UPSC CSE Prelims
Climatology Previous Year Questions (PYQs)

Detailed Explanation:

Correct Answer: ✅ Option 3 (I, II and III)

All three statements correctly describe the role of the atmosphere in maintaining Earth's temperature. The atmosphere acts like a thermal blanket by absorbing and trapping heat, preventing the Earth from becoming extremely cold. Greenhouse gases such as carbon dioxide, methane, and water vapour are especially effective in trapping outgoing infrared radiation.

✅ Statement I is Correct: Without the atmosphere, Earth would lose most of its heat to space, and average surface temperatures would be well below freezing.

✅ Statement II is Correct: The atmosphere absorbs and retains heat, maintaining a habitable average temperature through the natural greenhouse effect.

✅ Statement III is Correct: Greenhouse gases such as carbon dioxide, methane, and water vapour efficiently absorb and trap infrared radiation.

Short Notes: Greenhouse Effect

• The natural greenhouse effect keeps Earth warm enough to support life.

• Earth's average temperature is about 15°C with the atmosphere.

• Without the atmosphere, the average temperature would be around –18°C.

• Carbon dioxide (CO₂), methane (CH₄), and water vapour are major greenhouse gases.

• Greenhouse gases absorb outgoing infrared radiation emitted by Earth's surface.

• The greenhouse effect is a natural process, but excessive greenhouse gases enhance global warming.

• Water vapour is the most abundant natural greenhouse gas.

Detailed Explanation:

In January (Northern Hemisphere winter), land cools much faster than oceans because land has a lower heat capacity. As a result, continents become much colder than nearby oceans at the same latitude. This temperature contrast causes isotherms to bend equatorward over land and poleward over oceans.

✅ Statement I is Correct: In January, isotherms dip southward over cold landmasses and curve northward over relatively warm oceans in the Northern Hemisphere.

✅ Statement II is Correct: Oceans retain heat longer than land, making oceanic areas warmer than continental areas during winter.

✅ Statement II explains Statement I: The temperature difference between colder land and warmer oceans is the direct reason for the bending of isotherms.

Short Notes: Isotherms

• Isotherms are lines joining places with equal temperature.

• In January, Northern Hemisphere continents are colder than oceans.

• Land heats and cools faster because it has lower specific heat than water.

• Oceans act as heat reservoirs and lose heat slowly.

• Isotherms bend equatorward over land and poleward over oceans in winter.

• In July, the pattern is generally reversed in the Northern Hemisphere.

• The greatest isotherm deviations occur over large landmasses such as Eurasia and North America.

Detailed Explanation:

The concentration of atmospheric dust is generally higher in subtropical and temperate regions because these areas contain large deserts, semi-arid lands, sparse vegetation, and stronger dry winds that can easily lift dust particles into the atmosphere. Therefore, Statement I is correct. However, Statement II is incorrect because these regions experience more dry winds, not less.

✅ Statement I is Correct: Subtropical and temperate regions have higher dust content due to dry conditions, deserts, sparse vegetation, and wind action.

❌ Statement II is Incorrect: These regions experience more dry winds, which help transport dust particles over long distances.

Concept Note:

Dust particles (Aerosols) in the atmosphere originate from deserts, dry soils, volcanic eruptions, sea salts, and human activities. The world's major dust sources, such as the Sahara Desert, are located in subtropical regions, making these areas dust-rich.

Detailed Explanation:

Answer: Option 4 — Statement-I is incorrect, but Statement-II is correct

This question tests understanding of atmospheric heating mechanisms and the greenhouse effect. Statement-I incorrectly suggests that incoming solar radiation heats the atmosphere more than terrestrial radiation, while Statement-II correctly describes the greenhouse gas absorption properties.

❌ Statement-I – Incorrect: The atmosphere is heated more by terrestrial (long wave) radiation than by incoming solar (short wave) radiation. Solar radiation mostly passes through the atmosphere without being absorbed, whereas the Earth's surface absorbs solar energy and re-radiates it as long wave radiation, which is then absorbed by greenhouse gases in the atmosphere, heating it.

✅ Statement-II – Correct: Carbon dioxide and other greenhouse gases are indeed good absorbers of long wave (infrared) radiation emitted by the Earth's surface, which is the fundamental principle of the greenhouse effect.

