Atmosphere | ICSE Class 7 Geography Notes

This note covers the atmosphere and its importance, the composition and uses of its gases, its five layers, the greenhouse effect, global warming, ozone depletion, and ways to protect the atmosphere.

What is the atmosphere, and why is it essential for life?

Definition: The atmosphere is the huge blanket of air surrounding the Earth. Air is a mixture of gases, rather than a single gas.

The atmosphere supplies the air that living beings need. It also protects life from harmful effects of the Sun's rays and helps keep the Earth's temperature suitable for living organisms. Temperature means the degree of hotness or coldness.

Without this protective blanket, the contrast between daytime heating and night-time cooling would be extreme. The Earth's surrounding air therefore matters both as a supply of gases and as protection for life. Its importance extends beyond the act of breathing.

How do gases support living things?

Oxygen is the gas humans and animals take from air when they breathe. Carbon dioxide is a gas green plants use to make food. Nitrogen, the most plentiful atmospheric gas, is also needed by plants, although they cannot take it directly from air.

These gases have different roles. A gas present in a small quantity can still be important. Carbon dioxide helps plants make food and helps retain heat, while ozone, a form of oxygen found in small amounts, protects the surface from harmful solar radiation high in the atmosphere.

What else is present in air?

Air also contains tiny dust particles, which are small solid particles suspended in it. Water can be present as water vapour, its gaseous form. Thus, studying the atmosphere involves more than learning the names of its main gases.

The atmosphere can be studied through two different questions. Its composition tells us what it contains. Its structure tells us how its layers are arranged above the Earth. Keeping these ideas separate makes it easier to connect each gas and each layer with its function.

What is the composition of the atmosphere?

Nitrogen and oxygen make up the bulk of the atmosphere. Carbon dioxide, helium, ozone, argon and hydrogen occur in smaller quantities. Helium, argon and hydrogen are the names of other gases in the mixture; they do not replace the two main constituents.

A percentage, written with the symbol %, expresses a share out of one hundred. In the simplified composition chart below, nitrogen occupies the largest share, oxygen the next largest, and the remaining gases much smaller shares.

How should a composition chart be read?

A pie chart is a circle divided into sectors, or portions, to represent the parts of a whole. The large nitrogen sector and the smaller oxygen sector show their relative abundance. The very narrow remaining sectors represent gases present in small quantities.

The circular chart labels nitrogen 78%, oxygen 21%, argon 0.93%, carbon dioxide 0.03% and all others 0.04%. Nitrogen occupies most of the circle, while the smallest shares have labels outside it.

See Fig. 4.1 in your NCERT textbook

The chart answers a question about quantity, not about usefulness. Oxygen has a smaller share than nitrogen but is essential for breathing. Carbon dioxide occupies a much smaller share than either gas but has an important role in food-making and heat retention.

Do not interpret the “all others” category as empty space. It represents other constituents grouped together. Also keep the idea of gaseous composition separate from the presence of dust and water vapour: air around us contains more than the two largest gases shown in the chart.

How do nitrogen, oxygen and carbon dioxide support life?

How do plants obtain nitrogen?

Although nitrogen is abundant in air, plants cannot use it directly in that form. Bacteria, microscopic living organisms found in the soil and in the roots of some plants, change atmospheric nitrogen into a form plants can use.

Humans inhale some nitrogen along with the air and exhale it again. Simply breathing in the gas is different from making it available to plants. The action of bacteria explains why abundance in the atmosphere does not mean direct use by plants.

How are oxygen and carbon dioxide connected?

Photosynthesis is the process by which green plants use sunlight to make food from carbon dioxide and water. They release oxygen during this process. Humans and animals take oxygen from the air and release carbon dioxide.

The amount of carbon dioxide released by humans and animals seems to be equal to the amount used by plants. This describes a balance between the release and use of gases. Cutting trees disturbs the balance, as does adding carbon dioxide by burning fuels.

Coal and oil are fuels whose burning adds carbon dioxide to the atmosphere. These additional releases upset the balance between the gas entering the air and its use by plants. The resulting increase affects the Earth's weather and climate.

The useful connection to remember is therefore a relationship: plants take in carbon dioxide for food-making and release oxygen, while humans and animals use oxygen and release carbon dioxide. Nitrogen follows a different route involving bacteria. These processes should not be mixed into a single explanation.

