# The Atmosphere: Composition, Structure & the Heat Budget (UPSC Geography) Source: https://tracknation.in/story/the-atmosphere-composition-structure-the-heat-budget-upsc-geography (TrackNation — free UPSC notes with PYQs) > A UPSC note on the atmosphere — its composition (78% nitrogen, 21% oxygen, carbon dioxide, ozone, water vapour, dust), the five temperature-based layers (troposphere with the normal lapse rate, stratosphere and the ozone layer, mesosphere, thermosphere/ionosphere, exosphere), insolation, and the heat budget (35-unit albedo, 65 in = 65 out, greenhouse effect) — explaining why the atmosphere is heated from below. Anchors the real 2022–2024 Prelims PYQs with self-MCQs. ## What the Atmosphere Is 🎯 **Exam priority: Important.** *The atmosphere is one of UPSC's most dependable scoring grounds: Prelims has tested water vapour, troposphere thickness, insolation and the carbon-dioxide mechanism almost every year (2022–2024), and the 2024 paper carried Statement-I/II questions on both atmospheric heating and troposphere thickness. The single idea that unlocks most of it: the atmosphere is heated **from below** by the earth, not directly by the sun — so you must master short-wave in versus long-wave out.* The atmosphere is the thin envelope of gases held to the earth by gravity. We can survive minutes without food or water but not seconds without air — and almost the whole of it sits close to the ground: **99% of the atmosphere's mass lies within 32 km** of the surface. ## What the Atmosphere Is — and Why It Is Studied First in Climate - The atmosphere is a **mixture of gases, water vapour and dust particles** enveloping the earth. It is colourless and odourless, felt only when it moves as wind, and is an integral part of the earth's mass. - It carries the **life-giving gases** — oxygen for animals, carbon dioxide for plants — and shields and regulates life through ozone and water vapour. This is why NCERT opens the **Climate unit** with the atmosphere before moving to temperature, pressure, winds and rainfall. - The whole chapter rests on one energy principle: the earth **receives energy from the sun and radiates an equal amount back to space**, so over time it neither warms up nor cools down. Master this balance and the composition, the layers and the heat budget all fall into place. ## Composition of the Atmosphere By volume the air is overwhelmingly two gases, but the tiny variable constituents — carbon dioxide, ozone, water vapour and dust — do almost all the climatic work. ## Composition of the Atmosphere - The atmosphere has three categories of constituents: **gases, water vapour and dust particles**. By volume it is about **78% nitrogen** and **21% oxygen**, with carbon dioxide, ozone and other gases making up the small remainder. The proportions change with height: **oxygen becomes almost negligible at 120 km**, while **carbon dioxide and water vapour are found only up to about 90 km**. - **Carbon dioxide** is meteorologically the most important gas. It is **transparent to incoming short-wave solar radiation but opaque to outgoing long-wave terrestrial radiation** — it lets the sun's energy in but traps the earth's outgoing heat, absorbing part of it and re-radiating some back to the surface. It is largely responsible for the **greenhouse effect**; unlike other gases its volume has been rising for decades, chiefly from **burning fossil fuels**, raising air temperature. - **Ozone** is concentrated **between 10 and 50 km** above the surface. It acts as a **filter that absorbs the sun's ultraviolet (UV) rays** and prevents them from reaching the ground — without it, life on the surface would be exposed to intense, harmful radiation. - **Water vapour** is a variable gas: it **decreases with altitude** and from the equator to the poles. It can reach **up to 4% of the air by volume in the warm, wet tropics** but falls to **less than 1% in dry, cold deserts and polar regions**. It absorbs part of the incoming insolation and preserves the earth's radiated heat — acting like a **blanket** that keeps the earth neither too hot nor too cold — and it governs the stability and instability of the air. - **Dust and salt particles** (sea salt, fine soil, smoke-soot, ash, pollen, meteor fragments) are kept suspended in the lower air and lifted higher by convection. They are most concentrated over **subtropical and temperate regions** (dry winds) and least over the equator and poles. Crucially, they act as **hygroscopic nuclei around which water vapour condenses to form clouds** — without them, cloud and rainfall would be impaired. | **Constituent** | **Key data / location** | **Climatic