ATMOSPHERE AND SOLAR RADIATION
- The Earth is surrounded by an atmosphere.
- It is a blanket of gases and has no definite outer edge.
- It gradually becomes thinner and it merges into space.
- Over 80% of atmospheric gases are held by gravity within 20 kilometres of the Earth’s surface.
- The physical and chemical structure of the atmosphere, the way that the gases interact with solar energy and the physical and chemical interactions between the atmosphere, land and oceans all combine to make the atmosphere an integral part of the Earth system.
- The Sun is a star. It consists primarily of hydrogen at temperatures high enough to cause nuclear fusion.
- Some 120 million tons of matter, mostly hydrogen are converted into helium on the sun every minute.
- The size of the Sun determines its temperature and the amount of energy radiated.
- This electromagnetic energy from the Sun comes to Earth in the form of radiation.
- The Sun radiates energy equally in all directions and the Earth intercepts and receives part of this energy.
- The Sun’s energy is the primary source of energy for all surface phenomena and life on Earth.
- This energy is the main reason for the immense diversity of life forms that are found on Earth.
- The range of electromagnetic energy emitted by the Sun is known as the solar spectrum and lies mainly in three regions
- While the Sun does emit ultraviolet radiation, the majority of solar energy comes in the form of “light” and “heat” in the visible and infrared regions of the electromagnetic spectrum.
- Light is special to humans and many other animals due to the evolution of the eye, a sensory organ that detects this part of the solar spectrum.
- We are all familiar with the rainbow of colours, the range of different colours that makes up sunlight.
- The best way to visualize this concept and the most common scientific demonstration is the image of a glass prism splitting up white light into the colours.
- When raindrops act as prisms, we see a rainbow.
- While the eye effectively perceives and distinguishes visible light, infra-red is perceived as heat.
- This is also called “heat” radiation because although we cannot see infra-red we can feel its presence as heat.
- The skin converts the heat into the energy of the molecules.
- Infra-red plays an important role in the temperature of the Earth and its atmosphere and in turn the climate of the Earth.
- There are interactions occurring constantly in the atmosphere with the solar radiation.
- This results in a complex and delicately balanced system, which is crucial to the continuation of present life forms on Earth.
- The following two interactions are crucial to the maintenance of life on Earth.
- The atmosphere acts as a filter, absorbing and reflecting portions of the electromagnetic spectrum, such as the ultraviolet region that are harmful to humans and other life forms.
- The atmosphere provides a natural “warming effect”, maintaining the temperatures and climates in which life forms on the Earth have evolved to survive.
- The atmosphere controls the amount of solar radiation reaching the surface of the Earth and regulates the amount of radiation from the Earth escaping into space.
Chemical Composition of the Atmosphere
- The major constituents are oxygen (O2) and nitrogen (N2).
- Other gases such as argon, carbon dioxide, nitric oxide and ozone are produced in minute quantities in natural processes.
- Numerous other gases circulate particularly in the troposphere in small quantities.
- Apart from these gases water vapour is also present in the atmosphere.
- The water fraction in the atmosphere varies from place – to – place and day – to day.
- In the last century, industrial and other technological activities have introduced gases such as carbon monoxide and chlorofluorocarbons into the atmosphere.
- CFCs are a family of chemicals that do not occur in nature. These chemically inert compounds rise into the stratosphere and cause disruptions in the ozone layer.
- This disrupts the natural balance of circulation and radiation absorption in the troposphere.
- Effects of these changes range from local atmospheric problems to global climate change.
Layers of the Atmosphere
- The atmosphere consists of five layers
- The troposphere
- The stratosphere
- The mesosphere
- The thermosphere
- The exosphere
- The thickness of these layers is slightly different around the globe and also varies according to temperature and season.
- Human-made pollutants affect the function of the lower layers.
- The troposphere is the layer closest to the Earth’s surface.
- It is a layer of air approximately 10 to 15 kilometres thick.
- The conditions in this layer determine practically all of the global weather patterns.
- It consists of gases like nitrogen, oxygen and carbon dioxide.
- In addition to these gases, water vapour and dust are found in this year.
- Weather phenomena such as clouds, lightning, thunder, storms and rain take place in this layer.
- Carbon dioxide is much heavier than the other gases. So the amount of carbon dioxide is higher in this lower layer of the atmosphere.
- It tends to decrease as the altitude increases.
- The temperature decreases with the increasing altitude at the rate of 1°C per 165 metres. This ratio of decrease in temperature with the increase of altitude is called the lapse rate
- The stratosphere is the layer just above the troposphere.
- It is approximately 40 kilometres thick. In contrast to the troposphere water vapour is not found in this layer.
- Therefore weather changes do not take place in this layer.
- Pollutants tend to remain long periods of time in the stratosphere.
- The wind blows horizontally and therefore the conditions are conducive for aeroplanes to fly with great speed and a long distance.
- The mesosphere spreads above the stratosphere from 50 kms to 80 kms.
- The meteors which fall on the Earth due to the gravitational force burn due to the friction with the wind at this level.
- This layer protects the Earth from the falling meteors; otherwise, these meteors will damage the surface of the Earth.
- The thermosphere lies above the mesosphere. It extends approximately from 80 kms to 1600 kms.
- It contains hydrogen and helium in large amounts.
- Below this layer ions are found from 500 kms.
- Radio programs broadcast from the radio station only because of the reflection done by the ions.
- The exosphere extends from 1600 kms to 10,000 kms.
- Winds having very low density are found here.
- It mingles in the space and the only vacuum is found above this layer.
- In layers of the atmosphere, the stratosphere filters the ultraviolet rays and protects the Earth.