How are the five layers of the atmosphere arranged?

From the Earth's surface upwards, the five layers are the troposphere, stratosphere, mesosphere, thermosphere and exosphere. These names identify successive regions of the atmosphere. Learning them in order helps locate the air we breathe, the ozone layer and the uppermost thin air.

Height here means distance above the Earth's surface. The abbreviation km means kilometre, a unit of length. An average height and an upper height describe different things, so they must not be treated as interchangeable measurements.

The ionosphere is a region within the thermosphere that helps return radio waves to the Earth. It is not an extra sixth layer in this five-layer arrangement. The exosphere is the uppermost layer and contains very thin air.

How can the structure be shown in a diagram?

The diagram places the troposphere nearest the ground, followed upwards by the stratosphere, mesosphere, thermosphere and exosphere. It labels ozone within the stratosphere and includes an aeroplane and Mount Everest near the lower part.

See Fig. 4.2 in your NCERT textbook

To draw a simple structure diagram, place the Earth's surface at the bottom and label the five layers upwards in sequence. Mark ozone inside the stratosphere. Keep the ionosphere label within the thermosphere rather than adding it above the exosphere.

For a simple diagram without a measured scale, write “not to scale”, meaning that drawn distances do not represent actual distances proportionally. This prevents the width of a band on the page from being mistaken for the real thickness of an atmospheric layer.

Why are the troposphere and stratosphere important?

What happens in the troposphere?

The troposphere is the lowest atmospheric layer and contains the air we breathe. Its average height is 13 km. Almost all weather phenomena, including rainfall, fog and hailstorms, occur in this layer.

Weather means the hour-to-hour and day-to-day condition of the atmosphere. Rainfall is water falling as rain; fog is a cloud of tiny water droplets near the ground; a hailstorm brings falling balls or pieces of ice. These are examples of atmospheric conditions we experience near the surface.

The word “almost” matters. The troposphere is the main region of weather, but describing it as the place where absolutely every atmospheric event occurs would be too strong. Its importance comes from both its breathable air and the weather processes taking place within it.

Case study: How did the 1999 Odisha supercyclone affect life?

Odisha, on India's eastern seacoast, is prone to cyclones originating in the Bay of Bengal. A depression formed in the Gulf of Thailand on 25 October 1999, moved northwestwards and intensified into a supercyclone. It struck between Erasama and Balikuda in Odisha on 29 October.

Damage resulted from wind, rain and tidal surge. Winds reaching 260 km per hour lasted over 36 hours, uprooting trees and damaging houses. Three days of continuous heavy rain flooded major rivers. Tidal waves swept 20 km inland and damaged standing paddy crops.

About 13 million people were affected. Salt brought by the tidal surge made large areas of agricultural land infertile. Mangrove forests between Paradeep and Konark vanished. The event shows how severe weather can damage settlements, farming and vegetation.

On an outline map covering the Gulf of Thailand, the Bay of Bengal and India's eastern coast, label these places and Odisha. Mark the origin on 25 October 1999 and use a schematic northwestward arrow towards the landfall between Erasama and Balikuda on 29 October.

On an outline map of Odisha, mark Bhubaneshwar and Cuttack as affected cities. Label Paradeep and Konark and highlight the coastal stretch between them where mangrove forests vanished. Use separate key symbols for the cities and the affected mangrove stretch.

What distinguishes the stratosphere?

The stratosphere lies above the troposphere and extends up to a height of 50 km. It is almost free from clouds and associated weather phenomena. These conditions are most ideal for flying aeroplanes.

Another important feature is its ozone layer, a region containing ozone gas that protects the Earth from harmful effects of the Sun's rays. Its protective role is different from supplying the oxygen that humans and animals breathe near the ground.

When comparing the layers, match each feature to its location. Breathing and most familiar weather belong with the troposphere; ozone protection belongs with the stratosphere. Neither the amount of a gas nor the height of a layer alone explains all its importance.

What happens in the mesosphere, thermosphere and exosphere?

The three upper layers continue the sequence above the stratosphere. Their main features are different: incoming objects burn up in the mesosphere, temperature rises rapidly in the thermosphere, and the exosphere contains very thin air.

What is the mesosphere?

