Atmosphere and Climate

The One Thing

The atmosphere is a connected system that supports life, regulates temperature, and produces weather and climate. The Indian monsoon is central to the country’s environment and economy, while human-driven increases in greenhouse gases are intensifying climate change and increasing hazards such as floods and heat waves.

Who and What

• Atmosphere: The blanket of gases surrounding Earth, held by gravity. It supports life, controls temperature, and protects living organisms. It consists mainly of nitrogen, about 78%, and oxygen, about 21%; argon, carbon dioxide, water vapour, and other gases make up most of the remainder.
• Troposphere: The lowest and densest atmospheric layer, where humans live and most weather changes occur. It becomes thinner with height, and most clouds, storms, and rainfall occur in it. In the troposphere, temperature decreases by about 6.5 degrees Celsius for every 1 kilometre increase in altitude, according to the normal lapse rate.
• Stratosphere: The layer above the troposphere. It contains the ozone layer, which absorbs much of the Sun’s harmful ultraviolet radiation, and has relatively stable air.
• Mesosphere: The atmospheric layer where many meteoroids burn because of friction.
• Thermosphere: A very high, thin layer with extremely high temperatures. It includes parts of the ionosphere.
• Exosphere: The outermost atmospheric layer, which gradually merges with outer space.
• Weather: The day-to-day condition of the atmosphere at a particular place and time. It is short-term and variable.
• Climate: The average pattern of weather in a place over a long period, commonly about 30 years. One unusual weather event alone does not prove climate change.
• Temperature: The degree of hotness or coldness of the air.
• Air pressure: The force exerted by the weight of air on a surface. It is measured with a barometer and commonly expressed in hectopascals or millibars. Standard sea-level pressure is about 1013 hPa.
• Humidity: The amount of water vapour present in the air.

Relative humidity = (actual amount of water vapour in air ÷ maximum amount possible at that temperature) × 100.

• Wind: Moving air that generally flows from areas of high pressure to areas of low pressure. Its direction and speed are influenced by pressure differences, Earth’s rotation, friction, and local heating.
• Precipitation: Water falling from the atmosphere as rain, snow, sleet, or hail.
• Water cycle: The continuous movement of water through evaporation, condensation, cloud formation, precipitation, runoff, infiltration, and collection.
• Monsoon: A seasonal reversal of wind direction that produces distinct wet and dry seasons.
• Southwest Monsoon: India’s main summer monsoon. It generally arrives from June and brings most of the country’s annual rainfall. It usually reaches Kerala around 1 June, advances across India, and begins withdrawing from northwestern India around September. Its major branches are the Arabian Sea branch and the Bay of Bengal branch.
• Northeast or Retreating Monsoon: The winter monsoon period, during which winds generally blow from land toward the sea. It gives important rainfall to Tamil Nadu and nearby areas, particularly in October and November.
• Indian monsoon-type climate: A climate characterised by seasonal reversal of winds, a rainy season, a relatively dry season, and regional variations in temperature and rainfall. It is influenced by the Western Ghats, Himalayas, distance from the sea, altitude, relief, and seasonal winds.
• Mawsynram: A place in Meghalaya that is among the locations receiving the highest average annual rainfall in the world.
• Western Rajasthan: A region receiving very little rainfall, demonstrating the uneven distribution of rainfall in India.
• Greenhouse effect: The natural process in which gases such as carbon dioxide, methane, and water vapour trap some outgoing heat and warm Earth.
• Greenhouse gases: Gases including carbon dioxide, methane, nitrous oxide, water vapour, and some industrial gases.
• Global warming: The long-term rise in Earth’s average surface temperature, mainly due to increased greenhouse gases from human activities.
• Climate change: Long-term changes in temperature, rainfall, winds, and other climate patterns.
• Flood: The overflow of water onto land that is normally dry.
• Carbon footprint: The total greenhouse-gas emissions caused directly or indirectly by a person, activity, product, or organisation, usually measured in carbon-dioxide equivalents.

