UPSC CSE Prelims
Atmosphere Previous Year Questions (PYQs)

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 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 — 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 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 – Correct: The atmosphere is transparent to incoming solar radiation but is primarily heated from below by long-wave terrestrial radiation emitted from Earth's surface, making lower layers warmer.

❌ Statement 2 – Incorrect: Water vapor and moisture are concentrated in the lower atmosphere (troposphere), not the upper atmosphere, which is significantly drier.

✅ Statement 3 – Correct: At higher altitudes, air pressure decreases making air less dense; rising air expands and undergoes adiabatic cooling, and thinner air has lower capacity to absorb and retain heat.