The mesosphere is the third atmospheric layer, above the stratosphere. It extends up to a height of 80 km. Meteorites, rocky objects entering from space, burn up in this layer. This is its main identifying feature in the five-layer description.

Its position is easy to confuse with that of the thermosphere. Keep the sequence fixed: the mesosphere comes after the stratosphere and before the thermosphere. The ozone layer belongs lower down, within the stratosphere, rather than in the region identified with burning meteorites.

What is the thermosphere?

In the thermosphere, temperature rises very rapidly with increasing height. The ionosphere forms part of this layer. Radio transmission means sending signals using radio waves, a form of electromagnetic radiation that carries information.

The ionosphere extends between 80 and 400 km and helps radio transmission by reflecting radio waves from the Earth back towards it. In this description, the height range refers to the ionosphere. Do not turn it into a separate layer outside the five-layer sequence.

What is the exosphere?

The exosphere is the uppermost atmospheric layer. Its air is very thin. Light gases such as helium and hydrogen float into space from this region. Its position above the thermosphere is the main fact to show in a labelled structure diagram.

A useful comparison links each layer to its distinctive feature: mesosphere with burning meteorites, thermosphere with rapidly rising temperature and the ionosphere, and exosphere with very thin air. These connections give meaning to the order instead of leaving it as a list of unfamiliar names.

How does the greenhouse effect warm the Earth?

Definition: The greenhouse effect is the warming caused when gases in the atmosphere trap heat radiated from the Earth. A greenhouse gas is a gas that contributes to this heat retention.

Radiated heat means energy given out as radiation. Carbon dioxide is a greenhouse gas because it traps some of the heat given out by the Earth. This helps keep the planet warm enough for life; without its warming effect, the Earth would be too cold.

What is the sequence of warming?

1. Energy from the Sun reaches the Earth and warms it.
2. The warmed Earth gives out heat as radiation.
3. Carbon dioxide in the atmosphere traps some of this outgoing heat.
4. Retained heat helps keep the Earth's temperature suitable for living things.

The process involves heat given out by the Earth, so an explanation should include that step. Saying merely that carbon dioxide “makes the Sun hotter” would describe a different and incorrect process. The warming is connected with heat retention in the Earth's atmosphere.

When does additional heat retention become a problem?

When the amount of carbon dioxide rises because of releases such as factory smoke and car fumes, more heat is retained. The resulting rise in the Earth's temperature is called global warming. This connects a change in atmospheric composition with a change in temperature.

The ordinary greenhouse effect and increased warming should be distinguished. The atmosphere's warming role makes life possible, but additional greenhouse gas increases heat retention. Carbon dioxide therefore has both a necessary natural role and a role in warming when its level increases.

Draw the Sun, the Earth's surface and a band representing the atmosphere. Show incoming solar energy, heat leaving the warmed Earth, and some heat retained by greenhouse gases. Label every arrow with the process it represents.

What causes global warming, and how can it affect life?

Global warming is a rise in the Earth's temperature associated with increased heat retention by greenhouse gases. Burning coal and oil adds carbon dioxide to the atmosphere. These are examples of fossil fuels, fuels formed from the remains of organisms over very long periods.

Why does cutting forests matter?

Deforestation means clearing forests and using the land for other purposes. Fewer trees use less carbon dioxide in photosynthesis, so more remains in the atmosphere. This strengthens the connection between loss of forests and increased warming.

1. Forests are cleared, leaving fewer trees.
2. Less carbon dioxide is used up by trees in photosynthesis.
3. The amount of carbon dioxide in the atmosphere increases.
4. More heat is trapped, contributing to global warming.

What consequences can follow?

Rising temperature causes snow in the coldest parts of the world to melt. Melting ice caps, large masses of ice covering land, and rising sea level, the height of the sea surface, are important concerns. Rising sea levels cause floods in coastal areas.

Warming also affects patterns of temperature and precipitation, water falling from the atmosphere as rain, snow, sleet or hail. Climate means the average weather conditions of a place over a longer period. A changing climate involves more than one unusually hot day.

There may be drastic changes in a place's climate, leading to the extinction, or complete disappearance, of some plants and animals in the long run. The possibility should not be changed into a claim that every species will disappear or that every place will change identically.