role** | | Nitrogen (~78%) | Major gas by volume | Largely inert diluent | | Oxygen (~21%) | Negligible above 120 km | Respiration / combustion | | Carbon dioxide | Found up to ~90 km; rising | Greenhouse effect (traps long-wave) | | Ozone | 10–50 km (stratosphere) | Absorbs UV; shields life | | Water vapour | Up to 4% (tropics), <1% (poles) | Thermal blanket; stability/instability | | Dust / salt | Concentrated in lower air, subtropics | Condensation nuclei for clouds | ## Structure: The Five Layers Vertically, the air is stacked into five layers defined by how temperature behaves with height — and the geographer cares most about the lowest two. ## Structure of the Atmosphere: The Five Layers - Density is **highest at the surface and falls with altitude**. On the basis of temperature, the column is divided into five layers — **troposphere, stratosphere, mesosphere, thermosphere and exosphere** — with the ionosphere described within the thermosphere region. Although all influence us, geographers are concerned chiefly with the **first two**. - **1. Troposphere** — the lowermost layer, average height **13 km**: about **8 km at the poles and 18 km at the equator**, thickest at the equator because strong **convectional currents** carry heat to great heights (the basis of the 2024 Statement-I/II question). It holds the dust and water vapour, and **all weather and climate occur here** — making it the most important layer for biological activity and for human beings. Temperature falls with height at the **normal lapse rate of 1°C per 165 m**. - **Tropopause** — the boundary above the troposphere where temperature stops changing (hence the name "pause"). It is colder over the equator (**about −80°C**) than over the poles (**about −45°C**), precisely because equatorial convection pushes the tropopause higher and colder. - **2. Stratosphere** — above the tropopause, extending to about **50 km**. Its defining feature is the **ozone layer**, which absorbs UV radiation and shields life from intense, harmful energy. **3. Mesosphere** — up to about **80 km**, where temperature again decreases with height, reaching about **−100°C** at 80 km; its upper limit is the **mesopause**. - **Ionosphere** — lying **80–400 km** above the mesopause within the **thermosphere**, it contains electrically charged **ions**; temperature rises with height here, and it **reflects radio waves back to earth**, the basis of long-distance radio communication. **5. Exosphere** — the uppermost, extremely rarefied layer about which little is known, gradually merging with outer space. | **Layer** | **Upper limit / extent** | **Temperature behaviour** | **Hallmark feature** | | Troposphere | ~13 km (8 km poles, 18 km equator) | Falls (1°C / 165 m) | All weather; lapse rate | | Tropopause | boundary | Near-constant (−80°C eq., −45°C poles) | Temperature 'pause' | | Stratosphere | up to 50 km | Stable / rises | Ozone layer (UV filter) | | Mesosphere | up to 80 km (mesopause) | Falls to −100°C | Coldest layer | | Thermosphere / Ionosphere | 80–400 km | Rises with height | Ions reflect radio waves | | Exosphere | uppermost | — | Rarefied; merges with space | ## Insolation The energy that drives all of this is insolation — the incoming solar radiation — and how much a place gets depends mostly on the angle at which the sun's rays strike it. ## Insolation: How Much Solar Energy Reaches the Earth - **Insolation** is the incoming solar radiation the earth receives, mostly in **short wavelengths**. Because the earth is nearly spherical, the rays fall obliquely at the top of the atmosphere and the earth intercepts only a tiny portion of the sun's output — on average **1.94 calories per sq. cm per minute** at the top of the atmosphere (the solar constant). - The earth–sun distance varies through the year: the earth is farthest at **aphelion (152 million km, 4 July)** and nearest at **perihelion (147 million km, 3 January)**, so insolation on 3 January is slightly more — but this effect is **masked by the distribution of land and sea and by atmospheric circulation**, so it barely affects daily weather. The earth's axis is tilted at **66½° to the orbital plane**, which strongly shapes how much each latitude receives. - Insolation varies because of five factors: the **rotation of the earth, the angle of the sun's rays, the length of day, the transparency of the atmosphere and the configuration (aspect) of land** (the last two having less influence). The decisive one is the **angle of inclination, which depends on latitude**: the higher the latitude, the more slanting the rays. - Slant rays deliver less energy for two reasons: they **spread the same energy over a