📝 Short Notes: Atmospheric Heating and Earth's Heat Budget

• Insolation: Incoming solar radiation reaches Earth in short waves (0.2 to 4 micrometers). Most of it passes through the atmosphere with minimal absorption.
• Terrestrial Radiation: Earth's surface absorbs solar energy and re-radiates it as long wave infrared radiation (4 to 80 micrometers).
• Atmospheric Heating: The atmosphere is largely transparent to incoming short wave solar radiation but opaque to outgoing long wave terrestrial radiation due to greenhouse gases.
• Greenhouse Effect: CO₂, water vapor, methane, and other greenhouse gases absorb long wave radiation, warming the lower atmosphere. This is why the atmosphere is heated more from below (by terrestrial radiation) than from above (by solar radiation).
• Heat Budget Components: Of the solar radiation reaching Earth, approximately 35% is reflected back, 14% is absorbed by the atmosphere, and 51% is absorbed by the Earth's surface. The surface then heats the atmosphere through radiation, conduction, and convection.
• Albedo Effect: About 30% of incoming solar radiation is reflected by clouds, atmospheric particles, and the Earth's surface without heating the atmosphere significantly.

Detailed Explanation:

Answer: Option 4 — Marine West coast climate

The Marine West Coast climate (Cfb in Köppen classification) is characterized by low annual and daily temperature ranges due to oceanic moderation, year-round precipitation brought by prevailing westerly winds, and precipitation totals typically ranging from 50-250 cm. This climate is found on western coasts of continents between 40°-60° latitude, such as the Pacific Northwest of North America and Western Europe.

📝 Short Notes: Marine West Coast Climate

• Location: Found on western coasts of continents between 40°-60° latitude (e.g., Pacific Northwest USA, British Columbia, Western Europe, Southern Chile, Southeast Australia, New Zealand)
• Temperature Characteristics: Mild winters (above 0°C) and cool summers (below 22°C); narrow annual temperature range due to maritime influence; typically no month averages below -3°C
• Precipitation Pattern: Year-round rainfall with slight winter maximum; brought by mid-latitude cyclones and orographic lifting; ranges from 50-250 cm annually depending on topography
• Atmospheric Controls: Dominated by prevailing westerlies that carry moist oceanic air; frequent passage of frontal systems and cyclonic storms; maritime air masses moderate temperature extremes
• Vegetation: Dense evergreen and deciduous forests; includes temperate rainforests in areas with higher precipitation; characteristic species include Douglas fir, redwood, spruce, and beech
• Distinguishing Features: Cloudy, foggy conditions common; low sunshine hours; high relative humidity; minimal temperature extremes compared to continental interiors at similar latitudes
• Contrast with Other Climates: Unlike Mediterranean climate (dry summers), Marine West Coast has year-round precipitation; unlike Humid Subtropical (hot summers), it has cool summers; unlike Equatorial (high temperatures year-round), it has distinct seasonal temperature variation though mild

Detailed Explanation:

Answer: Option 1 — 1 only

Statement 1 is correct because water vapour concentration decreases with altitude as colder air at higher elevations has a reduced capacity to hold moisture. Statement 2 is incorrect because water vapour percentage is maximum near the equator (due to higher temperatures and evaporation) and minimum at the poles, not the other way around.

✅ Statement 1 – Correct: Water vapour is a gas that decreases with altitude because temperature drops at higher elevations, reducing the air's moisture-holding capacity.
❌ Statement 2 – Incorrect: Water vapour percentage is maximum at the equator (warm, high evaporation) and minimum at the poles (cold, low evaporation).

📝 Short Notes: Atmospheric Water Vapour

• Nature: Water vapour is an invisible gas and the most variable component of the atmosphere, ranging from 0% to 4% by volume.
• Vertical Distribution: Concentration decreases rapidly with altitude; about 50% of atmospheric water vapour is found below 2 km, and 90% below 6 km altitude.
• Horizontal Distribution: Maximum concentration occurs in the equatorial region (hot, high evaporation) and decreases towards the poles (cold, low evaporation).
• Temperature Dependency: Warm air can hold more moisture than cold air; at 30°C, air can hold about 4 times more water vapour than at 0°C.
• Role in Weather: Water vapour is crucial for cloud formation, precipitation, and latent heat transfer, which drives weather systems and storms.
• Greenhouse Gas: Water vapour is the most abundant greenhouse gas, contributing significantly to the natural greenhouse effect and Earth's temperature regulation.
• Sources: Primarily from evaporation of ocean water (oceans cover 71% of Earth's surface), followed by lakes, rivers, soil moisture, and transpiration from plants.

Detailed Explanation:

Answer: Option 1 — Both Statement-I and Statement-II are correct and Statement-II explains Statement-I

The troposphere is indeed thicker at the equator (approximately 16-18 km) compared to the poles (approximately 8-10 km) due to differential heating and thermal expansion. The intense solar radiation at the equator generates strong convectional currents that transport heat vertically to great heights, causing the air to expand and thereby increasing the thickness of the troposphere. Statement-II provides a direct causal explanation for the phenomenon described in Statement-I.