Causes and Consequences

• The atmosphere’s gases, together with temperature and air pressure, form a connected system. Differences in pressure produce winds; winds influence cloud formation and rainfall; and rainfall affects ecosystems, agriculture, water supplies, and human activities.

• The composition and structure of the atmosphere determine its functions. Nitrogen makes up about 78% of the atmosphere and oxygen about 21%, while the ozone layer in the stratosphere absorbs much of the Sun’s harmful ultraviolet radiation.

• Temperature decreases with altitude in the troposphere at the normal lapse rate of about 6.5 degrees Celsius per 1 kilometre. This variation, together with differences in air pressure and local heating, contributes to atmospheric movement and weather.

• Weather refers to short-term atmospheric conditions, whereas climate refers to the usual pattern of weather over about 30 years. Consequently, a single unusual weather event cannot, by itself, establish that climate change has occurred.

• The water cycle links evaporation, condensation, cloud formation, precipitation, runoff, infiltration, and collection. Changes in atmospheric conditions therefore affect both water availability and the likelihood of rainfall.

• India’s monsoon-type climate results from the seasonal reversal of winds. The southwest monsoon generally reaches Kerala around 1 June, spreads across the country through the Arabian Sea branch and Bay of Bengal branch, and begins withdrawing from northwestern India around September.

• The Indian monsoon is essential for agriculture, water supply, ecosystems, and the economy, but its rainfall is uneven and may vary from year to year. The Western Ghats, Himalayas, distance from the sea, altitude, relief, and seasonal winds create regional contrasts. Mawsynram in Meghalaya receives among the highest average annual rainfall in the world, whereas parts of western Rajasthan receive very little.

• The northeast or retreating monsoon brings important rainfall to Tamil Nadu and nearby areas. Tamil Nadu receives a large part of its annual rainfall during this period, especially in October and November.

• Floods may result from intense rainfall, overflowing rivers, cyclones, storm surges, rapid snowmelt, blocked drainage, and unplanned construction. Their effects are often intensified when wetlands and floodplains are occupied, forests are removed, drainage systems are blocked, or planning is weak.

• Natural climate variations occur, but the recent rapid warming of Earth is mainly linked to human activities that increase greenhouse-gas concentrations. Major human sources of carbon emissions include burning coal, petrol, diesel, and gas; deforestation; industries; transport; electricity generation; and some agricultural activities.

• The greenhouse effect is a natural process, but increased concentrations of greenhouse gases strengthen heat retention. This contributes to global warming and wider climate change involving temperature, rainfall, winds, and other climate patterns.

• Climate change raises the risks of floods, heat waves, and other hazards. Flood risk may increase when rainfall becomes more extreme, while poor land use, deforestation, blocked drainage, and inadequate preparedness increase damage.

• Carbon footprints can be reduced by saving electricity, using renewable energy, walking or cycling, using public transport, reducing waste, reusing materials, recycling, and planting and protecting trees.

• Flood preparedness reduces risks through early warning systems, evacuation plans, raised storage of important items, avoidance of floodwater, safe drinking water, and community disaster-response planning. Reducing carbon footprints and adapting to changing climate conditions are both necessary for protecting people, resources, and future generations.

What Gets Asked

• Compare weather and climate, including their timescales, variability, and the reason why one unusual weather event does not prove climate change.
• Explain how temperature, air pressure, humidity, wind, precipitation, and the water cycle operate as parts of a connected atmospheric system.
• Describe the layers of the atmosphere and evaluate the significance of the troposphere and the ozone layer in the stratosphere.
• Explain the causes, branches, seasonal sequence, and regional effects of the Indian monsoon, including the significance of Kerala, Mawsynram, western Rajasthan, and Tamil Nadu.
• Distinguish the natural greenhouse effect from human-driven global warming and climate change, identifying major greenhouse gases and human sources of emissions.
• Explain why floods occur and assess how land use, deforestation, blocked drainage, unplanned construction, early warning systems, evacuation plans, and community preparedness affect flood risk and damage.