Ozone layer and its depletion
- Ozone is a form of oxygen. Ozone is made of three oxygen atoms (O3).
- The oxygen we find in the atmosphere is made up of two oxygen atoms (O2).
- Its chemical formulation, a single atom of oxygen (O) is unstable.
- That is it wants to combine with something else.
- That is why oxygen is almost always found in pairs in its diatomic form where it is more stable.
- O3is are less stable than because it wants to return to the diatomic state by giving up an oxygen atom.
- When enough ozone molecules are present it forms a pale blue gas.
- Ozone has the same chemical structure whether it is found in the stratosphere or the troposphere.
- In the troposphere “bad” ozone is an air pollutant.
- It damages human health and vegetation. It is a key ingredient of urban smog.
- In the stratosphere the “good” ozone that protects life on earth from the harmful effects of the sun’s ultraviolet rays.
- Ozone is constantly being formed in the earth’s atmosphere by the action of the sun’s ultraviolet radiation on oxygen molecules.
- Ultraviolet light splits the molecules apart by breaking the bonds between the atoms.
- A highly reactive free oxygen atom then collides with another oxygen molecule to form an ozone molecule.
- Because ozone is unstable, ultraviolet radiation quickly breaks it up and the process begins again.
- About 90% of the zone in the earth’s atmosphere lies in the stratosphere.
- Ozone forms a very thin layer in the stratosphere, where it is more concentrated than anywhere else.
- While both oxygen and ozone together absorb 95 to 99.9% of the sun’s ultraviolet radiation, only ozone effectively absorbs the most energetic ultraviolet light, which causes biological damage.
Ozone – Depleting Substances
- A family of compounds known as chlorofluorocarbons (CFCs) have the most significant effect on the ozone layer.
- CFCs contain different proportions of three elements. Carbon (C), fluorine (F) and chlorine (c1).
- CFCs were produced and used extensively as refrigerants starting in the early 1930’s after a scientist named Medgley discovered this gas.
- At that time, ammonia and sulphur dioxide were widely used as refrigerants. But ammonia was undesirable because it is a strong eye and respiratory irritant.
- Chloro fluoro carbons were seen then as the ideal compounds because they were thought to be harmless.
- They are chemically inert, non-toxic and insoluble in water. For the past seventy years, CFCs were used extensively in aerosols, refrigerants and foams.
- Because of their non – reactive nature.
- CFSs are able to rise undisturbed into the atmosphere.
- When CFSs migrate high enough and are hit by enough ultraviolet radiation, they are broken down and release chlorine atoms.
- The chlorine atoms react with O3gas and make ozone molecules unavailable.
The cycle of ozone gas
- O2 + UV radiation (<175 nm) O + O
- + O2 O3 or O + O O2
- O3 + UV radiation (240 – 280 nm) O* + O2 gas
- O* + O O2 or O* + H2O 2OH and so on.
- This cycle repeats over millions of years and has reached a state of equilibrium.
- The net result of the above reactions is that O2 and O3 are constantly changing into each other.
- Each cycle takes up energy in the form of ultraviolet radiation resulting in a large reduction of the amount of ultraviolet radiation reaching the troposphere.
- These reactions result the higher concentration of ozone gas in the lower region of the stratosphere.
- The largest amount of O2 is found between 20 and 26 km above the Earth’s surface.
- This area is called the “ozone layer.”
- In general ultraviolet radiation damages, the skin and can initiate the process of skin cancer.
- Thus ozone layer forms a shield protecting us from receiving large amounts of UV
- One chlorine atom can destroy over 100, 00 molecules of ozone. On our planet, a temperature balance in nature is necessary to sustain living organisms.
- This balance occurs due to the absorption of short wave (visible) solar radiation by surfaces on the earth and the subsequent transformation of that radiation into longer – wave infrared.
- Infra-red is then absorbed and “trapped” by carbon dioxide causing the troposphere to maintain a significantly warmer temperature.
- Since the Industrial Revolution, there has been a high rate of increase in the concentration of carbon dioxide. This gas has risen by 30% since the late 1800s.
- Scientists predict that CO2concentrations will continue to rise, likely reaching 2 to 3 times the pre-industrial level by 2100.
- The carbon dioxide effect can become a problem when the amount of heat-absorbing gases in the atmosphere rapidly rises far above the levels at present.
- The warming of the atmosphere is seen not only at the global level but also at local levels.
- Local – Urban heat islands are one such example.
- “Urban heat islands” are a kind of localized enhanced warming phenomenon.
- They are simply built-up areas of a city that are significantly warmer than the surrounding area of the countryside.
Urban heat islands
- The difference in temperature comes from the fact that buildings, paved surfaces and other human-made structures absorb higher amounts of sunlight than most natural objects.
- This energy is re-radiated at longer wavelengths during the night.
- The atmospheric pollution in the form of carbon dioxide, a heat-absorbing gas forms a “local” atmosphere trapping in the heat.
- Meteorologists have noticed that metropolitan areas are creating their own weather patterns at night due to the collision of cool air from the surrounding area with the warmer city air.
- It is important to note that urban heat islands are a localized effect, whereas the general atmospheric effect is global in extent.
- The atmospheric phenomena and found that the current state of the atmosphere is the result of increased industrial activities.
- In the last century especially, the atmospheric system that evolved over billions of years has been subject to rapid changes.
- James Lovelock, author of Gaia proposes that the atmosphere owes its current composition to living systems.
- He remarks that life on Earth requires a particular atmospheric composition and this composition is in turn maintained by the interactions between biological systems and the atmospheric system.
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