Increased temperature can disturb the water cycle, the continuing movement of water between the Earth and atmosphere, and may reduce rainfall. This could cause droughts, prolonged periods of inadequate water availability associated with insufficient rain. If the Earth's temperature rises too high, it would become too warm for some crops to grow.

What is ozone depletion, and how is it different from global warming?

Ozone is a form of oxygen whose molecules each contain three oxygen atoms. An atom is a basic unit of an element; a molecule consists of atoms joined together. Ordinary oxygen molecules contain two oxygen atoms.

High in the atmosphere, ozone shields the Earth's surface from ultraviolet radiation, a form of solar radiation that can harm living organisms. Ozone itself is poisonous, so its protective role high above the ground does not make it a gas suitable for breathing.

What causes the protective layer to decline?

Ozone depletion means a decrease in the amount of ozone in the atmospheric ozone layer. This decline has been linked to synthetic chemicals called chlorofluorocarbons, abbreviated as CFCs. “Synthetic” means made by chemical processes rather than obtained directly in a natural form.

CFCs have been used as refrigerants, substances used for cooling in refrigeration equipment. Reducing the use of ozone-damaging chemicals and choosing CFC-free alternatives addresses this cause. A CFC-free refrigerator is one that does not use chlorofluorocarbons.

Case study: How did the 1987 agreement address ozone depletion?

Atmospheric ozone began to decline sharply in the 1980s. The decrease was linked to synthetic chemicals such as CFCs, used as refrigerants and in fire extinguishers. This connected damage to the protective ozone layer with chemicals used in everyday equipment.

In 1987, the United Nations Environment Programme (UNEP) secured an agreement to freeze CFC production at 1986 levels. The agreement addressed production of ozone-damaging chemicals. Using CFC-free refrigerators is another way to avoid these chemicals.

How should the two environmental problems be compared?

The greenhouse effect concerns retention of heat, while the ozone layer concerns protection from ultraviolet radiation. Keeping these functions distinct prevents the mistaken explanation that global warming simply happens because sunlight enters through a gap in the ozone layer.

Both problems involve human effects on the atmosphere, but that does not make their mechanisms identical. An answer about fossil-fuel burning should explain additional carbon dioxide and heat retention. An answer about ozone protection should identify ozone-damaging chemicals and the loss of protective ozone.

How can people help reduce global warming and protect the atmosphere?

Useful action follows from understanding causes. If fossil-fuel burning adds carbon dioxide, reducing unnecessary fuel use tackles an input to warming. If clearing forests reduces carbon dioxide uptake, protecting and restoring trees helps maintain that uptake.

How can forests and resources be conserved?

Reforestation means restocking destroyed forests by planting new trees. The planted trees should generally be of the same species found in that forest. A species is a particular kind of organism. At least as many trees should be planted as are cut.

Reforestation can also happen naturally if a deforested area is left undisturbed and the forest re-establishes itself. Protecting existing forests and restoring damaged ones are related actions. Tree planting should be understood alongside the need to reduce forest destruction.

Saving, reusing and recycling paper saves trees, energy and water. Reusing means using an item again; recycling means processing waste material so that it can be used again. These actions connect everyday resource use with forest conservation.

What everyday and collective actions help?

• Save energy: switch off unnecessary lights and fans, reducing avoidable energy use.
• Reduce avoidable fuel use: walking instead of making a scooter journey is an example of an environment-friendly choice.
• Protect forests: avoid wasteful use of paper and support the restoration of destroyed forests.
• Protect ozone: use alternatives to ozone-damaging chemicals, including CFC-free refrigeration.
• Build awareness: use posters and campaigns to explain causes, consequences and practical actions.

An awareness poster is stronger when it connects an action to a reason. A message about saving trees can explain that trees use carbon dioxide during photosynthesis. A message about avoiding unnecessary fuel burning can explain that burning coal and oil adds carbon dioxide.

These actions address different parts of atmospheric protection. Saving energy and conserving forests help address warming, while reducing ozone-damaging chemicals helps protect the ozone layer. Explaining the connection makes conservation more meaningful than simply memorising a list of instructions.