larger area** (so energy per unit area falls), and they **pass through a greater depth of atmosphere**, suffering more absorption, scattering and diffusion. Within the troposphere, **water vapour, ozone and other gases absorb much of the near-infrared**, and small particles **scatter the visible spectrum** — giving the sky its **blue colour** and the rising and setting sun its **red colour**. - At the surface, insolation ranges from about **320 W/m² in the tropics to about 70 W/m² at the poles**. The **maximum is over the subtropical deserts**, where cloud cover is least; the **equator gets comparatively less** than the tropics because of its heavy cloudiness, and at the same latitude the continents receive more than the oceans. ## Heating of the Atmosphere Once insolation arrives, it heats the ground first; the ground then heats the air above it — which is why the atmosphere is warmed from below. ## How the Atmosphere Is Heated — and Why from Below - The atmosphere is **largely transparent to short-wave solar radiation**, so most insolation passes through and heats the **earth's surface** first. The heated earth then becomes a radiating body and emits energy in **long-wave (terrestrial) form**, which is strongly absorbed by **carbon dioxide and other greenhouse gases**. Thus the atmosphere is heated **indirectly, from below** — the key idea behind the 2024 Statement-I/II PYQ and behind **Planck's law** (a hotter body radiates more energy at shorter wavelengths, so the hot sun emits short-wave and the cooler earth long-wave). - **Conduction** heats the lowest air by direct contact: when two bodies of unequal temperature touch, energy flows from warmer to cooler until they equalise. It is important only for the **lowest layers** in contact with the ground. - **Convection** is vertical heating: air in contact with the ground is warmed, rises in currents and carries heat upward. This transfer is **confined to the troposphere**. - **Advection** is the horizontal transfer of heat by moving air, and it is **more important than vertical movement**. In the middle latitudes most of the day-to-day weather change is caused by advection; in tropical India the hot summer wind **'loo'** is an example. | **Process** | **Direction / mechanism** | **Where it matters** | | Terrestrial radiation | Earth re-emits long-wave; absorbed by CO₂/GHGs | Heats whole atmosphere from below | | Conduction | Contact, warmer → cooler body | Lowest layers only | | Convection | Vertical rising currents | Confined to troposphere | | Advection | Horizontal air movement | Dominant in middle latitudes (e.g. 'loo') | ## The Heat Budget of the Earth Finally, the books are balanced: every unit of solar energy the earth keeps is eventually returned to space, which is why the planet maintains a constant temperature. ## The Heat Budget of the Earth - The **heat budget** is the balance between energy received and energy lost. Taking insolation at the top of the atmosphere as **100 units**, about **35 units are reflected straight back to space** before reaching the surface — **27 from cloud tops, 2 from snow and ice**, and the rest from the air. This reflected fraction is the earth's **albedo**. - The remaining **65 units are absorbed**: **14 within the atmosphere** and **51 by the earth's surface**. The earth then re-radiates all 51 units as long-wave terrestrial radiation — **17 units directly to space** and **34 units to the atmosphere** (6 absorbed directly, 9 via convection and turbulence, 19 via the latent heat of condensation). - The atmosphere therefore holds **48 units (14 from insolation + 34 from the earth)** and radiates them all back to space. The total returned is **17 + 48 = 65 units**, exactly balancing the 65 absorbed. So the earth **neither warms up nor cools down** — a global energy equilibrium. - The balance is not uniform by latitude: there is a **surplus of net radiation between 40°N and 40°S and a deficit near the poles**. This surplus tropical heat is **redistributed poleward** by winds and ocean currents, which is why the tropics do not keep heating up and the poles do not stay permanently frozen — the engine behind global winds (taken up in Article 11). - The **greenhouse effect** is simply this trapping mechanism: greenhouse gases (chiefly CO₂, plus water vapour and others) let short-wave sunlight in but absorb the outgoing long-wave radiation, keeping the lower atmosphere warm enough for life. Rising CO₂ from fossil fuels intensifies it, the link to **global warming**. | **Stage** | **Units** | **Detail** | | Insolation at top | 100 | Reference total | | Reflected (albedo) | 35 | 27 cloud tops + 2 snow/ice + rest | | Absorbed by atmosphere | 14 | From insolation | | Absorbed by surface | 51 | Heats the ground | | Earth → space (direct) | 17 | Long-wave to space | | Earth → atmosphere | 34 | 6 direct + 9 convection + 19 latent heat | | Atmosphere → space | 48 | 14 + 34, re-radiated | | Total back to space | 65 | 17 + 48 = balances 65 absorbed | ## Why this matters for UPSC - Prelims tests this area relentlessly — **water vapour (2024), troposphere thickness (2024), atmospheric heating / greenhouse gases (2024), insolation (2023) and the heat budget / clouds (2022)**. Two anchors carry you through almost all of it: (1) the atmosphere is heated **from below by long-wave terrestrial radiation**, not directly by the sun; and (2) the heat budget balances at **65 in = 65 out** with a **35-unit albedo**. Memorise the layer limits, the lapse rate (1°C/165 m), and the CO₂ "transparent-in, opaque-out" mechanism. ## Further Reading - Standard NCERT-level geography textbooks and reference books, plus a good atlas (any UPSC reading list). ## Test Yourself: Practice Questions & PYQs Test yourself on atmospheric composition, the five layers, insolation and the heat budget — keep returning to the two anchors: heated from below, and 65 in = 65 out. Attempt each before revealing the answer. ## Practice Questions **Q1.** By volume, the two most abundant gases in the atmosphere are: - (a) Nitrogen and oxygen - (b) Carbon dioxide and water vapour - (c) Oxygen and carbon dioxide - (d) Nitrogen and ozone **Answer: (a)** — Nitrogen (~78%) and oxygen (~21%) together make up about 99% of the air by volume; carbon dioxide, ozone and water vapour are minor but climatically vital constituents. **Q2.** Which gas is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation? - (a) Argon - (b) Oxygen - (c) Nitrogen - (d) Carbon dioxide **Answer: (d)** — Carbon dioxide lets short-wave solar energy pass in but absorbs and re-radiates the earth's outgoing long-wave radiation, which is the basis of the greenhouse effect. **Q3.** The troposphere is thickest at the equator (~18 km) mainly because: - (a) Strong convectional currents carry heat to great heights - (b) The earth bulges at the equator - (c) Gravity is weakest at the equator - (d) Ozone is concentrated there **Answer: (a)** — Intense equatorial heating drives strong convection that pushes the troposphere up to ~18 km, against ~8 km at the poles — and also makes the tropopause colder over the equator (−80°C). **Q4.** Oxygen becomes almost negligible at a height of about: - (a) 32 km - (b) 400 km - (c) 50 km - (d) 120 km **Answer: (d)** — Oxygen becomes almost negligible at about 120 km; by contrast carbon dioxide and water vapour are found only up to about 90 km, and 99% of the atmosphere's mass lies within 32 km. **Q5.** The normal lapse rate in the troposphere is a temperature decrease of about: - (a) 10°C per 165 m - (b) 6.5°C per 100 m - (c) 1°C per 1000 m - (d) 1°C per 165 m **Answer: (d)** — In the troposphere temperature falls at roughly 1°C for every 165 m of ascent — the normal lapse rate. (The figure of 6.5°C per 1000 m is the broadly equivalent average rate.) **Q6.** In the earth's heat budget, the albedo (radiation reflected back to space before reaching the surface) is about: - (a) 14 units of 100 - (b) 65 units of 100 - (c) 51 units of 100 - (d) 35 units of 100 **Answer: (d)** — Of 100 units of insolation, about 35 are reflected back (27 from cloud tops, 2 from snow/ice, rest from air) — this is the albedo. The remaining 65 are absorbed (14 by the atmosphere, 51 by the surface). **Q7.** The atmosphere is heated mainly by: - (a) Reflected sunlight from clouds - (b) Long-wave terrestrial radiation from below - (c) Short-wave solar radiation directly - (d) Scattered visible light **Answer: (b)** — Solar radiation largely passes through the transparent atmosphere and heats the ground; the ground re-radiates long-wave energy that CO₂ and other greenhouse gases absorb, so the atmosphere is heated from below. **Q8.** Net radiation is in surplus between 40°N and 40°S and in deficit near the poles. The chief consequence is: - (a) Insolation becomes equal at all latitudes - (b) Surplus heat is redistributed poleward by winds and currents - (c) The tropics keep heating up indefinitely - (d) The poles become permanently frozen with no exchange **Answer: (b)** — The latitudinal imbalance drives a poleward transfer of surplus tropical heat by winds and ocean currents, preventing the tropics from overheating and the poles from permanently freezing. ### UPSC Previous Year Questions (PYQs) *These are genuine UPSC Prelims questions on this exact theme — note how heavily 2022–2024 leaned on it.