Looking for complete, textbook-verified solutions for SEBA Class 9 Social Science Geography Chapter 2 (Atmosphere: Structure, Air Pressure and Wind System / বায়ুমণ্ডল: গঠন, বায়ুৰ চাপ আৰু বায়ু প্ৰবাহ) for the 2026–27 academic session? Access step-by-step solutions to all official textbook exercises—including Very Short Answer (VSA), Short Answer (SA), and Long Answer (LA) questions. Master high-scoring answers covering the gaseous composition and layered structure of the atmosphere (Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere); the concept of normal lapse rate and the protective ozone layer; factors influencing atmospheric pressure (altitude, temperature, and moisture); distribution of the global air pressure belts (Equatorial Low, Subtropical Highs, Subpolar Lows, and Polar Highs); Ferrel’s Law and the Coriolis force; classification of winds into planetary winds (Trade Winds, Westerlies, Polar Easterlies), periodic winds (monsoons, land and sea breezes), and local winds; the formation and characteristics of cyclones and anticyclones; and point-wise answers formatted strictly according to the Assam Board (SEBA) examination pattern.
Q.1. What is atmosphere? Write in brief with data about its structure.
Answer: Atmosphere is the continuous, gaseous envelope of air that surrounds the earth from its surface up to around 10,000 km altitude, held in place by gravitational pull.
- Gaseous Data: Nitrogen accounts for 78.08%, Oxygen makes up 20.94%, Argon is 0.93%, Carbon Dioxide is 0.036%, and trace gases (neon, helium, methane, ozone, krypton, hydrogen, xenon) comprise 0.014%.
- Mass Concentration: About 99% of atmospheric mass lies within the first 32 km.
- Thermal Structure: From bottom to top, it is structured into the Troposphere (up to 12 km), Stratosphere (up to 50 km), Mesosphere (up to 80 km), Thermosphere (up to 400 km), and Exosphere (up to 10,000 km).
Q.2. What are the main layers of the atmosphere based on chemical composition of the gases? Write briefly about the characteristics of these layers.
Answer: Based on chemical composition, the atmosphere is divided into two primary divisions:
- Homosphere:
- Extends from the ground surface up to roughly 80 km.
- The gaseous proportion remains practically identical throughout this zone.
- Contains virtually all atmospheric water vapour, dust particles, and the bulk of heavier gases like nitrogen, oxygen, and carbon dioxide.
- Heterosphere:
- Extends above 80 km up to around 10,000 km.
- The chemical composition varies with height, stratifying into four atomic layers based on molecular weight: Nitrogen layer (80–200 km), Oxygen layer (200–1,125 km), Helium layer (1,125–3,540 km), and Hydrogen layer (3,540–10,000 km).
Q.3. Write with diagram about the layers of the atmosphere based on the variations in altitude and temperature and write the characteristics of each layer in brief.
Answer: Textbook Structure & Temperature Profile:
- Troposphere (0–12 km): Contains 75% of atmospheric air, almost all dust and water vapour; site of all clouds, storms, and precipitation; experiences a temperature lapse rate of 6.5°C per km descent.
- Stratosphere (12–50 km): Weather-free layer preferred by aircraft; houses the ozone band (40–50 km) that absorbs solar UV rays, heating the air up to 0°C at the stratopause.
- Mesosphere (50–80 km): Temperature steadily drops with height, reaching the lowest atmospheric value of -100°C at the mesopause.
- Thermosphere (80–400 km): Temperature increases dramatically up to 1650°C; contains electrically charged ions (ionosphere) that reflect radio signals.
- Exosphere (400–10,000 km): Outermost rarefied margin composed of light helium and hydrogen gas; temperatures soar past 5550°C.
Q.4. Discuss with examples the factors responsible for variation in atmospheric pressure.
Answer: Atmospheric pressure varies mainly due to two fundamental factors:
- Air Temperature: Heating expands air, decreasing its molecular density and causing it to rise, creating low pressure (e.g., intense heat creates equatorial low pressure). Cold air contracts, increases in density, and sinks, creating high pressure (e.g., freezing polar high pressure). Furthermore, warm air holds moisture which lowers air density, while cold, dry air is heavy and maintains higher pressure.