Glossary

• Atmosphere — The blanket of air surrounding the Earth that supports and protects living things.
• Composition — The substances making up a mixture and their relative amounts within it.
• Photosynthesis — The process in which green plants use sunlight to make food from carbon dioxide and water.
• Troposphere — The lowest atmospheric layer, containing breathable air and almost all weather phenomena.
• Stratosphere — The layer above the troposphere that contains ozone and is almost free from clouds.
• Mesosphere — The atmospheric layer above the stratosphere where incoming meteorites burn up.
• Thermosphere — The atmospheric layer where temperature rises rapidly with height and which includes the ionosphere.
• Exosphere — The uppermost atmospheric layer, containing very thin air and light gases.
• Greenhouse effect — Warming caused by atmospheric gases trapping heat radiated from the Earth.
• Global warming — A rise in the Earth's temperature associated with increased retention of heat by greenhouse gases.
• Deforestation — Clearing forests and using the cleared land for purposes other than forest.
• Precipitation — Water falling from the atmosphere as rain, snow, sleet or hail.
• Ozone depletion — A decrease in the amount of ozone in the protective atmospheric ozone layer.
• Reforestation — Restocking destroyed forests by planting new trees to restore forest cover.

Common errors and misconceptions

• Misconception: Air is mainly oxygen. Correct: Nitrogen is the most plentiful gas; oxygen is the second most plentiful.
• Misconception: Plants take nitrogen directly from air. Correct: Bacteria in soil and the roots of some plants change it into usable forms.
• Misconception: The troposphere is exactly 13 km high everywhere. Correct: The stated 13 km is its average height.
• Misconception: The stratosphere has absolutely no clouds. Correct: It is almost free from clouds and associated weather phenomena.
• Misconception: The ionosphere is a sixth layer above the exosphere. Correct: It forms part of the thermosphere in the five-layer arrangement.
• Misconception: The greenhouse effect is entirely harmful. Correct: Heat retention helps make the Earth liveable; increased greenhouse gases cause additional warming.
• Misconception: Global warming and ozone depletion are the same process. Correct: One concerns increased heat retention; the other concerns declining protective ozone.
• Misconception: Warming will certainly destroy every plant and animal. Correct: Climate changes may lead to extinction of some plants and animals in the long run.

Exam-style questions with model answers

Q1. A composition chart gives nitrogen 78%, oxygen 21%, argon 0.93%, carbon dioxide 0.03% and all others 0.04%. Identify the most plentiful gas and the second most plentiful gas, giving their shares. [2 marks]

1. Nitrogen is the most plentiful gas in the chart, with a share of 78%.
2. Oxygen is the second most plentiful gas, with a share of 21%.

Q2. Green plants use carbon dioxide to make food and release oxygen during photosynthesis. Humans and animals take oxygen from air and release carbon dioxide. Plants cannot take nitrogen directly from air, but bacteria change atmospheric nitrogen into forms plants can use. Using this information, state one role each for carbon dioxide, oxygen and bacteria. [3 marks]

1. Carbon dioxide supplies a gas that green plants use to make food during photosynthesis; it is taken from the air for this process.
2. Oxygen released by green plants is the gas that humans and animals take from the air when they breathe.
3. Bacteria change atmospheric nitrogen into a usable form, allowing plants to obtain nitrogen even though they cannot take it directly from air.

Q3. The troposphere has an average height of 13 km and contains almost all weather phenomena. The stratosphere lies above it, extends up to 50 km, is almost free from clouds, and contains ozone. Give four comparisons using position, height, weather and ozone location. [4 marks]

1. The troposphere lies below the stratosphere, while the stratosphere is the atmospheric layer immediately above the troposphere.
2. The troposphere has an average height of 13 km; the stratosphere extends up to a height of 50 km.
3. Almost all weather phenomena occur in the troposphere, whereas the stratosphere is almost free from clouds.
4. The ozone layer is located in the stratosphere, rather than in the troposphere described in the question.

Q4. Solar energy warms the Earth, which then radiates heat. Carbon dioxide traps some of that heat. Burning coal and oil adds carbon dioxide, increasing heat retention and the Earth's temperature. Explain this sequence in five points, ending with global warming. [5 marks]

1. Solar energy first reaches and warms the Earth. This provides the initial heating in the sequence described in the question.
2. The warmed Earth then gives out heat as radiation. An explanation must include this outgoing heat as well as incoming solar energy.
3. Carbon dioxide in the atmosphere traps some of the heat radiated from the Earth. This retention of heat is the greenhouse effect.
4. Burning coal and oil adds carbon dioxide to the atmosphere. With more carbon dioxide, the amount of heat retained increases.
5. The increased retention of heat raises the Earth's temperature. This rise in temperature is called global warming, the final result in the sequence.

