Layers of the atmosphere

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Sd-Questions asked in class

Vgd-Things taught in class

___-Important points

Layers of the atmosphere

The layers of the atmosphere are separated by "pauses," where there is no change in temperature with altitude change: the tropopause (between the troposphere and the stratosphere), the stratopause (between the stratosphere and the mesosphere), and the mesopause (between the mesosphere and the thermosphere).

Troposphere

The word troposphere derives from the Greek: tropos for "turning" or "mixing”. This is reflected in the fact that convective currents provide mixing of air masses with different temperatures in the troposphere.

In class, we have learnt that temperatures in the troposphere decreases with altitude. But by how much? Temperatures decrease by 6.3 °C with every 1000m increase in altitude. And why does this happen? This is because the Earth’s surface is warmed by solar radiation, which in turns heats the air around it by conduction, convection, and radiation.

At the utmost layer of the troposphere, the air temperature is about 76 degrees below
zero. This is very significant as it allows water vapour to be changed into ice, resulting in the formation of the cold trap .The cold trap prevents water molecules from rising beyond the troposphere. If no cold trap is formed, the water molecules would rise and be broken down into oxygen and hydrogen. Due to its small and light nature, the hydrogen molecules can then escape into space, causing Earth to lose its water. 

However, the temperatures in the troposphere do not always decrease with altitude, but increase. Such cases are known as temperature inversions where warm air is on top of cooler air. This situation prevent the vertical mixing of air, leading to air pollution when air pollutants released at ground level are trapped under the temperature inversion.

Refer to http://www.wrh.noaa.gov/slc/climate/TemperatureInversions.php for information on the occurrence of temperature inversions and their impact on the air quality.

Stratosphere

The stratosphere is characterized by a steady increase in temperature with altitude, caused by the concentration of ozone. Ozone absorbs ultra-violet radiation from the sun, heating the air around them.

The stratosphere allows for long-distant flight as it is above stormy weather and has strong, steady, horizontal winds. It also acts as a protective shield against meteorites which usually burn as they enter the earth’s gravitational field.

How does the ozone protect life from UV rays?

UV-C rays do not reach the Earth’s surface due to the ozone layer. When UV-C rays meet the ozone molecules, the energy inherent in them is sufficient to break apart the bond of the molecule and absorb the energy. Hence, no UV-C rays reach Earth. 

Some of UV-B rays reaches the Earth’s surface while some are absorbed by the stratospheric ozone by undergoing homolytic(decomposing into two uncharged atoms or radicals) fission to form an oxygen and an oxygen molecule.

UV-A rays reaches the earth’s surface almost unfiltered. This is because they do not have enough energy to split an ozone molecule and cannot be absorbed fully. Hence, it is important that we protect ourselves from these harmful rays.

Ozone can be both beneficial and harmful!

Naturally occurring ozone in the stratosphere helps to protect life on Earth by absorbing ultraviolet radiation from the sun.

However, ozone in the troposphere can be harmful to life on earth. Ozone causes damage to humans, vegetation and materials such as rubber and textiles.

The endangered ozone layer

With the lack of vertical convection in the stratosphere, materials that get into the stratosphere can stay there for long periods. An example is the  ozone-destroying chemicals called CFCs (chlorofluorocarbons). This can cause the depletion of the ozone layer when the CFC molecules are broken down by ultraviolet radiation, releasing their constituent chlorine atoms. These chorine atoms then react with the ozone molecules, resulting in their removal.

Do you know how is ozone formed?

In the stratosphere, ultraviolet light has enough energy to bring about photochemical reactions that convert dioxygen,  into ozone.

In the troposphere, ozone is formed from oxygen by photochemical reactions when oxides of nitrogen and hydrocarbons are emitted by motor vehicles, power stations and many other sources.

Mesosphere

Temperature in the mesosphere decreases with altitude as there is no water vapour, cloud, dust or ozone to absorb incoming radiation. It experiences the atmosphere’s lowest temperature of -90°C and strongest wind of about 3000km/hr.

The mesosphere protects life on earth by destroying meteors that fall on earth. (Did you know that about 40 tons of meteors fall on earth every day?) Without the mesosphere, we would not be able to survive. Meteors collide with gas particles in the mesosphere and burn, resulting in what we call the ‘shooting stars’.

In the troposphere, carbon dioxide and methane heat up the troposphere and cause the greenhouse effect. However, in the mesosphere, as heat is released from the mesosphere into the space, the carbon dioxide cools down. The methane gas is decomposed due to the solar light. (Learning point: Due to different properties of the layers, the same material can result in a different effect)

The night sky is completely dark. It surprises me how this is related to the properties of the mesosphere. The nightglow is formed by the sodium layer located just below the mesopause. The explanation to this mechanism can be found on http://en.wikipedia.org/wiki/Airglow under ‘Description’.

Thermosphere

Temperature rise rapidly with height in the thermosphere. This is due to an increasing proportion of atomic oxygen in the atmosphere which, like ozone, absorbs ultraviolet radiation. Although temperature can reach up to 2000°C, the gas molecules making up the air are so far apart the temperature is not felt. This explains why you would freeze to death in the thermosphere although the temperature at the thermosphere can reach beyond 1200°C.

Feelings

I was not aware of the importance of the layers of the atmosphere until we were taught about it. Life on earth is actually very dependent on the layers of the atmosphere which provide us with protection from the harmful rays and meteorites. These layers work together to ensure the survival of living things on earth and the absence of any of the layers would definitely result in the breakdown of the entire system. I now know how to appreciate the structure of the atmosphere.