Class 11 Geography Notes Chapter 10 (Atmospheric circulation and weather systems) – Fundamental of Physical Geography Book

Detailed Notes with MCQs of Chapter 10: Atmospheric Circulation and Weather Systems from our Physical Geography textbook. This chapter is fundamental to understanding the 'why' behind our daily weather and larger climate patterns. It's a crucial topic for your government exam preparations, so pay close attention.

We'll break down the complex movements of air in our atmosphere and how these movements create the weather systems we experience.

Chapter 10: Atmospheric Circulation and Weather Systems - Detailed Notes

1. Atmospheric Pressure

• Definition: The weight of the column of air contained in a unit area from the mean sea level to the top of the atmosphere. Measured in millibars (mb) or Pascals (Pa).
• Measurement: Using a Barometer (Mercury Barometer or Aneroid Barometer).
• Isobars: Lines on a map connecting places having equal atmospheric pressure at a given level (usually sea level). The spacing of isobars indicates the pressure gradient. Close isobars = Steep pressure gradient = Strong winds. Widely spaced isobars = Weak pressure gradient = Light winds.
• Vertical Variation: Pressure decreases rapidly with increasing altitude. Why? Because the density of air decreases upwards, meaning less air mass is pressing down from above.
• Horizontal Distribution: Varies across the globe due to differential heating. This variation is the primary cause of air motion (wind).
  • Factors Affecting Horizontal Distribution:
    • Temperature: Warm air is less dense and exerts lower pressure. Cold air is denser and exerts higher pressure. (Inverse relationship)
    • Altitude: Higher altitudes generally have lower pressure. Pressure readings are adjusted to sea level for comparison on weather maps.
    • Water Vapour: Moist air is less dense than dry air (molecular weight of H₂O is less than N₂ or O₂), so higher water vapour content can lead to lower pressure, temperature remaining constant.
    • Earth's Rotation: Influences wind direction (Coriolis effect), indirectly affecting pressure distribution over time.

2. Forces Affecting Wind Velocity and Direction

• Wind: Horizontal movement of air from high pressure to low pressure. Vertical air movement is called 'air current'.
• Key Forces:
  • Pressure Gradient Force (PGF): The primary force driving wind. It's caused by differences in atmospheric pressure over distance. Air moves from High Pressure (HP) to Low Pressure (LP). The steeper the gradient (closer isobars), the stronger the PGF and the faster the wind. PGF acts perpendicular to isobars.
  • Coriolis Force: An apparent force caused by the Earth's rotation. It deflects moving objects (like wind and ocean currents) to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
    • Its effect is zero at the Equator and maximum at the Poles.
    • It acts perpendicular to the wind direction (and PGF).
    • Its magnitude increases with wind velocity.
    • It only affects wind direction, not speed.
    • Geostrophic Wind: When isobars are straight and friction is negligible (upper atmosphere), PGF and Coriolis force balance each other, resulting in wind blowing parallel to the isobars.
  • Frictional Force: Acts opposite to the wind direction, primarily near the Earth's surface (up to ~1-3 km). It slows down the wind speed. This reduction in speed also reduces the Coriolis effect, causing winds near the surface to cross isobars at an angle (towards the low pressure). Friction is negligible over sea surfaces compared to land surfaces with irregular terrain.