- Altitude / Elevation: Higher elevations have shorter overlying columns of air. Density and weight decrease rapidly with elevation at a rate of 1 inch of mercury (about 34 mb) per 900 feet ascent (e.g., mountain peaks experience far lower air pressure than coastal lowlands).
Q.5. With the help of diagram discuss the characteristics of the major pressure belts of the world.
Answer: Textbook Global Pressure Belts:
- Equatorial Low Pressure Belt (10° N to 10° S): Perpetual vertical sun rays cause continuous air heating, expansion, and convection; calm air with virtually no surface horizontal wind earned it the name “Doldrums.”
- Sub-Tropical High Pressure Belts (25° to 35° N & S): Air masses lifted at the equator cool aloft and sink here, converging with descending upper air to create dense, descending high pressure zones termed “Horse Latitudes.”
- Sub-Polar Low Pressure Belts (60° to 70° N & S): The earth’s rotational spin throws and deflects air away from high latitudes toward sub-tropical regions, creating low pressure belts.
- Polar High Pressure Belts (90° N & S): Frigid solar angles keep air intensely cold, dry, and dense year-round, generating permanent high surface pressure.
Class 9 Social Science Geography Chapter 2 Question Answer Assam Board
Q.6. Write the importance of atmosphere towards creation of a favourable physical environment on the earth.
Answer: 1. Sustains Respiration and Photosynthesis: Supplies vital oxygen for animal and human life, and carbon dioxide for green plant photosynthesis.
2. Regulates Planetary Temperature: Carbon dioxide and water vapour absorb outbound terrestrial radiation, keeping the mean surface temperature at a congenial 15°C and averting extreme planetary freezing.
3. Radiation Shield: The stratospheric ozone layer absorbs lethal solar ultraviolet radiation, shielding living tissue from harm.
4. Drives the Hydrological Cycle: Holds moisture and suspended dust particles that condense into clouds to generate rainfall and sustain the biosphere.
5. Distributes Heat: Atmospheric wind systems constantly circulate surplus thermal energy and moisture from equatorial zones toward colder polar regions.
Q.7. What is wind? Discuss the factors of origin of wind.
Answer: Wind is the horizontal movement of atmospheric air parallel to the earth’s surface.
- Factors of Origin:
- Unequal Heating: Non-uniform reception of solar radiation across different latitudes creates uneven air temperatures.
- Pressure Differences: Unequal temperatures cause regional variations in air density, producing low pressure where air is hot and high pressure where air is cold.
- Pressure Gradient: Air naturally flows from areas of high pressure toward areas of low pressure to re-establish atmospheric equilibrium.
Q.8. What is the most important reason for the origin of wind? Briefly discuss the factors determining velocity and direction of wind.
Answer: The primary, most essential reason for the origin of wind is the atmospheric pressure difference between two regions.
- Factors Governing Velocity and Direction:
- Pressure Gradient Force: The steepness of pressure change over distance determines the wind’s initial speed and direction; steeper gradients generate higher wind velocity.
- Coriolis Force: Caused by planetary rotation; it deflects wind paths to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
- Frictional Force: Resistance created by rugged land surfaces, vegetation, and hills reduces wind speed and alters its path near the ground.
Q.9. What do you mean by ‘Coriolis Force’? With the help of diagram briefly describe its contribution in determining the direction of wind.
Answer: The Coriolis force is an apparent deflecting force produced entirely by the rotational movement of the earth on its axis.
Wind Deflection Diagram:
- Role in Wind Direction: It deflects winds to the right of their path in the Northern Hemisphere (clockwise) and to the left in the Southern Hemisphere (anticlockwise), as stated by Ferrel’s Law. It is zero at the equator and increases progressively with latitude, reaching its maximum strength at the poles.
Q.10. What is meant by ‘Pressure Gradient Force’? What is its contribution in air movement?
