Air & Water | CTET SST P2

Composition and Layers of the Atmosphere

The atmosphere is the envelope of gases held to the Earth by gravity. It extends from the surface to about 10,000 km, though more than 99% of its mass lies within 50 km of the ground. By volume, dry air near sea level contains about 78% nitrogen, 21% oxygen and 1% other gases — argon, carbon dioxide, neon, helium, methane and ozone — plus water vapour and dust particles in varying amounts.

Nitrogen is essential for plant growth through the nitrogen cycle. Oxygen sustains respiration and combustion. Carbon dioxide, although less than 0.04%, is critical — it traps heat (the greenhouse effect) and is the raw material of photosynthesis. Water vapour is the source of all clouds, rain, dew and snow. Dust particles act as nuclei around which water droplets condense.

The atmosphere is divided into five layers on the basis of temperature change with height. The troposphere, from the surface to about 8–18 km, contains 75% of the air mass and almost all the water vapour; this is where weather happens, and temperature falls with height (about 6.5°C per km). The stratosphere reaches up to 50 km; it contains the ozone layer that absorbs harmful ultraviolet rays. Aircraft fly here because the air is dry and stable. The mesosphere extends up to 80 km; meteoroids entering from space burn up here, producing 'shooting stars'. The thermosphere reaches about 400 km and contains the ionosphere, which reflects radio waves and enables long-distance communication. The auroras occur here. Beyond is the exosphere, the thinnest outer layer, where the atmosphere gradually merges with space.

Two CTET-favourite facts: (1) the troposphere is the lowest layer and the seat of all weather, and (2) meteors burn up in the mesosphere. The atmosphere protects life by burning meteors, filtering ultraviolet radiation in the ozone layer, holding warmth through the greenhouse effect and providing oxygen.

Weather and Climate

Weather is the day-to-day state of the atmosphere at a place — it includes temperature, humidity, rainfall, wind, cloudiness and air pressure. Weather can change in a few hours. Climate, on the other hand, is the average weather conditions of a place over a long period, usually at least 30 years. A single sunny afternoon is weather; the fact that Jodhpur is hot and dry most of the year is climate.

The major elements of weather and climate are the same — temperature, humidity, precipitation, pressure and wind. Insolation, the incoming solar energy, is the basic driver. Insolation is highest near the Equator (where Sun rays fall vertically) and least at the poles (where rays are slanted and spread over a larger area). This unequal heating creates differences in air pressure that drive the global winds.

Climate of a place is controlled by latitude (distance from the Equator), altitude (height above sea level — temperature falls by about 6.5°C per km), distance from the sea (coastal places have moderate climate; interiors have continental extremes), ocean currents, and winds. India has a tropical monsoon climate with four broad seasons — winter (December–February), summer (March–May), south-west monsoon (June–September) and the retreating monsoon or post-monsoon (October–November).

Pedagogically, the distinction between weather and climate is a high-yield topic. Children confuse the two because both use the same words. A good classroom technique is to keep a 'weather diary' for a month (daily temperature, rainfall, cloud cover) and then compare it with the average for that month over 30 years from a state climate atlas — the children themselves see that one is a snapshot, the other an average. CTET often frames questions with insolation, monsoon winds (south-west during summer; north-east during winter), and the difference between weather and climate.

Wind Systems and Pressure Belts

Air has weight, and the weight of the column of air above a unit area at the surface is called atmospheric pressure. Pressure is measured by a barometer in millibars (mb). Average sea-level pressure is about 1013 mb. Air moves from areas of high pressure to areas of low pressure — this horizontal movement of air is wind. The greater the pressure difference, the stronger the wind.

The unequal heating of the Earth and its rotation produces seven pressure belts: the equatorial low (the Doldrums, around 0°), two sub-tropical high pressure belts (around 30° N and 30° S — the Horse Latitudes), two sub-polar low pressure belts (around 60° N and 60° S) and two polar highs at the poles.

From the pressure belts arise the three permanent (planetary) wind systems. The trade winds blow from the sub-tropical highs towards the equatorial low; in the Northern Hemisphere they are north-east trades, in the Southern Hemisphere south-east trades. The westerlies blow from the sub-tropical highs towards the sub-polar lows — south-westerly in the north, north-westerly in the south. The polar easterlies blow from the polar highs towards the sub-polar lows. The deflection of the winds from a straight path is caused by the Coriolis force, an effect of Earth's rotation.