3. General Circulation of the Atmosphere: Pressure Belts and Winds

• The global pattern of pressure and winds is driven by differential heating between the equator and poles, modified by Earth's rotation.
• Pressure Belts (Idealized):
  • Equatorial Low Pressure Belt (ITCZ - Inter Tropical Convergence Zone / Doldrums): (Approx. 5°N - 5°S). Intense heating leads to rising air (convection). Zone of calm or light variable winds. Thermally induced.
  • Sub-Tropical High Pressure Belts (Horse Latitudes): (Approx. 30°N and 30°S). Air rising at the equator cools, becomes heavy, and sinks here. Zone of descending air, leading to calm conditions and dry weather (major deserts located here). Dynamically induced.
  • Sub-Polar Low Pressure Belts: (Approx. 60°N and 60°S). Air moving poleward from Sub-Tropical Highs (Westerlies) meets cold air moving equatorward from Polar Highs (Polar Easterlies). The warmer, lighter air is forced to rise. Dynamically induced.
  • Polar High Pressure Belts: (Approx. 90°N and 90°S). Intense cold leads to sinking dense air. Thermally induced.
• Planetary Winds (Prevailing Winds): Winds blowing consistently throughout the year in particular directions over large areas.
  • Trade Winds (Tropical Easterlies): Blow from Sub-Tropical Highs towards the Equatorial Low. Deflected by Coriolis force, becoming North-East Trades (NH) and South-East Trades (SH). Very consistent.
  • Westerlies: Blow from Sub-Tropical Highs towards the Sub-Polar Lows. Deflected by Coriolis force, becoming South-Westerlies (NH) and North-Westerlies (SH). Stronger and more variable than Trade Winds, especially in SH ("Roaring Forties", "Furious Fifties", "Shrieking Sixties").
  • Polar Easterlies: Blow from Polar Highs towards the Sub-Polar Lows. Deflected by Coriolis force, becoming North-East winds (NH) and South-East winds (SH). Very cold and dry.
• Tri-Cellular Meridional Circulation: Explains the global circulation pattern.
  • Hadley Cell: (0°-30° N/S) Air rises at the Equator, flows poleward aloft, sinks at 30°, returns equatorward near the surface (Trade Winds). Thermally direct cell.
  • Ferrel Cell: (30°-60° N/S) Air sinks at 30°, flows poleward near the surface (Westerlies), rises at 60°, returns equatorward aloft. Indirectly driven (thermally indirect) by the Hadley and Polar cells.
  • Polar Cell: (60°-90° N/S) Air sinks at the Poles, flows equatorward near the surface (Polar Easterlies), rises at 60°, returns poleward aloft. Thermally direct cell.
• Seasonal Shifting: Pressure belts and wind systems shift North and South following the apparent movement of the Sun (roughly 5°-10° latitude shift). This impacts regional climates (e.g., Mediterranean climate receiving winter rain from shifting Westerlies).

4. Local Winds

• Winds produced by local variations in temperature and pressure, affecting smaller areas for shorter durations.
• Land and Sea Breezes: Coastal phenomena.
  • Sea Breeze: During the day, land heats faster than the sea. Low pressure develops over land, high pressure over the sea. Wind blows from sea to land.
  • Land Breeze: At night, land cools faster than the sea. High pressure develops over land, low pressure over the sea. Wind blows from land to sea.
• Mountain and Valley Breezes:
  • Valley Breeze (Anabatic): During the day, mountain slopes heat up faster than the valley floor. Air on slopes rises, drawing air up from the valley.
  • Mountain Breeze (Katabatic): At night, slopes cool rapidly (radiation). Dense, cold air drains down the slopes into the valley.
• Examples of Specific Local Winds: (Know a few key examples)
  • Loo: Hot, dry wind in Northern India/Pakistan during summer afternoons.
  • Foehn/Chinook: Warm, dry winds descending the leeward side of mountains (Foehn in Alps, Chinook in Rockies). Snow-eater effect.
  • Mistral: Cold, dry wind blowing from the Alps towards the Mediterranean coast of France.
  • Bora: Cold, dry wind blowing from Eastern Europe towards the Adriatic Sea.

