Atmosphere and its gases
The sky looks empty, but wind blows, a kite flies and we breathe; so there is an envelope of air around us. That is called the atmosphere.
The gaseous envelope around the Earth is called the atmosphere. Because of it we are saved from the Sun’s heat and get a constant supply of air needed for life.
Key idea 1 : Nitrogen and oxygen — Nitrogen is about 78 per cent; it controls combustion caused by oxygen and is essential for compounds of living things. Oxygen is about 21 per cent; it is needed for respiration and combustion and plants release it into the air.
Key idea 2 : Other gases — Nitrogen and oxygen together make 99 per cent of air by volume. In the remaining about 1 per cent there are gases like argon (0.93 %), carbon dioxide (0.04 %), neon, helium, krypton, xenon, hydrogen and methane.
Key idea 3 : Carbon dioxide — The proportion of carbon dioxide increases by respiration, combustion, volcanoes and decay of vegetation. Plants absorb it in sunlight with the help of chlorophyll and release oxygen. So it is important for plants.
- Easy : Gas in the largest amount in air? — Nitrogen (78 %).
- Medium : Which gas for respiration? — Oxygen.
- Exam-type : Which gas do plants absorb? — Carbon dioxide.
In the figure: Proportion of gases in the atmosphere.
78 + 21 + 1 = 100.
It seems air has most oxygen; but it has most nitrogen (78 %), oxygen is 21 %.
Memory clue: Nitrogen 78, oxygen 21, rest 1. Memory clue: "78, 21, 1".
Spot the difference
| Gas | Share | Importance |
|---|---|---|
| Nitrogen | About 78 % | Controls combustion |
| Oxygen | About 21 % | Respiration, combustion |
| Carbon dioxide | About 0.04 % | Food of plants |
Practice questions
- 1What is the atmosphere?
- 2State the two main gases of the atmosphere and their proportion.
- 3Write the importance of nitrogen.
- 4How is oxygen added and how is it used?
- 5Write four reasons for increase of carbon dioxide.
- 6What do plants do with carbon dioxide?
- 7What would have happened if there were no atmosphere?
- 8Write three measures for purity of air in your village.
Show answers (try them yourself first)
- 1The gaseous envelope around the Earth.
- 2Nitrogen 78 %, oxygen 21 %.
- 3Controls combustion and forms compounds of living things.
- 4Plants release it; used in respiration and combustion.
- 5Respiration, combustion, volcanoes, decay of vegetation.
- 6They absorb it in sunlight and release oxygen.
- 7There would be no protection from heat and no air for breathing.
- 8Plant trees, save water, dispose waste properly.
Water vapour and dust particles
When wet clothes dry in the sun the water goes into the air; in the morning sunlight we see dust floating in the air. So air has both vapour and dust.
Vapour is the gaseous form of water. It comes into the air through evaporation from seas, rivers, lakes and land. Dust particles are very tiny solid particles floating in the air.
Key idea 1 : Water vapour — Vapour in the atmosphere is up to about 4 per cent. In desert regions it is very low. It is found up to about 8 to 12 km above the ground and decreases with height. At height, because of low temperature, vapour turns into water droplets or ice particles.
Key idea 2 : Importance of vapour — Vapour gives humidity to the atmosphere, keeps temperature balanced and forms clouds. Its role in the water cycle is important; clouds, rain, snow, hail and fog all form from vapour.
Key idea 3 : Dust particles — Soil particles, pollen, fungal spores, dry particles of vegetation, industrial pollution and volcanic ash are all dust particles. They are so tiny that about one lakh particles fit in one centimetre length. Although some particles are harmful to health, they are important for the balance of the environment; they help in cloud formation and temperature control.
- Easy : Is vapour more or less in deserts? — Less.
- Medium : Fog forms from what? — Vapour.
- Exam-type : Volcanic ash is which component? — Dust particle.
In the figure: From vapour to cloud and rain.
Vapour → cloud → rain.
Not all dust is bad; it helps in cloud formation.
Memory clue: Vapour = water gas; dust = tiny particles in air. Memory clue: "Clouds from vapour, drops around particles".
Spot the difference
| Point | Vapour | Dust particles |
|---|---|---|
| Form | Gas | Tiny solid particles |
| Source | Evaporation | Soil, pollen, ash, pollution |
| Use | Clouds, rain, humidity | Help in cloud formation |
Practice questions
- 1What is water vapour?
- 2How does vapour come into the atmosphere?
- 3Write the importance of vapour in the atmosphere.
- 4Of what are dust particles made?
- 5Write advantages and disadvantages of dust particles.
- 6Why is vapour less in deserts?
- 7What happens when vapour goes to height?
- 8Write the experiment of observing dust particles by shaking soil in a glass jar.
Show answers (try them yourself first)
- 1Gaseous form of water.
- 2By evaporation from seas, rivers, lakes and land.
- 3Humidity, temperature balance, clouds and rain.
- 4Soil, pollen, spores, ash, pollution.
- 5Advantages: clouds, temperature; disadvantages: harm to health.
- 6There is less water and fewer sources for evaporation.
- 7It becomes droplets or ice particles due to cold.
- 8After shaking, heavy particles settle at the bottom and fine ones float.
Layers of the atmosphere
On going up a high mountain it gets colder and breathing becomes harder. So air changes as we go up; therefore different layers of the atmosphere are considered.
In the empty space around the Earth the atmosphere spreads up to about 1000 km above the surface. It is dense near the ground and becomes thinner with height, merging with space. According to temperature it is divided into five layers.