✅ Statement-I – Correct: The troposphere's thickness at the equator (16-18 km) is significantly greater than at the poles (8-10 km) due to greater heating and air expansion.
✅ Statement-II – Correct: Strong convectional currents at the equator, driven by intense solar heating, transport heat vertically to great heights, directly causing the increased tropospheric thickness.

📝 Short Notes: Troposphere and Atmospheric Structure

• Troposphere Characteristics: The lowest layer of the atmosphere where all weather phenomena occur, containing approximately 75% of the atmosphere's mass and almost all water vapor.
• Thickness Variation: Troposphere thickness varies from 8-10 km at the poles to 16-18 km at the equator, averaging about 13 km globally.
• Equatorial Heating: The equator receives more direct and intense solar radiation throughout the year, causing maximum surface heating and air temperature.
• Convectional Currents: Hot air at the equator rises rapidly due to lower density, creating strong vertical currents that transport heat to higher altitudes (part of the Hadley Cell circulation).
• Thermal Expansion: Warmer air occupies greater volume, causing the atmospheric column to expand vertically at the equator, increasing tropospheric height.
• Temperature Lapse Rate: Temperature decreases with altitude in the troposphere at an average rate of 6.5°C per kilometer until reaching the tropopause.
• Tropopause Variation: The tropopause (boundary between troposphere and stratosphere) is higher and colder at the equator (-80°C) than at the poles (-45°C).

Detailed Explanation:

Answer: Option 4 — Statement-I is incorrect but Statement-II is correct

Statement-I claims that temperature contrast between continents and oceans is greater during summer, which is incorrect. In reality, the contrast is greater during winter because land cools rapidly while oceans retain heat, creating a large temperature difference. During summer, both land and ocean warm up, resulting in a smaller temperature contrast.

❌ Statement-I – Incorrect: Temperature contrast between continents and oceans is greater during winter, not summer, as land loses heat quickly in winter while oceans remain relatively warm.

✅ Statement-II – Correct: Water has a higher specific heat capacity than land, meaning it heats up and cools down more slowly, which explains the moderating influence of oceans on temperature.

📝 Short Notes: Differential Heating of Land and Water

• Specific Heat Capacity: Water has about 4 times the specific heat capacity of land, requiring more energy to change its temperature.
• Summer Pattern: Land heats up quickly and becomes warmer than oceans; temperature difference is moderate (typically 5-10°C).
• Winter Pattern: Land cools rapidly and becomes much colder than oceans; temperature difference is large (can exceed 20-30°C), creating maximum contrast.
• Continentality Effect: Areas far from oceans experience extreme temperature variations due to lack of moderating oceanic influence.
• Maritime Climate: Coastal regions have smaller annual temperature ranges due to oceanic influence.
• Monsoon Formation: Differential heating between land and ocean drives monsoon circulation, especially prominent in winter when contrast is maximum.

Detailed Explanation:

Answer: Option 3 — Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.

Water vapour, being a major greenhouse gas concentrated in the troposphere (lower atmosphere), effectively absorbs infrared radiation emitted by the Earth's surface. This absorption is crucial for the greenhouse effect and temperature regulation on Earth.

❌ Option 1 – Incorrect: The equator receives approximately 2.5 times more insolation than the poles, not 10 times. The difference is due to the angle of incidence and Earth's curvature.

❌ Option 2 – Incorrect: Visible light constitutes about 44% of insolation, infrared about 49%, and ultraviolet about 7%. Infrared does not constitute two-thirds of insolation.

✅ Option 3 – Correct: Water vapour in the lower atmosphere (troposphere) is the primary absorber of infrared radiation, making this the correct statement.

❌ Option 4 – Incorrect: Infrared waves have wavelengths longer than visible light (700 nm to 1 mm) and are not part of the visible spectrum (380-700 nm).

📝 Short Notes: Solar Radiation and Atmosphere

• Insolation Composition: Ultraviolet rays (7%), Visible light (44%), Infrared rays (49%)
• Atmospheric Absorption: Ozone absorbs most UV radiation in the stratosphere; water vapour and CO₂ absorb infrared radiation in the troposphere
• Latitude Variation: Equatorial regions receive 2-2.5 times more insolation than polar regions due to angle of incidence
• Greenhouse Gases: Water vapour (most significant), CO₂, methane, and ozone absorb outgoing terrestrial infrared radiation
• Electromagnetic Spectrum: Visible light (380-700 nm), Infrared (700 nm-1 mm), shorter wavelengths have higher energy

Detailed Explanation:

Answer: Option 4 — Neither 1 nor 2

Both statements are incorrect regarding the radiative effects of high and low clouds. High clouds are thin and primarily trap outgoing infrared radiation, causing warming rather than cooling. Low clouds are thick and primarily reflect incoming solar radiation, causing cooling rather than warming.

❌ Statement 1 – Incorrect: High clouds (like cirrus) are thin and allow solar radiation to pass through but trap outgoing infrared radiation from Earth's surface, thereby warming the surface rather than cooling it.

