GS Notes

1.5 - Geography Notes 24

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Elaborate Notes

Westerlies

  • Definition and Path: The Westerlies are prevailing winds that blow from the subtropical high-pressure belts (located around 30°-35° latitude) towards the subpolar low-pressure belts (located around 60°-65° latitude) in both the Northern and Southern Hemispheres. Due to the Coriolis effect, they are deflected to the right in the Northern Hemisphere, thus blowing from the southwest (South-westerlies), and to the left in the Southern Hemisphere, blowing from the northwest (North-westerlies).
  • Hemispheric Variation:
    • Northern Hemisphere: The presence of vast continental landmasses (North America, Europe, and Asia) and significant mountain ranges (like the Rockies and Himalayas) obstructs the flow of the Westerlies. This friction and topographic barrier cause the winds to be more variable and less consistent, often developing into large-scale eddies and Rossby waves, which influence mid-latitude weather systems.
    • Southern Hemisphere: This hemisphere is dominated by oceans between 40° and 60° south latitude. The absence of significant landmasses allows the Westerlies to blow with great force and consistency. This led mariners during the Age of Sail to give them specific names based on their ferocity at different latitudes.
      • Roaring Forties: Found between 40°S and 50°S latitude, known for their strong and steady winds, which were historically crucial for trade routes.
      • Furious Fifties: Between 50°S and 60°S latitude, the winds are even stronger and associated with more frequent storms.
      • Shrieking (or Screaming) Sixties: South of 60°S, these winds are extremely powerful and dangerous, marked by massive waves and harsh weather conditions as they blow unimpeded around the Antarctic continent.

Polar Easterlies

  • Definition and Path: These are cold, dense winds that blow from the polar high-pressure cells (located over the poles) towards the subpolar low-pressure belts. The Coriolis effect deflects them, causing them to blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.
  • Characteristics: Originating over the ice-covered polar regions, these winds are extremely cold, stable (as cold air is dense and tends to sink), and carry very little moisture (dry).
  • Climatic Influence: Polar Easterlies are responsible for bringing frigid temperatures to polar and subpolar regions. The zone where these cold winds meet the warmer Westerlies is known as the Polar Front. This convergence leads to the formation of temperate cyclones (or mid-latitude depressions), which are a primary driver of weather in the mid-latitudes.

Zones of Planetary winds

Inter-Tropical Convergence Zone (ITCZ)

  • Formation: The ITCZ is a low-pressure belt near the Equator where the Northeast Trade Winds of the Northern Hemisphere and the Southeast Trade Winds of the Southern Hemisphere converge. This convergence forces the air to rise.
  • Location and Shift:
    • It is generally centered on or near the geographical equator. During the equinoxes (March and September), it lies roughly at 5°N-5°S of the Equator.
    • The ITCZ migrates seasonally with the apparent movement of the sun. During the Northern Hemisphere’s summer, it shifts northward (up to 20°-25°N over the Indian subcontinent), and during the Northern Hemisphere’s winter, it shifts southward. This seasonal shift is a fundamental driver of monsoonal climates.
  • The Doldrums: The zone is characterized by ascending air, leading to very light and variable surface winds. This condition of calm is known as the Doldrums. Historically, this posed a significant challenge for sailing ships, which could be stranded for weeks due to the lack of wind.
  • Weather Conditions: As the warm, moist air from the tropics rises, it cools and condenses, leading to high humidity, thick cloud cover, and heavy convective rainfall, often in the form of daily thunderstorms.

Horse Latitudes

  • Location and Formation: These are belts of high pressure located around 30° North and South of the Equator, corresponding to the Subtropical High-Pressure Belts. They are formed by the descending air from the upper atmosphere in the Hadley and Ferrel cells.
  • Characteristics: The subsiding air is compressed and warmed, which inhibits cloud formation and precipitation. This results in clear skies, calm or light variable winds, and arid or semi-arid conditions. Most of the world’s major hot deserts, such as the Sahara, Arabian, and Australian deserts, are located in these latitudes.
  • Etymology: The term is believed to have originated in the 17th century when Spanish sailing ships transporting horses to the Americas were often becalmed in this zone. To conserve dwindling water supplies, the crew would be forced to throw the horses overboard.

Tri-cellular Meridional Circulation

This is a globally accepted model describing the circulation of the Earth’s atmosphere. It posits three distinct convection cells in each hemisphere that circulate air along meridians (north-south).

  • Hadley Cell:
    • Scientist: Proposed by George Hadley in 1735 to explain the Trade Winds.
    • Circulation: It is a thermally direct cell. Warm, moist air rises at the Equator (ITCZ), travels poleward in the upper troposphere, cools and descends around 30° N/S (Horse Latitudes), and then flows back towards the Equator as the surface Trade Winds. It is the most dominant of the three cells.
  • Ferrel Cell:
    • Circulation: Located between the Hadley and Polar cells (approx. 30° to 60° N/S). It is a thermally indirect cell, acting like a gear driven by the motions of the two adjacent cells. At its southern margin (in NH), air sinks with the Hadley Cell, moves poleward at the surface as the Westerlies, and rises at the Polar Front (around 60° N/S) where it meets the cold polar air.
  • Polar Cell:
    • Circulation: The northernmost and southernmost cell (approx. 60° to 90° N/S). It is a thermally direct but weak cell. Extremely cold, dense air descends over the poles (creating the Polar High), flows equatorward as the Polar Easterlies, and rises at the subpolar low (Polar Front) upon meeting the warmer Westerlies.