Key idea 1 : Troposphere and stratosphere — The troposphere is the layer near the ground; air density is highest here, temperature decreases with height, and vapour, clouds, rain, snow, hail and fog form here. In the stratosphere above it temperature is more than in the troposphere; here the ozone layer absorbs the Sun’s ultraviolet rays.
Key idea 2 : Mesosphere and ionosphere — In the mesosphere temperature decreases with height up to about −100 °C. Meteorites coming towards the Earth burn and are destroyed here. In the ionosphere (thermosphere) the air is very thin and temperature rises rapidly; because of ions here radio waves are reflected back to the Earth.
Key idea 3 : Exosphere and human use — The exosphere is the outermost layer of the atmosphere; air density is extremely low and temperature extremely high. This layer is useful for space shuttles and artificial satellites. Radio (Akashvani), television (Doordarshan) and satellite communication are possible because of the ionosphere.
- Easy : In which layer are clouds? — Troposphere.
- Medium : In which layer is the ozone layer? — Stratosphere.
- Exam-type : In which layer do meteorites burn? — Mesosphere.
In the figure: Layers of the atmosphere.
As per the textbook the atmosphere spreads up to about 1000 km; the ‘Kármán line’ taken at about 100 km is used as the boundary of space, and many sources say the atmosphere has no definite upper limit (as per the textbook; check with the book).
Tr-St-Me-Io-Ex.
Temperature does not decrease in all layers with height; it decreases in the troposphere, increases in the stratosphere, decreases again in the mesosphere.
Memory clue: From bottom: troposphere → stratosphere → mesosphere → ionosphere → exosphere. Memory clue: "Tro-Stra-Meso-Iono-Exo".
Spot the difference
| Layer | Feature | Use |
|---|---|---|
| Troposphere | Highest density; clouds, rain | Life and weather |
| Stratosphere | Ozone layer | Protection from UV rays |
| Ionosphere | Radio waves reflected | Radio, TV, satellite |
Practice questions
- 1State the five layers of the atmosphere from bottom upward in order.
- 2Write the features of the troposphere.
- 3In which layer is the ozone layer and what is its work?
- 4What is the benefit of the mesosphere to the Earth?
- 5How is the ionosphere used for communication?
- 6For what is the exosphere used?
- 7In which layers does temperature increase and in which decrease?
- 8Draw the layers of the atmosphere and label them.
Show answers (try them yourself first)
- 1Troposphere, stratosphere, mesosphere, ionosphere, exosphere.
- 2Highest density; temperature decreases with height; vapour, clouds, rain.
- 3Stratosphere; absorbs ultraviolet rays.
- 4Meteorites burn and are destroyed.
- 5Radio waves are reflected back to the Earth.
- 6Space shuttles and artificial satellites.
- 7Increases: stratosphere, ionosphere; decreases: troposphere, mesosphere.
- 8Student draws own.
Ozone and atmospheric phenomena
If we stay long in the sun our skin burns. A gas in the atmosphere blocks such harmful rays of the Sun and protects us; that gas is ozone.
Ozone is a gas that is a form of oxygen. Its proportion is low near the ground; it is maximum at about 25 to 40 km above sea level, so that part is called the ‘ozone layer’.
Key idea 1 : Importance of ozone — The ozone layer absorbs the Sun’s ultraviolet rays. So living things are protected, temperature is balanced, destruction of vegetation is avoided and environmental balance is kept. Without ozone skin cancer, eye diseases and harm to plants and animals would have increased.
Key idea 2 : Atmospheric phenomena — Many phenomena occur in the atmosphere such as clouds, lightning, wind, rain, fog, hail, heat waves and storms. Some are useful to people, while heavy rain, snowfall, storms, lightning, fog, heat waves and hailstorms disrupt life. Dense fog increases accidents, reduces visibility, cancels flights and stops water transport.
Key idea 3 : Our responsibility — Planting trees, saving water and disposing of waste properly help keep air pure. To create awareness about ozone, 16 September is celebrated as World Ozone Day.
- Easy : Which rays does the ozone layer block? — Ultraviolet rays.
- Medium : World Ozone Day? — 16 September.
- Exam-type : Effect of dense fog? — More accidents, less visibility.
In the figure: Protection by the ozone layer.
The UN declared 16 September as the ‘International Day for the Preservation of the Ozone Layer’ because on this date in 1987 the Montreal Protocol, reducing substances harmful to the ozone layer, was signed (as per several sources; check with the book).
Ozone = umbrella.
Ozone is useful in the upper layer; near the ground it can be a pollutant.
Memory clue: Ozone = umbrella from the Sun’s rays. Memory clue: "25 to 40 km – umbrella open".
Spot the difference
| Point | If ozone is present | If ozone is absent |
|---|---|---|
| UV rays | Less effect | More effect |
| Temperature | Balanced | Disturbed |
| Vegetation | Safe | Destroyed |
Practice questions
- 1Where is the ozone layer?
- 2Write the importance of the ozone layer.
- 3What would have happened if there were no ozone?
- 4Write five examples of atmospheric phenomena.
- 5State the effect of dense fog on human life.
- 6When is World Ozone Day celebrated?
- 7What will you do to keep air pure?
- 8Collect newspaper cuttings of atmospheric phenomena and discuss.
Show answers (try them yourself first)
- 1At about 25 to 40 km above sea level.
- 2Absorbs UV rays; protects living things and plants.
- 3Skin cancer, eye diseases, destruction of vegetation would have increased.
- 4Lightning, storm, fog, hail, rain.
- 5More accidents; low visibility; flights cancelled; effect on breathing.
- 616 September.
- 7Plant trees, save water, dispose waste.
- 8Student does own.