Q5. Ozone high in the atmosphere protects the surface from harmful ultraviolet radiation. Ozone decline has been linked to chlorofluorocarbons, called CFCs, used as refrigerants. CFC-free alternatives avoid these chemicals. Global warming involves increased heat retention by greenhouse gases. State ozone's role, a cause of depletion, a protective action and the difference from warming. [4 marks]

1. Ozone high in the atmosphere protects the Earth's surface from ultraviolet radiation, a harmful component of radiation received from the Sun.
2. The decline in ozone has been linked to chlorofluorocarbons, or CFCs, chemicals used as refrigerants in cooling equipment.
3. Using CFC-free alternatives avoids these ozone-damaging chemicals and is an action directed towards protection of the ozone layer.
4. Ozone depletion concerns the loss of protective ozone, whereas global warming concerns an increase in temperature through greater heat retention.

Q6. Trees use carbon dioxide in photosynthesis. Forest clearing leaves fewer trees, so less carbon dioxide is used and more remains in the atmosphere. Carbon dioxide traps heat. Restoring destroyed forests means planting new trees. Explain the link between forest clearing and warming in four points, then give one corrective action. [5 marks]

1. Forest clearing removes trees from the land. This leaves fewer trees available to carry out the photosynthesis described in the question.
2. Because trees use carbon dioxide during photosynthesis, fewer trees mean that less of this gas is taken up from the atmosphere.
3. With less carbon dioxide being used by trees, more remains in the atmosphere. Forest loss therefore changes the balance involving this gas.
4. Carbon dioxide traps heat, so an increased amount in the atmosphere increases heat retention. This contributes to a rise in the Earth's temperature.
5. Restoring destroyed forests by planting new trees is a corrective action called reforestation. It addresses the loss of trees that use carbon dioxide.

Key takeaways

• The atmosphere supplies gases needed by living things and helps protect the Earth from extreme heating and cooling.
• Nitrogen and oxygen make up the bulk of air, but gases present in smaller quantities also perform important functions.
• Plants cannot take nitrogen directly from air; bacteria change it into forms that plants can use.
• The five layers upwards are troposphere, stratosphere, mesosphere, thermosphere and exosphere; the ionosphere forms part of the thermosphere.
• Almost all weather occurs in the troposphere, while the stratosphere contains the ozone layer and is almost free from clouds.
• The greenhouse effect helps keep the Earth liveable, but additional carbon dioxide increases heat retention and contributes to global warming.
• Global warming can affect ice, sea level and climate; there may be extinction of some plants and animals in the long run.
• Conserving forests and reducing unnecessary fuel use address warming; reducing ozone-damaging chemicals helps protect the ozone layer.

Test yourself

What is the difference between atmospheric composition and structure?

Composition describes the substances making up the atmosphere; structure describes the arrangement of its layers above the Earth.

Why can plants not simply use the abundant nitrogen in air?

Plants cannot take nitrogen directly from air. Bacteria in soil and the roots of some plants change it into usable forms.

Which layer contains ozone, and why does it matter?

The stratosphere contains the ozone layer, which protects the Earth's surface from harmful ultraviolet radiation from the Sun.

Does “average height of 13 km” mean the troposphere has that height everywhere?

No. It states an average height, not a fixed height that must apply everywhere.

Where does the ionosphere belong in the five-layer arrangement?

The ionosphere forms part of the thermosphere and helps radio transmission by reflecting radio waves back towards the Earth.

Why is the greenhouse effect necessary even though warming is a concern?

The greenhouse effect retains heat and helps keep the Earth liveable. Increased greenhouse gases cause additional heat retention and warming.

How can deforestation contribute to global warming?

Fewer trees use less carbon dioxide in photosynthesis. More remains in the atmosphere and traps heat, contributing to global warming.

What is the difference between reforestation and deforestation?

Deforestation clears forests for other uses. Reforestation restores destroyed forests by planting new trees.