Apart from the planetary winds there are seasonal winds like the monsoons of South Asia (the south-west monsoon from June–September brings 75–90% of India's annual rainfall, while the north-east monsoon in October–December brings rain to Tamil Nadu) and local winds like the loo (hot dry summer wind of north India), mango showers (pre-monsoon shower in Kerala–Karnataka), Kal Baisakhi (north-westers of Bengal) and land–sea breezes along coasts. CTET regularly tests the names of the three planetary wind systems and the direction of the monsoon winds.

Distribution of Water on Earth

Water covers about 71% of the Earth's surface, which is why the planet looks blue from space. The total volume of water is enormous, but most of it is unusable for daily human needs.

About 97% of Earth's water is the saline water of the oceans and seas. Only about 3% is freshwater. Of this 3%, about two-thirds is locked in glaciers, ice caps and polar ice sheets — chiefly in Antarctica and Greenland. A small portion is stored as groundwater in aquifers. Less than 1% of all the Earth's water is available as liquid freshwater in lakes, rivers and the soil — the water humans actually use for drinking, irrigation and industry.

This distribution explains the global water crisis. Although water is abundant, freshwater is scarce; and although freshwater exists on every continent, its distribution is uneven. Some regions (the Amazon basin, the monsoon belt of South Asia) have a surplus; others (the Sahara, Central Asia, parts of Australia) are chronically water-short.

India has about 4% of the world's freshwater for about 18% of the world's population. The major sources are the Himalayan rivers (Ganga, Yamuna, Brahmaputra — perennial, snow-fed) and the Peninsular rivers (Godavari, Krishna, Kaveri, Mahanadi, Narmada, Tapi — rainfall-fed and seasonal). Groundwater is heavily used in the northern plains. Water pollution, over-extraction of groundwater and unequal access make conservation an urgent topic for the social studies classroom. Two facts CTET likes to test: the 97% saline / 3% fresh ratio, and the fact that more than two-thirds of freshwater is locked in ice — so less than 1% of Earth's total water is the liquid freshwater we actually use.

The Water Cycle

The continuous movement of water between the oceans, the atmosphere and the land is called the water cycle or hydrological cycle. It is driven by the energy of the Sun and by gravity, and it keeps the total quantity of water on Earth constant.

The cycle has four main processes. Evaporation: solar heat converts liquid water from oceans, lakes, rivers and soil into water vapour. Transpiration from plants adds more water vapour to the air — together evaporation and transpiration are called evapotranspiration. The water vapour rises with warm air. Condensation: as the rising air cools, the vapour condenses around dust particles into tiny droplets, forming clouds. When the droplets become heavy enough, they fall as precipitation — rain, snow, hail, sleet or dew. A part of this water flows back to the oceans through rivers (runoff), part seeps into the ground (infiltration) to become groundwater, and part is again evaporated or used by plants. The cycle then begins again.

The water cycle has many natural functions — it distributes freshwater across land, sculpts landforms through erosion and deposition, maintains the salinity of oceans, and regulates climate. Without the cycle, all freshwater would soon drain to the sea and no rain would fall.

Human actions disturb the cycle. Deforestation reduces transpiration and increases runoff, causing floods downstream and droughts upstream. Concrete cities reduce infiltration and lower the water table. Air pollution can change the chemistry of precipitation (acid rain). Global warming intensifies the cycle — more evaporation, heavier rain in some regions and longer droughts in others. Classroom activities such as a boiling-water-on-a-cold-plate demonstration, drawing a labelled diagram of the cycle, or tracking rainwater in the schoolyard make the abstract cycle concrete. CTET questions in this section often link the cycle to local examples — clouds forming over the Western Ghats, monsoon rain feeding the rivers of peninsular India.

Oceans and Ocean Currents

The world has five oceans. In order of size they are the Pacific (the largest, about one-third of Earth's surface; the Mariana Trench, the deepest known point, lies here at about 11,000 m), the Atlantic (S-shaped, between the Americas and Eurasia–Africa), the Indian (the only ocean named after a country), the Southern or Antarctic (officially recognised by the IHO in 2000 — it surrounds Antarctica) and the Arctic (the smallest and shallowest, around the North Pole and partly covered by ice). The Indian Ocean borders India on three sides — south, east (Bay of Bengal), and west (Arabian Sea).

Ocean water is salty mainly because rivers carry dissolved salts from rocks, and seas have collected these salts over millions of years. Average salinity is about 35 g per litre. Salinity is higher in landlocked seas with high evaporation (Dead Sea, Red Sea) and lower where many rivers enter (Baltic Sea, Bay of Bengal).