Answer: Pressure Gradient Force is the rate of change of atmospheric pressure per unit of horizontal distance between high-pressure and low-pressure zones.
- Contribution: It initiates the motion of air and dictates its initial velocity. When isobars are spaced tightly together, the pressure gradient is steep and produces high-velocity winds; when isobars are far apart, the gradient is gentle, producing light breezes.
SEBA Class 9 Geography Chapter 2 Solutions
Q.11. Briefly discuss the contribution of the earth’s gravitational force in air movement.
Answer: 1. Holds the gaseous mass securely to the globe, preventing it from escaping into space.
2. Compresses air molecules close to sea level, making lower air layers denser and heavier than upper layers.
3. Drives convective circulation by pulling down cold, dense air, which displaces and forces warm, buoyant air to rise vertically.
Q.12. What do you mean by naming of wind? How the wind blowing over a place is named?
Answer: Naming of wind refers to the established geographic convention used to identify a wind’s identity and direction.
- Rule of Naming: A wind is invariably named after the specific direction from which it blows, not the direction it travels toward. For example, a wind originating from the east and moving west is designated an Easterly wind, while a wind blowing out of the southwest is called a South-Westerly wind.
Q.13. How the velocity of wind is determined? What are the units of wind velocity?
Answer: 1. Determination: Wind velocity is measured with an Anemometer, or estimated visually on land and sea using the Beaufort Scale based on observed physical effects.
2. Units of Velocity: Commonly recorded in Knots (nautical miles per hour), Kilometres per hour (km/h), or
3. Miles per hour (mph).
Q.14. How the classification of wind is done? Briefly discuss with examples.
Answer: Wind systems are classified based on their scale, permanence, and geographical extent into three broad categories:
- Primary (Planetary) Circulation: Permanent, global wind belts driven by global pressure cells (e.g., Trade Winds, Westerlies, Polar Easterlies).
- Secondary Circulation: Regional, seasonal, or cyclical wind patterns created by land-sea contrasts and seasonal pressure shifts (e.g., Monsoons, Cyclones, Anticyclones, Air Masses).
- Tertiary (Local) Circulation: Localized winds confined to small areas caused by local topography and diurnal heating cycles (e.g., Land and Sea breezes, Mountain and Valley breezes, Loo, Chinook).
Q.15. What do you mean by primary circulation of wind? Show distribution of primary circulations in a diagram. Mention its important characteristics.
Answer: Primary circulation refers to the continuous, planet-wide wind systems that blow throughout the year in fixed latitudinal belts, governed directly by the earth’s global pressure belts.
Distribution Diagram:
- Characteristics:
- Blow consistently in predictable global directions across vast oceanic and continental expanses.
- Main components include the Trade Winds (blowing toward the equator), Westerlies (blowing poleward toward sub-polar zones), and Polar Easterlies.
- They transfer vast amounts of heat and moisture between tropical and polar regions.
Assam Board Class 9 Geography Chapter 2 Solved Textbook Solutions PDF
Q.16. What is secondary circulation of wind? Discuss its contribution in determining the climate of a place with examples.
Answer: Secondary circulation comprises regional, seasonal, and periodical wind systems generated by uneven heating of land and sea masses, topographical barriers, and shifting pressure centres.
- Climatic Contribution:
- Monsoons: Seasonal reversal of winds brings widespread torrential rains to South Asia, determining agricultural seasons and climate.
- Cyclones: Depressions moving inland produce violent gales, thick cloud covers, and extreme precipitation, drastically altering regional weather.
- Anticyclones: Divergent high pressure brings dry, stable air, calm conditions, and clear weather (cold waves in winter, hot waves in summer).
Q.17. What is local wind? Briefly discuss with examples how local wind determines the weather condition of a place.
Answer: Local winds are winds confined to small, specific geographic districts, produced by local differences in daily temperature, topography, and surface heating.
- Impact on Weather:
- Land and Sea Breezes: Moderate diurnal temperatures along coastal fringes, providing cooling onshore breezes during hot afternoons.