Seasonal winds

  • Definition: These are winds that undergo a seasonal reversal in direction due to the differential heating and cooling of large land and sea masses. Unlike planetary winds, they are not constant throughout the year and are regional, not global, in extent.
  • Mechanism: In summer, land heats up faster than the sea, creating a low-pressure area over the land and high pressure over the sea, causing winds to blow from sea to land (e.g., wet monsoon). In winter, the pattern reverses.
  • Example: The South-West Monsoon in the Indian subcontinent is a classic example. The intense heating of the Tibetan Plateau and the Indian landmass during summer creates a strong low-pressure zone, which pulls in moisture-laden winds from the Indian Ocean. This is further influenced by the northward shift of the ITCZ.

Local winds

These winds are generated by local or regional geographic features and diurnal (daily) or annual thermal differences.

Land and Sea Breeze

  • Mechanism: A diurnal cycle driven by the differential heating of land and water. Land has a lower specific heat capacity than water, meaning it heats up and cools down faster.
  • Sea Breeze (Daytime): During the day, land heats up more quickly than the adjacent sea. This creates a low-pressure zone over the land and a relative high-pressure zone over the sea. Air flows from high to low pressure, resulting in a cool breeze from the sea to the land.
  • Land Breeze (Nighttime): At night, the land cools down faster than the sea. A high-pressure zone develops over the land and a low-pressure zone over the sea. The wind direction reverses, flowing from land to sea.
  • Significance: This phenomenon moderates coastal temperatures and is traditionally used by coastal fishermen to sail out to sea with the land breeze at night and return to shore with the sea breeze during the day.

Mountain and Valley Breeze

  • Mechanism: A diurnal cycle in mountainous regions driven by differential heating of slopes and valley floors.
  • Valley Breeze (Anabatic Winds - Daytime): During the day, mountain slopes receive more direct solar radiation and heat up faster than the valley floor. The air in contact with the slopes becomes warmer and less dense, and it rises up along the slope. This creates a breeze blowing from the valley towards the mountains.
  • Mountain Breeze (Katabatic Winds - Nighttime): At night, the slopes cool down rapidly through radiation. The air in contact with them becomes colder and denser. This heavy, cold air then flows down the slope into the valley under the influence of gravity. Katabatic winds can be very strong if channeled through narrow valleys.

Local Winds of the World

Hot Winds

  • Chinook: (North America) A warm, dry, foehn-type wind that descends on the leeward (eastern) side of the Rocky Mountains. It is called the “snow eater” because its warmth can cause rapid melting of snow, which is significant for winter wheat cultivation in the Prairies.
  • Fohn: (Europe) A warm, dry wind that descends the leeward side of the Alps, primarily affecting Switzerland, Austria, and Germany. Similar to the Chinook, it causes rapid snowmelt and creates warmer local conditions.
  • Harmattan: (West Africa) A dry, dusty northeasterly wind blowing from the Sahara Desert over West Africa. It occurs during the winter (November to March). Because its low humidity provides relief from the oppressive coastal humidity, it is known as the “Doctor wind”.
  • Haboob: (Sudan, Sahara, Arabia) An intense sandstorm or dust storm caused by strong downdrafts from a collapsing thunderstorm. It is not a prevailing wind but a storm event.
  • Sirocco: A hot, humid, and often dusty wind originating over the Sahara and blowing northward across the Mediterranean Sea into Southern Europe. As it picks up moisture over the sea, it can cause fog and rain. It is called “Blood Rain” when it carries red Saharan dust.
    • Khamsin: The name for the Sirocco in Egypt.
    • Gibli: The name for the Sirocco in Libya.
  • Berg: (South Africa) A hot, dry wind that blows from the interior plateau down the coastal escarpment towards the ocean. It is a foehn-type wind.
  • Samoon: (Iran, Iraq, Arabian Desert) An extremely hot, dry, and often violent, dust-laden wind. Its name translates to “poison wind” due to its ability to cause severe heatstroke.
  • Loo: (North India, Pakistan) A hot, dry, and strong westerly wind that blows over the northern plains during the afternoon in the summer months (May-June), causing extremely high temperatures.
  • Karaburan: (“Black storm”) A strong, superheated, dust-laden wind in the deserts of Mongolia and Northern China (Central Asia).
  • Brickfielder: (Southern Australia) A hot, dry, and dusty northerly wind that originates from the desert interior and blows towards the southern coastal areas during the summer.

