere, unless there is a thrust of gases such as that during the highly explosive volcanic eruptions. The inability of the air to mix in vertical direction is also the principal reason why the Ozone depleting Chloro-Fluoro Carbons take so long to reach the altitudes where the Sun’s energy is sufficient enough to break them apart. This also implies that some of the ozone depleting substances will still be there a centuries later from now.
Water vapor Methane Interaction in Stratosphere
The source of methane in Earth’s atmosphere can be traced to its release at the surface through a variety of sources such as wood combustion, coal mining, oil and gas drilling and refining, landfills, wetland rice cultivation, crop residue burning, industrial activities and the digestive action by grazing animals (such as cow flatulence) and to some extent human flatulence because around half of us produce methane in farts!
The tropopause is the very cold boundary between the troposphere and the stratosphere. Due to this, the water vapour is frozen out when moist air is lofted upward through the tropopause. This means that the air that enters stratosphere is almost dry. On the other hand, methane remains unaffected by the cold temperatures as it passes through this boundary. Only when methane reaches the upper stratosphere, it is depleted via oxidation reactions with OH. These reactions lead to the production of water vapour molecules. Indeed, each methane molecule eventually is converted in to two molecules of water vapour in the middle to upper stratosphere via the following reaction in which methane is converted into water vapour by a reaction with the hydroxyl radical OH.
CH4+OH →CH3 +H2O
The second reaction involves a series of steps that begins with the methane reacting with the free oxygen form a hydroxyl radical (OH). This hydroxyl radical is then able to interact with non-soluble compounds like chlorofluorocarbons, and UV light breaks off chlorine radicals (Cl). These chlorine radicals break off an oxygen atom from the ozone molecule, creating an oxygen molecule (O2) and a hypochlorite radical (ClO). The hypochlorite radical then reacts with atomic oxygen creating another oxygen molecule and another chlorine radical, thereby preventing the reaction of monatomic oxygen with O2 to create natural ozone. This way, methane plays a role in hindering the formation of the Ozone layer. Above about 65 km, photodissociation of methane becomes an important mechanism for Ozone loss.
The temperature stratification in Stratosphere
In the stratosphere, temperature has a tendency to rise. This is due to the presence of Ozone. The first thing we have to note is that the air is highly rarefied and there are only eight ozone molecules to a million. The ozone (O3) here absorbs high energy Ultraviolet energy waves from the Sun and is broken down into atomic oxygen (O) and diatomic oxygen (O2). Atomic oxygen is found prevalent in the upper stratosphere due to the bombardment of UV light and the destruction of both ozone and diatomic oxygen. The mid stratosphere has less UV light passing through it, O and O2 are able to combine, and is where the majority of natural ozone is produced. It is when these two forms of oxygen recombine to form ozone that they release the heat found in the stratosphere. The lower stratosphere receives very low amounts of UV, thus atomic oxygen is not found here and ozone is not formed (with heat as the byproduct). This vertical stratification, with warmer layers above and cooler layers below, makes the stratosphere dynamically stable: there is no regular convection and associated turbulence in this part of the atmosphere. The top of the stratosphere is called the stratopause, above which the temperature decreases with height.
Aviation & Jet Streams in Stratosphere
Stratosphere is free from the violent weather changes which occur below in the Troposphere. So, it is preferred by commercial airliners. The commercial airliners typically cruise at altitudes of 9–12 km in the lower reaches of the stratosphere. They do this to optimize fuel burn. Jet liners, however, face another menace in stratosphere, namely jet streams. Jet streams are high velocity horizontalair currents. The main jet streams are located near the tropopause, the transition between the troposphere (where temperature decreases with altitude) and the stratosphere (where temperature increases with altitude). The location of the jet stream is extremely important for aviation. Jet streams are NOT always harmful for aviation. They are beneficial and used commercially as it reduced the trip time and fuel consumption. Commercial use of the jet stream began in 1950s when an aeroplane flew from Tokyo to Honolulu at an altitude of 7,600 meters cutting the trip time by over one-third. It also nets fuel savings for the airline industry.
Ozone Layer in stratosphere
As discussed above, the Ozone layer is contained within the stratosphere. In this layer ozone concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from about 15–35 km, though the thickness varies seasonally and geographically. About 90% of the ozone in our atmosphere is contained in the stratosphere. The Ozone layer absorbs ultraviolet radiation from the sun andc onverts it into heat and chemical energy. It is this activity that is responsible for the rise in temperature.The layer is NOT of uniform thickness. Height at the equator is maximum and lowest at the poles.
The mesosphere extends from the stratopause to 80–85 km. Most meteoroids get burnt in this layer.