* **Q9.** With reference to “water vapour”, which of the following statements is/are correct? 1. It is a gas, the amount of which decreases with altitude. 2. Its percentage is maximum at the poles. (UPSC Prelims 2024) - (a) Both 1 and 2 - (b) 1 only - (c) Neither 1 nor 2 - (d) 2 only **Answer: (b)** — Water vapour decreases with altitude (1 correct), but it is maximum in the warm wet tropics (up to ~4%) and minimal over cold dry polar regions, so 2 is wrong. **Q10.** Statement-I: Thickness of the troposphere at the equator is much greater as compared to poles. Statement-II: At the equator, heat is transported to great heights by strong convectional currents. (UPSC Prelims 2024) - (a) Both are correct, but Statement-II does not explain Statement-I - (b) Statement-I is correct, but Statement-II is incorrect - (c) Both Statement-I and Statement-II are correct and Statement-II explains Statement-I - (d) Statement-I is incorrect, but Statement-II is correct **Answer: (c)** — The troposphere is ~18 km thick at the equator vs ~8 km at the poles precisely because strong convectional currents carry heat to great heights, so II correctly explains I. **Q11.** Statement-I: The atmosphere is heated more by incoming solar radiation than by terrestrial radiation. Statement-II: Carbon dioxide and other greenhouse gases in the atmosphere are good absorbers of long-wave radiation. (UPSC Prelims 2024) - (a) Both are correct, but Statement-II does not explain Statement-I - (b) Statement-I is correct, but Statement-II is incorrect - (c) Statement-I is incorrect, but Statement-II is correct - (d) Both Statement-I and Statement-II are correct and Statement-II explains Statement-I **Answer: (c)** — The atmosphere is heated mainly by terrestrial (long-wave) radiation, not directly by solar radiation, so Statement-I is wrong; CO₂ and other GHGs are good absorbers of long-wave radiation, so Statement-II is correct. **Q12.** With reference to the Earth's atmosphere, which one of the following statements is correct? (UPSC Prelims 2023) - (a) The total insolation received at the equator is roughly about 10 times that received at the poles - (b) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere - (c) Infrared rays constitute roughly two-thirds of insolation - (d) Infrared waves are a part of the visible spectrum of solar radiation **Answer: (b)** — Water vapour, concentrated in the lower atmosphere, strongly absorbs and re-emits infrared (long-wave) radiation; equatorial insolation is only about 5x (not 10x) the poles, and infrared is not part of the visible spectrum. **Q13.** Consider the following statements about clouds and the heat budget: 1. High clouds primarily reflect solar radiation and cool the Earth's surface. 2. Low clouds have a high absorption of infrared radiation from the surface and thus cause warming. Which is/are correct? (UPSC Prelims 2022) - (a) Both 1 and 2 - (b) 1 only - (c) 2 only - (d) Neither 1 nor 2 **Answer: (d)** — It is low thick clouds that reflect solar radiation and cool the surface, while high thin clouds trap outgoing infrared and warm it — both statements reverse the cloud types, so neither is correct. **Q14.** Why are dewdrops not formed on a cloudy night? (UPSC Prelims 2019) - (a) Clouds reflect back the Earth's radiation - (b) The Earth's surface would have low temperatures on cloudy nights - (c) Clouds deflect the blowing wind to ground level - (d) Clouds absorb the radiation released from the Earth's surface **Answer: (a)** — Clouds reflect terrestrial (long-wave) radiation back to the ground, keeping nights warmer; the surface never cools to the dew point, so dew does not form — a direct application of the heat-budget idea. ### Mains Practice Questions Use these to frame full-length answers. You don't have to answer one exactly — they show the angles UPSC tests, so let them guide which points you cover. - Explain why the atmosphere is heated from below rather than directly by the sun, and the role of greenhouse gases. (cf. UPSC Prelims 2024) - Describe the five-layered structure of the atmosphere and the temperature behaviour that defines each layer. - Discuss the heat budget of the earth and explain how it keeps global temperature constant. - Account for the variation of insolation with latitude and the factors that control it. - Examine the climatic roles of carbon dioxide, ozone, water vapour and dust particles in the atmosphere. ### UPSC Mains — Previous Years’ Questions on this topic (last 10 years) No direct GS Mains question has been asked specifically on this topic in the last decade — it is primarily a **Prelims** area. Use the practice prompts above to stay exam-ready.