- Mountain and Valley Breezes: Modulate daily temperatures and drainage of cold air in mountain valleys.
- Hot/Cold Local Winds: The Loo brings scorching summer afternoon conditions across northern India; the Chinook melts snow rapidly on North American mountain slopes.
Q.18. What is a cyclone? What are its types? Mention briefly how it influences the climate of a place?
Answer: A cyclone is a weather system with an intense low-pressure centre surrounded by progressively higher pressure outward, causing winds to spiral inward (anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere).
- Types: Tropical Cyclones and Temperate (Extra-Tropical) Cyclones.
- Influence: Induces sudden atmospheric instability, heavy cloud cover, violent squalls, severe rainfall, hail, and maritime storm surges, often followed by sudden drops in temperature.
Q.19. What is an air mass? How it is classified? What is its contribution in determining the climate of a place?
Answer: An air mass is an extensive body of air spanning thousands of square kilometres with generally uniform temperature, pressure, and humidity characteristics acquired from its underlying source region.
- Classification:
- Tropical Continental (cT) – Warm and dry
- Tropical Maritime (mT) – Warm and humid
- Polar Continental (cP) – Cold and dry
- Polar Maritime (mP) – Cold and humid
- Contribution: As an air mass migrates, it carries the thermal and moisture traits of its origin region, dramatically modifying local temperatures, humidity, cloudiness, and rainfall along its path.
Q.20. What do you mean by ‘Front’? How it is formed? Briefly discuss the relationship between climate and front.
Answer: A Front is a sloping boundary surface or line of discontinuity separating two approaching air masses that possess sharply contrasting temperatures, densities, and moisture levels.
- Formation: Requires two conditions: (1) marked contrasts in physical properties (one air mass distinctly colder/heavier than the other), and (2) convergent horizontal wind movement forcing them together without immediate mixing.
- Relationship with Climate: Frontal convergence forces lighter warm air to rise rapidly over cold air, resulting in adiabatic cooling, condensation, cloud bands, thunderstorms, and frontal precipitation.
Atmosphere Structure Air Pressure and Wind System Textual Questions SEBA 2026-27
Q.21. What do you mean by ‘Monsoon Wind’? How it is formed? In which areas of the earth its impact is quite distinct?
Answer: Monsoon wind is a periodic, seasonal wind system characterised by a complete reversal of prevailing wind direction between summer and winter.
- Formation: Generated by differential heating and cooling rates of extensive continental landmasses compared to adjacent oceans, producing deep low pressure over land in summer (drawing moist oceanic winds) and high pressure over land in winter (pushing dry winds seaward).
- Distinct Areas: South Asia (most notably India), Southeast Asia, and parts of East Asia.
Q.22. Write with reasons :
Answer: (a) Why do all weather phenomena occur mainly in troposphere?
Almost all atmospheric water vapour, dust particles, and three-fourths of total gaseous mass are concentrated within the troposphere, providing the physical constituents required for evaporation, condensation, cloud development, storms, and precipitation.
(b) What would be the consequence of the increase in the amount of carbon dioxide?
Excess carbon dioxide traps more reflected longwave terrestrial radiation, intensifying the greenhouse effect, raising average global temperatures (Global Warming), and disrupting worldwide weather regimes.
(c) What is the reason behind the prevalence of high pressure of dry air than moist air?
Dry air is largely composed of heavier nitrogen and oxygen molecules, whereas moist air contains water vapour, which has a lower molecular weight than dry air. Consequently, dry air has higher density and exerts greater pressure.
(d) How does the ozone layer help the living-beings?
The stratospheric ozone molecules absorb solar ultraviolet (UV) radiation, preventing these rays from reaching the surface where they could cause skin cancers, cataracts, and genetic harm to life.
(e) Why is the air pressure highest at the sea level?
Sea level lies at the base of the entire atmospheric column, bearing the full weight of overlying gases, and the earth’s gravitational attraction compresses air molecules most densely near the surface.
(f) What is the reason behind variation in vertical extent of the troposphere in the polar region and equatorial region?