Ocean water moves in three ways — waves caused by wind, tides caused by the gravitational pull of the Moon and the Sun, and currents caused by winds, salinity and temperature differences. Warm currents flow from the Equator towards the poles (e.g. the Gulf Stream in the Atlantic, the Kuroshio in the Pacific) and raise the temperature of coasts they pass; the British Isles are mild for their latitude because of the North Atlantic Drift. Cold currents flow from the poles towards the Equator (the Labrador, the Humboldt off Peru, the Benguela off Namibia); they bring rich nutrients but cool coastal climates. Where warm and cold currents meet, fishing grounds are exceptionally rich — for example, off Newfoundland (the Grand Banks) and Japan.

Tides cause the daily rise and fall of sea level. Spring tides are highest, on full-moon and new-moon days when Sun, Earth and Moon are aligned. Neap tides are smallest, when Sun and Moon are at right angles to the Earth. Tides help in navigation, in fishing, in flushing estuaries and now in generating tidal electricity.

Climate Change and Global Warming

Climate change means a long-term shift in average weather patterns. Global warming is one particular kind of change — the steady rise in Earth's average surface temperature observed since the late 19th century. Earth's average temperature has risen by about 1.1°C since pre-industrial times, and the rate of warming has accelerated since 1970.

The main cause is the enhanced greenhouse effect. Burning of coal, oil and natural gas in factories, power plants, vehicles and homes releases huge quantities of carbon dioxide. Methane comes from paddy fields, cattle and landfills. Deforestation reduces the natural absorption of CO₂. These gases trap more outgoing heat in the lower atmosphere — like an extra blanket on the planet.

The visible signs are everywhere. Melting glaciers in the Himalayas and the polar regions; rising sea levels threatening low-lying coasts including Mumbai, Kolkata and the Sunderbans; more frequent extreme weather — heatwaves, cloudbursts, cyclones (e.g. Amphan 2020, Tauktae 2021), prolonged droughts in some regions and floods in others; shifting monsoon patterns; loss of crops; and threat to species like the snow leopard, polar bear and coral reefs.

The international response is built around the UN Framework Convention on Climate Change (UNFCCC) agreed at the Rio Earth Summit (1992) and the Paris Agreement (2015), where countries pledged to keep warming well below 2°C and to aim for 1.5°C. India has committed to net-zero emissions by 2070 and has launched the International Solar Alliance with France. At the personal level, the '3 Rs' — reduce, reuse, recycle — plus saving electricity, planting trees, using public transport and reducing meat consumption all help. The SST classroom should treat climate change not as a distant future issue but as a present reality already affecting Indian rivers, crops and coasts. CTET often pairs this topic with a pedagogy item asking how to teach climate change without inducing despair — the answer is to focus on action and solutions, not only on threats.

Teaching Air and Water Through Experiments

Air and water are tangible but invisible (air) or constantly changing form (water). The best pedagogy is to make them visible through hands-on experiments rather than only describing them in words.

To prove that air has weight and exerts pressure: blow up two balloons of equal size and hang them from the ends of a rod balanced at the middle — they balance. Now burst one balloon; the rod tilts towards the inflated one because the air inside has weight. To show pressure, fill a glass to the brim with water, cover it with a stiff card, invert it — the card does not fall because the atmospheric pressure from below is greater than the weight of the water. To show that air takes up space: push an inverted glass straight down into a basin of water; the water does not enter because air is already inside.

For the water cycle: a 'mini-cycle' in a bowl — place a small cup of water inside a large bowl, cover with cling film, put a stone on top to make a depression, leave in sunlight. Water evaporates, condenses on the film and drips into the empty area. Children see evaporation, condensation and precipitation in one device.

For weather observation, a class can build a simple rain gauge from a graduated bottle, a wind vane from a paper arrow on a pin, and a thermometer chart for daily readings. A month of data turns into a small bar graph — integrating geography with mathematics.

For ocean currents: a tray of cold water with a few drops of warm coloured water released in one corner shows that warm fluids rise and move along the surface — a model of how warm currents like the Gulf Stream move. Field trips to a local pond, river or tank, video clips of the Indian monsoon, and the daily newspaper weather report make textbook learning feel real. NCF 2005 emphasises that activity, observation and discussion — not memorisation of definitions — are the heart of geography teaching. The CTET pedagogy MCQs in this chapter usually reward options that involve student investigation, drawing of diagrams, and connecting concepts to local experience.