5. Air Masses

• Definition: A large body of air having little horizontal variation in temperature and moisture at any given level.
• Source Region: The extensive area over which an air mass originates and acquires its characteristics. Should be a uniform surface (ocean or flat land) with stagnant air conditions.
• Classification: Based on source region (determining temperature and moisture).
  • Based on Latitude (Temperature):
    • P: Polar (Cold)
    • A: Arctic (Extremely Cold)
    • T: Tropical (Warm)
    • E: Equatorial (Warm/Hot)
  • Based on Surface (Moisture):
    • m: Maritime (Moist - originates over ocean)
    • c: Continental (Dry - originates over land)
• Major Types:
  • mT: Maritime Tropical (Warm, Moist) - e.g., Gulf of Mexico, Tropical Atlantic/Pacific
  • mP: Maritime Polar (Cool/Cold, Moist) - e.g., North Atlantic, North Pacific
  • cT: Continental Tropical (Hot, Dry) - e.g., Sahara Desert, SW USA, Thar Desert
  • cP: Continental Polar (Cold, Dry) - e.g., Interior Canada, Siberia
  • cA: Continental Arctic (Very Cold, Dry) - e.g., Arctic regions, Antarctica

6. Fronts

• Definition: The boundary zone separating two different air masses with contrasting characteristics (temperature, density, moisture). Frontogenesis is the formation of a front; Frontolysis is the dissipation.
• Types of Fronts:
  • Cold Front: Cold air mass advances and actively lifts the warmer air mass. Steep slope. Associated with rapid temperature drop, pressure rise, cumulonimbus clouds, heavy but short-lived precipitation (showers, thunderstorms), squall lines. Symbol: Line with blue triangles pointing in the direction of movement.
  • Warm Front: Warm air mass advances and gently overrides the colder air mass (which is retreating). Gentle slope. Associated with gradual temperature rise, pressure fall then steadying, layered clouds (stratus, nimbostratus, altostratus, cirrus), steady and prolonged light-to-moderate precipitation. Symbol: Line with red semi-circles pointing in the direction of movement.
  • Stationary Front: Boundary between two air masses where neither is advancing significantly. Weather is often similar to a warm front (cloudy, light precipitation) but can persist for days. Symbol: Alternating blue triangles and red semi-circles on opposite sides.
  • Occluded Front: Forms when a faster-moving cold front overtakes a warm front, lifting the warm air completely off the ground. Complex weather, often involving clouds and precipitation from both warm and cold front types. Symbol: Line with alternating purple triangles and semi-circles pointing in the direction of movement.

7. Extra-Tropical Cyclones (Temperate Cyclones / Mid-Latitude Cyclones / Depressions)

• Formation: Develop in mid-latitudes (35°-65° N/S) along the polar front (boundary between cP/mP and mT air masses). Explained by the Polar Front Theory (Bjerknes). Form due to instability and wave development along the front.
• Characteristics:
  • Large systems (hundreds to thousands of km diameter).
  • Have distinct warm and cold fronts, forming a warm sector.
  • Move from West to East (driven by Westerlies).
  • Pressure is lowest at the center (Low pressure system). Isobars are often elliptical or V-shaped.
  • Produce moderate to heavy precipitation over large areas, associated with frontal lifting. Weather changes are predictable as the system passes (warm front weather -> warm sector weather -> cold front weather).
  • Energy derived from the temperature contrast between air masses.
  • Formation often occurs over oceans in winter when temperature contrasts are strongest. Can also form over land.

8. Tropical Cyclones (Hurricanes, Typhoons, Cyclones)

• Formation Conditions:
  • Large, continuous warm sea surface (Temperature > 27°C) to a significant depth.
  • Presence of Coriolis force (hence, they don't form right at the equator, typically poleward of 5° N/S).
  • Pre-existing weak low-pressure area or cyclonic circulation.
  • Low vertical wind shear (little change in wind speed/direction with height).
  • Upper-level divergence (air spreading out aloft) to help draw air upwards from the surface.
• Characteristics:
  • Intense low-pressure systems originating over tropical oceans.
  • Smaller than extra-tropical cyclones (few hundred km diameter) but much more intense and destructive.
  • No distinct fronts. Isobars are nearly circular and closely packed (very steep pressure gradient).
  • Fueled by latent heat released during condensation of moist air drawn into the system.
  • Structure:
    • Eye: Calm, clear center with sinking air (10-50 km diameter). Lowest pressure.
    • Eyewall: Ring of intense thunderstorms surrounding the eye. Strongest winds and heaviest rainfall occur here.
    • Spiral Rainbands: Bands of thunderstorms spiraling outwards from the eyewall.
  • Movement: Generally Westward initially (driven by Trade Winds), then often curve Poleward and Eastward (influenced by Westerlies and Sub-Tropical Highs).
  • Dissipation: Weaken rapidly over land (cut off from warm, moist air supply) or over cold water.
• Regional Names:
  • Hurricanes: Atlantic Ocean, Caribbean Sea, Eastern Pacific
  • Typhoons: Western Pacific Ocean (China Sea, Philippines, Japan)
  • Cyclones: Indian Ocean (Bay of Bengal, Arabian Sea)
  • Willy-willies: Off the coast of Australia
• Destruction: Caused by high winds, torrential rainfall (leading to floods), and storm surge (abnormal rise in sea level).