Temperature decreases with height in the mesosphere. The mesopause, the temperature minimum that marks the top of the mesosphere, is the coldest place around Earth and has an average temperature around −85 °C. At the mesopause, temperatures may drop to −100 °C. Due to the cold temperature of the mesosphere, water vapour is frozen, forming ice clouds. These clouds are called noctilucent clouds. This implies that the noctilucent clouds are the highest clouds in the Earth’s atmosphere, located in the mesosphere at altitudes of around 76 to 85 kilometers (47 to 53 mi). They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the Earth’s shadow. Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon. Mesopause, a thin layer of extremely cold atmosphere, separates the mesosphere from the Ionosphere above.
Ionosphere is called so because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. Ionosphere stretches from 50 to 1,000 km and typically overlaps both the exosphere and the thermosphere. It has practical importance because it influences, for example, radio propagation on the Earth. It is also responsible for auroras.
Temperature in Ionosphere
Ionosphere is also known as THERMOSPHERE because of the high temperatures because of the high temperatures prevailing there as much as 870°C over the equator and 1427°C over the north pole, the temperature near the upper boundary of the thermosphere may become higher than 1000-1500°C. Along with temperature rise sharp changes caused by the corpuscular and ultraviolet solar radiation are observed in it.
Various Layers in Ionosphere
We note that the ionization depends primarily on the Sun and its activity. This means that the amount of ionization in the ionosphere varies greatly with the amount of radiation received from the Sun. This is the reason that there are changes in the Ionosphere and there are diurnal effect and seasonal effects. The activity of the Sun is associated with the position of earth in the revolutionary orbit, sunspot cycle, with more radiation occurring with more sunspots. Radiation received also varies with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions). There are also mechanisms that disturb the ionosphere and decrease the ionization. There are disturbances such as solar flares and the associated release of charged particles into the solar wind which reaches the Earth and interacts with its geomagnetic field. Accordingly, Ionosphere has been divided into different sets of layers during day and night which are shown in this graphic:
The D layer explains why the AM Radio gets disturbed during day time, but quite smooth in night time. We see in the above graphics that the D layer is the innermost layer, 60 km to 90 km above the surface of the Earth. At this layer, the net ionization effect is low, but loss of wave energy is great due to frequent collisions of the electrons. This is the reason that the high-frequency (HF) radio waves are not reflected by the D layer but suffer loss of energy therein. The absorption is small at night and greatest about midday. This causes the disappearance of distant AM broadcast band stations in the daytime.
The E layer is the middle layer, 90 km to 120 km above the surface of the Earth, with primary source of ionization being soft X-ray (1-10 nm) and far ultraviolet (UV) solar radiation ionization of molecular oxygen (O2). This layer disappears in the night because primary source of ionization is no longer present. The practical value of this layer is that it reflects long radio-waves back to earth, which enables them to be received at a distance, rather than disappear into space. It is also known as HEAVYSIDE-KENNELY LAYER.
Importance of E-Layer
The E layer is a region of the ionosphere which influences long-distance communications by strongly reflecting radio waves in the 1-3 megahertz. It is also called E region, Heaviside layer, or Kennelly-Heaviside layer. This region reflects radio waves of medium wavelength and allows their reception around the surface of the Earth. The layer approaches the Earth by day and recedes from it at night. In technical terms, it is a cylinder of relativistic electrons gyrating in the magnetic field, which produces a self-field strong enough to dominate the externally applied field and produces half reversal in the system. Since the mid ’20s, another connection regarding the ionosphere has been hypothesized that lightning can interact with the lower ionosphere. According to this theory, thunderstorms could modulate the transient, localized patches of relatively high-electron density in the mid-ionosphere E layer, which significantly affects radio wave propagation.
The F LAYER extends from about 200 km to more than 500 km above the surface of Earth. The E- layer allows the penetration of short-radio waves, which continue until they reach the APPLETON LAYER. . Appleton layer reflects short-radio waves (which have penetrated the HEAVYSIDE- KENNELY LAYER) back to earth. This is also supposed to be the region where polar AURORAS occur and where most of the meteors burn themselves out.
Concept of Aurora
The luminous effect of electro-magnetic phenomena in the ionosphere is known as Aurora, visible in high latitudes as red, green and white arcs, draperies, streamers, rays and sheets in the night sky, best developed at a height of about 90 km. Probably, aurora is the result of magnetic storms and of electrical discharges from the sun during periods of sun-spot activity, causing ionization of gases, though this is still a matter of research. It is called the Aurora Borealis (or northern lights) in the northern hemisphere and the Aurora Australis in the southern hemisphere. Occasionally the Aurora borealis is seen in England, but it is more common in northern Scotland, presents a magnificent spectacle in northern Scandinavia and northern Canada.
The exosphere lies above the altitude of 800 kilometer and it needs further studies. Characteristic of exosphere is an extreme rarefaction of the air; gas particles, moving with tremendous velocities, nearly fail to meet one another and there takes place an outflow of gas particles into the interpreter space.