CLASS 9 SOCIAL SCIENCE · CHAPTER 3 · STUDY NOTES

Atmosphere and Climate

Understand atmosphere and climate through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.

EXPLORE

Air is a mixture, not one gas

The blanket of gases around Earth supports life and weather.

Dry air: compare the main shares

Read the bars: Height represents the approximate share by volume, not importance. Other gases include argon and carbon dioxide.

Dry-air composition: about 78% nitrogen, 21% oxygen and 1% other gases by volume. The 1% remainder is shown separately for visibility; water vapour varies.

Atmosphere: The gases surrounding Earth.
Dry air: Air composition described without its variable water vapour.

In dry air, roughly 78% is nitrogen, 21% oxygen and just under 1% argon. Carbon dioxide and other gases form a small remainder. Small proportions can still have important effects.

Water vapour varies greatly, from near zero to about 4% locally. Dust and other particles also vary. Do not treat the book’s narrow water-vapour range as a fixed worldwide value.

Take a moment. What do you think?

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Question

Is the atmosphere 21% water vapour?

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NCERT reference: chapter PDF pages 1, 2, 18, 19.

EXPLORE

Temperature changes help define the layers

Layer boundaries vary with place and season.

Layers above the surface

Named atmospheric layers, shown as a schematic guide. They have very different thicknesses and are not separated by solid walls.

Troposphere: The lowest atmospheric layer, where most weather occurs.
Stratosphere: The layer above the troposphere, containing much of the ozone layer.
Mesosphere: The layer above the stratosphere, where most incoming meteors burn up.
Thermosphere and exosphere: Very high regions; the thermosphere absorbs energetic radiation, and the exosphere grades into space.

The book’s approximate boundaries of 12, 50, 80 and 700 km are a learning guide, not fixed walls. Air usually gets thinner with height. High temperature in very thin upper air does not mean it transfers heat like an oven.

Ozone absorbs much harmful ultraviolet radiation. The ionosphere is an electrically charged region overlapping several layers, not simply another name for the thermosphere. Some radio frequencies interact with it.

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Question

Where does most cloud and rain weather occur?

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NCERT reference: chapter PDF pages 3, 4, 18, 19.

EXPLORE

One wet day is weather; the long pattern is climate

Ask which time scale the statement describes.

Two different windows of time

A day’s conditions and repeated observations over many years answer different questions.

Weather: Atmospheric conditions at a place over a short time.
Climate: Long-term patterns of weather, including averages and variation.

“It is raining this afternoon” describes weather. “This region usually has rainy summers” describes climate. Climate summaries often use 30-year periods.

Climate is not only an average temperature: seasonal rain, ranges and extremes also matter. A cold day does not by itself disprove long-term warming.

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Question

Is “today’s temperature is 28°C” a climate statement?

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NCERT reference: chapter PDF pages 5, 18, 19.

EXPLORE

Sunlight angle helps explain temperature

The same sunlight can spread over different areas.

Compare the same incoming beam

Two beams have the same width measured across the beam. A more oblique beam spreads the same incoming energy over a wider ground area. This is an angle model; the ground and beam colours are schematic.

Latitude: Angular distance north or south of the equator.
Torrid, temperate and frigid zones: Broad latitude belts separated by the tropics and polar circles.

Nearer the equator, sunlight is often more direct. Oblique rays spread energy over a larger surface. Altitude, cloud, winds and land–sea conditions also affect temperature.

The tropics lie near 23½° north and south; polar circles near 66½°. Earth’s tilted axis helps cause seasons as Earth orbits the Sun. Seasons are not mainly caused by changing distance from the Sun.

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Question

Why can mountains be cool even at low latitude?

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NCERT reference: chapter PDF pages 5, 6, 7.

EXPLORE

Water vapour and relative humidity are different

Temperature changes the saturation condition.

An illustrative comparison at two temperatures

Illustrative fractions only, not measurements. The vapour content is unchanged; the saturation condition differs. Air is not literally a bucket.

Humidity: Water vapour present in air.
Relative humidity: Actual water-vapour pressure as a percentage of saturation vapour pressure at that temperature.

Air at a higher temperature has a higher saturation vapour pressure. The same amount of water vapour can therefore have a lower relative humidity when the air warms.