Intense equatorial solar heating generates strong vertical convection currents that push the tropopause upward to 16 km, whereas severe cold and sinking dense air at the poles compress the troposphere down to roughly 8 km.
(g) Why is horizontal movement of wind parallel to the earth’s surface not felt in the equatorial low pressure belt?
Extreme solar heating warms the equatorial air continuously, causing it to expand and ascend vertically as convection currents, leaving the horizontal surface air calm and stagnant (Doldrums).
Q.23. Give short answer to the following questions :
Answer: (a) What is the vertical extent of the atmosphere?
Around 10,000 kilometres from the earth’s surface.
(b) Up to what altitude from the earth’s surface does chemical composition of gases remain almost same?
Up to an altitude of approximately 80 kilometres (the Homosphere).
(c) What is the name of the boundaryline between homosphere and heterosphere?
The boundary is called the Mesopause (located at about 80 km altitude).
(d) What is Ferrell’s Law?
Ferrel’s Law states that moving air is deflected to its right in the Northern Hemisphere and to its left in the Southern Hemisphere due to the earth’s rotation (Coriolis force).
(e) What is Beaufort Scale? Write briefly about the utility of this scale with example.
The Beaufort Scale is an empirical system ranging from 0 (calm) to 12 (hurricane) designed by Sir Francis Beaufort to estimate wind speed from observable physical effects.
Example: Scale 0 denotes calm air where smoke rises straight up, while Scale 8 indicates a gale where twigs break off trees and walking against the wind is difficult.
(f) Write in brief about Lapse Rate.
Lapse Rate is the rate at which atmospheric temperature drops with increasing elevation in the troposphere, averaging 6.5°C per kilometre of ascent.
Q.24. Write short note :
Answer: (a) Homosphere: The lower atmospheric division extending from ground level up to 80 km. The proportions of main constituent gases (nitrogen, oxygen, argon, carbon dioxide) remain constant throughout due to continuous convective mixing.
(b) Heterosphere: The upper atmospheric realm extending from 80 km to 10,000 km, where gases settle into distinct stratified layers based on atomic weights (nitrogen, oxygen, helium, and hydrogen).
(c) Air temperature, earth’s surface height and air pressure relationship: Temperature and surface elevation are inversely related to air pressure. Warm air expands and creates low pressure, whereas cold air sinks and generates high pressure; similarly, higher elevations reduce the atmospheric column, lowering air pressure by 34 mb per 900 ft.
(d) Polar High Pressure Belt: A permanent high-pressure zone located around both polar caps, created by year-round sub-zero temperatures, intense air contraction, high density, and lack of moisture.
(e) Wind system: The broad atmospheric mechanism involving global, regional, and local air currents that move heat, moisture, and momentum across the planet to restore thermal balance.
(f) Pressure Gradient: The rate of atmospheric pressure decline over a horizontal distance between two points, acting as the driving force that initiates wind.
(g) Trade wind: Permanent planetary winds blowing steadily from the sub-tropical high-pressure belts towards the equatorial low-pressure belt, blowing from the northeast in the Northern Hemisphere and southeast in the Southern Hemisphere.
(h) Horse latitude: Calm, subtropical high-pressure zones located between 25° and 35° latitudes in both hemispheres, historically named because becalmed Spanish sailors threw horses into the sea to preserve water.
(i) Roaring Forties: Strong, uninterrupted Westerly winds blowing across the open oceans of the Southern Hemisphere between 40° S and 50° S latitudes.
(j) Anticyclone: A high-pressure system with highest pressure at its centre, surrounded by divergent, outward-blowing winds accompanied by clear, dry weather.
(k) Jet Stream: Fast-flowing, narrow air currents blowing from west to east at speeds up to 640 km/h in the upper levels of the troposphere.
(l) Isobar: An imaginary line drawn on a meteorological map connecting geographical points that record equal atmospheric pressure reduced to sea level.
(m) Tropical Cyclone: A violent, circular storm originating over warm tropical oceans characterized by a deep low-pressure centre, steep pressure gradients, and destructive gale winds and rainfall.
