Facts About Stratosphere

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 horizontal air 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 and converts 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.