High relative humidity slows evaporation from clothes and skin. Wind, sunlight and fabric also affect drying. Warm air does not automatically have higher relative humidity.

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Question

If air warms without gaining water vapour, must relative humidity rise?

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NCERT reference: chapter PDF pages 6, 7.

EXPLORE

Vapour must become droplets or ice first

Invisible vapour does not simply fall as rain.

From moist air to falling water

A simplified water route. Not every cloud produces rain, and frozen precipitation follows additional processes.

Condensation: Water vapour changing into liquid water.
Precipitation: Water falling from clouds as rain, snow, sleet or hail.

Rising air often cools as it expands. Water vapour can condense on tiny particles or form ice, creating clouds. Cloud particles must grow enough before they fall to the ground.

Rain, snow and hail depend on temperatures and processes within clouds and below them. A cloud is made of tiny droplets or ice crystals, not only invisible water vapour.

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Question

What change forms the liquid droplets in a cloud?

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NCERT reference: chapter PDF pages 6, 7.

EXPLORE

Pressure differences can set air in motion

Wind is named for where it comes from.

A pressure difference drives a response

Schematic pressure-gradient direction. It is not a complete weather map or a claim that real winds always blow in straight lines.

Air pressure: Force per unit area exerted by air.
Wind: Air movement, usually described by its horizontal direction and speed.

Pressure generally falls with altitude because less air lies above. A pressure difference tends to accelerate air from higher toward lower pressure. Earth’s rotation, friction and geography affect the actual route.

A north wind comes from the north and blows toward the south. A wind vane shows direction; an anemometer measures speed. Low pressure can favour rising air and cloud, but it is not a guarantee of rain.

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Question

A wind comes from the south-west. What is it called?

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NCERT reference: chapter PDF pages 7, 8.

EXPLORE

Land and sea heat and cool at different rates

Compare daytime and nighttime near a calm coast.

The coastal circulation reverses

Side-view coastal model with a return route aloft. Arrows show air circulation, not ocean currents.

Sea breeze: A local wind from sea toward warmer land, commonly by day.
Land breeze: A local wind from land toward relatively warmer sea, commonly at night.

By day, land can heat faster. Rising air over land helps draw cooler surface air inland from the sea. Return flow aloft forms part of the circulation.

At night, land can cool faster and the surface flow may reverse. Larger weather systems can override these local breezes; they are not compulsory every day.

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Question

During the basic daytime model, where does the surface breeze go?

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NCERT reference: chapter PDF pages 8, 19.

EXPLORE

Season names organise a changing year

Different calendars group seasons differently.

A year is grouped into four statistical windows

Band widths represent month counts, not equal-sized seasons or universal regional weather.

Meteorological season: A period used to summarise weather statistics.
Ṛtu: A season in traditional Indian calendars.

IMD season	Months	Common pattern; varies locally
Winter	Jan–Feb	Cooler conditions; some northern rain
Pre-monsoon	Mar–May	Heating; thunderstorms in some areas
South-west monsoon	Jun–Sep	Major rainy season for much of India
Post-monsoon	Oct–Dec	Withdrawal; important rain on the south-east coast

For IMD climate statistics, winter is January–February, pre-monsoon March–May, south-west monsoon June–September and post-monsoon October–December. These standard groups correct the overlapping month ranges printed in the chapter.

The six traditional ṛtus are vasanta, grīṣma, varṣā, śarad, hemanta and śiśira. Approximate modern-month correspondences vary by calendar and region. Seasons do not arrive everywhere on one exact date.

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Question

Which IMD statistical season includes May?

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NCERT reference: chapter PDF pages 9, 10, 18, 19.

Atmosphere and Climate Class 9 Notes

Chapter notes

Atmosphere and Climate

Air is a mixture, not one gas

Dry air: compare the main shares

Temperature changes help define the layers

Layers above the surface

One wet day is weather; the long pattern is climate

Two different windows of time

Sunlight angle helps explain temperature

Compare the same incoming beam

Water vapour and relative humidity are different

An illustrative comparison at two temperatures

Vapour must become droplets or ice first

From moist air to falling water

Pressure differences can set air in motion

A pressure difference drives a response

Land and sea heat and cool at different rates

The coastal circulation reverses

Season names organise a changing year

A year is grouped into four statistical windows

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