Q.25. Write the differences:
(a) Horizontal Wind vs. Vertical Wind
Answer:
| Feature / Aspect | Horizontal Wind | Vertical Wind (Air Current) |
| Direction of Motion | Moves parallel to the earth’s surface. | Moves vertically upward or downward. |
| Primary Cause | Caused mainly by horizontal pressure differences. | Caused by heating (convection) or cooling of air. |
| Volume Involved | Involves a vast volume of atmospheric air. | Involves a comparatively smaller volume of air. |
| Main Function | Transports heat, moisture, and weather conditions across regions. | Responsible for vertical cloud formation, instability, and convection. |
(b) Wind vs. Air Mass
Answer:
| Feature / Aspect | Wind | Air Mass |
| Definition | Air in active motion moving horizontally across the earth’s surface. | An immense body of air covering thousands of kilometres. |
| Internal Properties | Temperature and moisture change rapidly along its path. | Exhibits homogeneous (uniform) temperature and humidity throughout. |
| Primary Driver | Driven directly by horizontal pressure gradient forces. | Formed when air remains stagnant over a uniform source region. |
| Lifespan / Nature | Dynamic, localized, or regional air flow. | Broad, semi-permanent body that modifies weather where it migrates. |
(c) Tropical Cyclone vs. Extra-Tropical Cyclone
Answer:
| Feature / Aspect | Tropical Cyclone | Extra-Tropical (Temperate) Cyclone |
| Region of Origin | Develops exclusively over warm tropical oceans. | Forms in mid-to-high latitudes (temperate zones). |
| Frontal System | Does not have any front system. | Formed by the convergence of contrasting air masses along fronts. |
| Isobar Shape & Gradient | Nearly circular with very steep pressure gradients. | More elongated/V-shaped with comparatively moderate gradients. |
| Impact & Intensity | Extremely violent, causing catastrophic winds and coastal destruction. | Covers a much larger area with moderate to heavy rain, moving southwest to northeast. |
(d) Sea Breeze vs. Land Breeze
Answer:
| Feature / Aspect | Sea Breeze | Land Breeze |
| Time of Occurrence | Blows during the daytime. | Blows during the nighttime. |
| Direction of Blow | Blows onshore from the sea toward the land. | Blows offshore from the land toward the sea. |
| Thermal Cause | Land heats up faster than water, creating lower pressure over land. | Land cools down faster than water, creating higher pressure over land. |
| Effect on Coast | Brings cool, refreshing maritime air to the warm coastline. | Carries drier, cooler air from the land out to the sea. |
(e) Mountain Wind vs. Valley Wind
Answer:
| Feature / Aspect | Mountain Wind (Katabatic) | Valley Wind (Anabatic) |
| Time of Occurrence | Blows during the nighttime. | Blows during the daytime. |
| Direction of Flow | Blows downhill from high mountain slopes into the valley. | Blows uphill from the valley floor along the mountain slopes. |
| Thermal Mechanism | Mountain slopes cool rapidly by radiation; cold, dense air drains downward. | Mountain slopes heat up quickly in the sun, causing warm air to rise up the slopes. |
| Characteristics | Cold, heavy, and settles as a dense layer in valley basins. | Warm, buoyant, and often produces afternoon cumulus clouds near peaks. |
(f) Cyclone vs. Anticyclone
Answer:
| Feature / Aspect | Cyclone | Anticyclone |
| Centre Pressure | Low pressure at the centre with high pressure outside. | High pressure at the centre with low pressure outside. |
| Wind Movement | Convergent (blows inward towards the centre). | Divergent (blows outward away from the centre). |
| Hemispheric Rotation | Anticlockwise in Northern Hemisphere; Clockwise in Southern Hemisphere. | Clockwise in Northern Hemisphere; Anticlockwise in Southern Hemisphere. |
| Associated Weather | Overcast skies, violent winds, storms, and heavy precipitation. | Calm air, clear skies, dry conditions, and fair weather. |