9. Other Weather Phenomena

• Thunderstorms: Localized storms produced by strong upward motion of warm, moist air (convection), leading to cumulonimbus clouds, lightning, thunder, heavy rain, sometimes hail and strong winds. Can occur individually, in clusters, or along fronts (especially cold fronts).
• Tornadoes: Violently rotating columns of air extending from a cumulonimbus cloud to the ground. Extremely destructive due to very high wind speeds and extreme pressure drop. Most common in USA (Tornado Alley), associated with severe thunderstorms. Waterspouts are tornadoes over water.

Multiple Choice Questions (MCQs)

1. Which force is primarily responsible for initiating the movement of air (wind)?
   a) Coriolis Force
   b) Frictional Force
   c) Gravitational Force
   d) Pressure Gradient Force

2. In the Northern Hemisphere, winds are deflected to the ______ due to the Coriolis effect.
   a) Left
   b) Right
   c) East
   d) West

3. The idealized global pressure belt located around 30° N and 30° S latitudes, characterized by sinking air and calm conditions, is known as:
   a) Doldrums / ITCZ
   b) Sub-polar Lows
   c) Horse Latitudes / Sub-tropical Highs
   d) Polar Highs

4. Which atmospheric circulation cell operates between the Equator and approximately 30° latitude?
   a) Ferrel Cell
   b) Polar Cell
   c) Hadley Cell
   d) Walker Cell

5. A large body of air characterized by relatively uniform temperature and humidity is called:
   a) A Front
   b) An Air Mass
   c) A Cyclone
   d) An Anticyclone

6. Which type of front involves a cold air mass actively lifting a warmer air mass, often resulting in heavy showers and thunderstorms?
   a) Warm Front
   b) Stationary Front
   c) Occluded Front
   d) Cold Front

7. Extra-tropical cyclones typically form along the:
   a) Equator
   b) Polar Front
   c) Sub-tropical Jet Stream
   d) Inter Tropical Convergence Zone (ITCZ)

8. Which of the following is NOT a necessary condition for the formation of Tropical Cyclones?
   a) Warm sea surface temperature (> 27°C)
   b) Presence of strong vertical wind shear
   c) Presence of Coriolis force
   d) Upper-level divergence

9. The calm, clear center of a mature Tropical Cyclone is known as the:
   a) Eyewall
   b) Spiral Rainband
   c) Eye
   d) Warm Sector

10. A warm, dry wind descending the leeward side of the Rocky Mountains is known as:
    a) Foehn
    b) Mistral
    c) Chinook
    d) Loo

Answer Key:

1. d) Pressure Gradient Force
2. b) Right
3. c) Horse Latitudes / Sub-tropical Highs
4. c) Hadley Cell
5. b) An Air Mass
6. d) Cold Front
7. b) Polar Front
8. b) Presence of strong vertical wind shear (Low shear is required)
9. c) Eye
10. c) Chinook

Remember to visualize these concepts. Draw diagrams of the pressure belts, wind systems, fronts, and cyclone structures. Understanding the interplay between pressure, temperature, moisture, and Earth's rotation is key. Good luck with your preparation!