❌ Statement 2 – Incorrect: Low clouds (like stratus and cumulus) are thick and excellent reflectors of solar radiation, reflecting sunlight back to space, thereby causing a cooling effect rather than a warming effect.

📝 Short Notes: Cloud Types and Their Radiative Effects

• Albedo Effect: Low clouds have high albedo (30-60%), reflecting significant solar radiation, while high clouds have low albedo (10-30%).
• Greenhouse Effect: High clouds trap infrared radiation more effectively due to their cold tops and thin structure, enhancing the greenhouse effect.
• Climate Impact: Overall, low clouds have a net cooling effect on climate (~-30 W/m²), while high clouds have a net warming effect (~+10 W/m²).
• Cloud Feedback: Changes in cloud cover and types are among the largest uncertainties in climate change predictions.

Detailed Explanation:

Answer: Option 3 — 2 only

Only statement 2 is correct. Jet streams occur in both hemispheres, not just the Northern Hemisphere. The eye of a cyclone is warmer than its surroundings due to adiabatic heating from subsiding air, not cooler. Only strong tropical cyclones develop a well-defined eye structure.

✅ Statement 1 – Incorrect: Jet streams occur in both the Northern and Southern Hemispheres at high altitudes (10-15 km), not exclusively in the Northern Hemisphere.
✅ Statement 2 – Correct: Only some cyclones, particularly intense tropical cyclones (hurricanes/typhoons), develop a well-defined eye at their center.
❌ Statement 3 – Incorrect: The eye of a cyclone is warmer (2-10°C higher) than the surroundings due to adiabatic heating from subsiding air, not cooler.

📝 Short Notes: Tropical Cyclones and Jet Streams

Detailed Explanation:

Answer: Option 2 — Clouds reflect back the Earth's radiation.

Dewdrops form when the Earth's surface cools below the dew point through radiation heat loss on clear nights. On cloudy nights, clouds act as a blanket by reflecting the Earth's longwave radiation back to the surface, preventing sufficient cooling. This keeps the temperature above the dew point, thus preventing dew formation.

📝 Short Notes: Dew Formation and Radiation

• Dew Formation: Dew forms when water vapor in the air condenses on cool surfaces when the surface temperature drops to or below the dew point.
• Radiational Cooling: On clear nights, the Earth's surface radiates heat (longwave infrared radiation) into space, causing surface temperatures to drop rapidly.
• Role of Clouds: Clouds contain water droplets that absorb and reflect back the Earth's outgoing longwave radiation, acting as an insulating blanket.
• Greenhouse Effect: This reflection by clouds is similar to the greenhouse effect, where certain atmospheric components trap heat near the surface.
• Clear vs Cloudy Nights: Clear nights experience maximum cooling and dew formation, while cloudy nights remain warmer with minimal or no dew.
• Dew Point: The temperature at which air becomes saturated (100% relative humidity) and water vapor begins to condense.

Detailed Explanation:

❌ Statement 1 – Incorrect: Westerlies blow between 30° and 60° latitudes in both hemispheres (i.e., 30°–60° N and 30°–60° S separately), not between 30° N and 60° S as stated.

✅ Statement 2 – Correct: Western disturbances originating over the Mediterranean region bring winter rains to North-Western India and are carried by westerlies.

Detailed Explanation:

✅ Option 2 – Correct: The Inter-Tropical Convergence Zone (ITCZ) remains predominantly north of the equator and seldom migrates into the South Atlantic and South Eastern Pacific regions, depriving them of the necessary low-level atmospheric instability and converging trade winds required for cyclone genesis.

❌ Option 1 – Incorrect: While cold ocean currents (Benguela Current in South Atlantic, Humboldt Current in South Eastern Pacific) do lower temperatures, sea surface temperatures are not the primary limiting factor.

❌ Option 3 – Incorrect: Coriolis force is adequate at these tropical latitudes (generally above 5° latitude) for cyclone formation; it is not too weak.

❌ Option 4 – Incorrect: Presence or absence of land does not prevent cyclone origination over ocean basins; cyclones form over open waters.

Detailed Explanation:

The passage describes the Equatorial climate which experiences convectional rainfall with a predictable daily pattern.

Morning: Clear, bright weather with sea breeze due to proximity to oceans and low pressure.

Afternoon: Intense solar heating causes rapid convection currents, forming cumulonimbus clouds that produce short, intense thunderstorms with lightning.

Evening: Rain clears quickly, returning to calm conditions — this daily convectional cycle repeats year-round in regions near the Equator (between 5°N and 5°S), unlike Savannah (distinct wet/dry seasons), Monsoon (seasonal wind reversal), or Mediterranean (dry summers, wet winters).