(g) Cold Front vs. Warm Front
Answer:
| Feature / Aspect | Cold Front | Warm Front |
| Advancing Air Mass | Cold air mass actively moves forward. | Warm air mass actively moves forward. |
| Air Interaction | Dense cold air undercuts and forcibly lifts the warm air mass. | Lighter warm air gradually ascends over the wedge of retreating cold air. |
| Slope & Speed | Steep slope and moves relatively fast. | Gentle slope and moves relatively slowly. |
| Weather & Clouds | Vertical clouds (cumulonimbus), sharp showers, and sudden thunderstorms. | Extensive layered clouds (stratus), overcast skies, and steady, prolonged rain. |
(h) Cold Wave vs. Hot Wave
Answer:
| Feature / Aspect | Cold Wave | Hot Wave |
| Season of Occurrence | Occurs during the winter season. | Occurs during the summer season. |
| Source / Driver | Sourced from high-latitude cold-core anticyclones. | Sourced from warm, semi-permanent subtropical anticyclones. |
| Nature of Air | Inflow of extremely cold, freezing, dense air (e.g., Blizzard conditions). | Inflow of excessively dry, scorching, and intensely heated air. |
| Temperature Effect | Causes temperatures to drop sharply below normal levels. | Causes temperatures to surge abnormally high above seasonal averages. |
(i) Troposphere vs. Stratosphere
Answer:
| Feature / Aspect | Troposphere | Stratosphere |
| Vertical Extent | Lowermost layer extending from ground level up to ~12 km (8 km poles, 16 km equator). | Extends from the tropopause (12 km) up to ~50 km. |
| Temperature Trend | Temperature decreases with altitude at the lapse rate of 6.5°C per km. | Temperature increases with altitude from -60°C up to around 0°C. |
| Constituents | Contains ~75% of atmospheric gases and almost all water vapour and dust. | Very dry with negligible moisture and dust; contains the protective Ozone layer. |
| Weather Activity | Venue of all active weather (clouds, rainfall, cyclones, storms). | Virtually free from clouds and storm activity; preferred zone for jet flights. |
Q.26. Find out the correct answer :
(a) Where is Ozone layer located?
(1) Troposphere
(2) Stratosphere
(3) Mesosphere
(4) Thermosphere
Answer: (2) Stratosphere
Q.26. (b) What is amount of oxygen in the atmosphere in terms of volume?
(1) 20.94%
(2) 29.01%
(3) 32.47%
(4) 78.08%
Answer: (1) 20.94%
Q.26. (c) The most important reason for air movement is:
(1) Humidity difference
(2) Pressure difference
(3) Gravitational Force
(4) Centrifugal Force
Answer: (2) Pressure difference
Q.26. (d) The instrument used for determination of wind velocity is:
(1) Wind vane
(2) Anemometer
(3) Beaufort scale
(4) Hydrometer
Answer: (2) Anemometer
Q.26. (e) The unit of wind velocity is:
(1) Knot
(2) Millibar
(3) Percentage
(4) Degree
Answer: (1) Knot
Q.26. (f) Monsoon wind belongs to which of the following class?
(1) Local winds
(2) Primary circulation
(3) Secondary circulation
(4) Permanent wind
Answer: (3) Secondary circulation
Q.26. (g) The name of the cyclone formed in the coastal region of the Pacific Ocean in the east is:
(1) Cyclone
(2) Hurricane
(3) Willy-Willy
(4) Typhoon
Answer: (4) Typhoon
🌤️ Class 9 Social Science Solutions Notice: This page features the complete SEBA Class 9 Social Science (Geography Section) Chapter 2: Atmosphere: Structure, Air Pressure and Wind System Official Textual Questions and Answers updated for the 2026–27 academic session. Formatted according to the official Assam Board evaluation marking scheme, every core climatology topic—from the vertical layers of the atmosphere, thermal lapse rate, and global pressure belts to planetary winds, Coriolis deflection, and cyclonic systems—is presented with point-wise clarity, schematic descriptions, and structured subheadings to help you secure full marks in your school assessments, half-yearly, and final annual examinations.
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