Cold Winds

  • Blizzard: A severe snowstorm characterized by strong sustained winds, low visibility, and lasting for a prolonged period. Common in North America (USA, Canada) and polar regions.
    • Burran: The name for a blizzard in Siberia and Central Asia.
  • Norte/Norther: A strong, cold wind that blows from the north in the USA (Texas), the Gulf of Mexico, and Mexico during the winter.
  • Pamperos: A cold, strong south-westerly wind in the Pampas grasslands of Argentina and Uruguay, often associated with a squall line and a drop in temperature.
  • Mistral: A cold, dry, and powerful north-westerly wind that blows from southern France into the Gulf of Lion in the northern Mediterranean. It is channeled through the Rhône Valley.
  • Levant: An easterly wind that blows in the western Mediterranean Sea, particularly strong through the Strait of Gibraltar.
  • Bora: A cold, dry, and violent north-easterly katabatic wind that blows along the coast of the Adriatic Sea, affecting regions of Croatia, Italy, and Slovenia.

Prelims Pointers

  • Westerlies: Blow from Subtropical High to Subpolar Low Pressure Belts.
  • Westerlies in the Southern Hemisphere are named: Roaring Forties (40°S), Furious Fifties (50°S), and Shrieking Sixties (60°S).
  • Polar Easterlies: Blow from Polar High to Subpolar Low Pressure Belts; they are cold and dry.
  • ITCZ: Inter-Tropical Convergence Zone; convergence of Trade Winds; also known as Doldrums due to calm winds.
  • Horse Latitudes: Located at 30°N and 30°S latitudes; zone of Subtropical High Pressure; characterized by calm winds and descending air.
  • Tri-cellular Meridional Circulation: Comprises three cells per hemisphere: Hadley, Ferrel, and Polar.
  • Hadley Cell: Operates between the Equator and 30° N/S.
  • Ferrel Cell: Operates between 30° and 60° N/S.
  • Polar Cell: Operates between 60° and 90° N/S.
  • Seasonal Winds: Reverse direction seasonally, e.g., South-West Monsoon.
  • Anabatic Wind: Upslope wind during the day; also known as Valley Breeze.
  • Katabatic Wind: Downslope wind at night; also known as Mountain Breeze.

Match the following (Local Winds and Region):

  1. Chinook: North America (Rockies) - Hot, “Snow Eater”
  2. Fohn: Europe (Alps) - Hot
  3. Harmattan: West Africa - Hot, “Doctor Wind”
  4. Sirocco: Mediterranean region - Hot, causes “Blood Rain”
  5. Khamsin: Egypt - Hot (Sirocco type)
  6. Loo: North India/Pakistan - Hot
  7. Brickfielder: Australia - Hot
  8. Blizzard: North America/Siberia (Burran) - Cold
  9. Mistral: France (Rhône Valley) - Cold
  10. Bora: Adriatic Coast - Cold
  11. Pamperos: Argentina (Pampas) - Cold
  12. Norte: USA/Mexico - Cold

Mains Insights

GS Paper I (Geography & Society)

  1. Influence of Planetary Winds on Global Climate Distribution:

    • Cause-Effect: The Tri-cellular Meridional Circulation is the primary engine distributing solar energy from the tropics to the poles. The descending limbs of the Hadley Cell (at 30° N/S) are directly responsible for the formation of the world’s major hot deserts due to atmospheric stability and lack of precipitation. Conversely, the rising limbs at the Equator (ITCZ) and the Polar Front (60° N/S) create zones of high rainfall.
    • Analytical Perspective: This model explains why climatic zones are arranged in latitudinal bands, forming the basis of global biome distribution (e.g., tropical rainforests at the equator, deserts at subtropics, temperate forests in mid-latitudes).

  2. Monsoons as a Modification of the Planetary Wind System:

    • Debate & Historiography: The understanding of monsoons has evolved from Halley’s (1686) simple thermal concept (differential heating of land and sea) to modern dynamic concepts. Today, monsoons are understood as a complex seasonal phenomenon involving the seasonal shift of the ITCZ, the role of the Tibetan Plateau as an elevated heat source, and the influence of upper-air circulation like the Sub-Tropical and Tropical Easterly Jet Streams.
    • Significance: This complex interaction makes the monsoon a unique and powerful seasonal wind, fundamentally shaping the climate, economy (agriculture), and culture of South and Southeast Asia.

  3. Human-Environment Interaction through Local Winds:

    • Adaptation and Livelihood: Human activities have historically adapted to wind patterns. Land and sea breezes support coastal fishing communities. Foehn winds like the Chinook and Fohn create favorable microclimates for agriculture (e.g., viticulture on the slopes of the Alps, wheat cultivation in the Prairies) by melting snow and extending the growing season.
    • Challenges and Hazards: Local winds also pose risks. The ‘Loo’ in North India causes severe heatwaves and health issues. The Sirocco’s dust (“Blood Rain”) can damage crops and machinery. Cold winds like the Mistral and Bora can cause rapid temperature drops, damaging agriculture and posing challenges to infrastructure.

GS Paper III (Economy & Disaster Management)

  1. Economic Implications of Wind Systems:

    • Agriculture: The Indian economy is heavily reliant on the South-West Monsoon, making it a “gamble on the monsoon.” Variations in its arrival or intensity can lead to droughts or floods, severely impacting agricultural output and GDP.
    • Energy: Prevailing winds like the Westerlies are now crucial for renewable energy generation. The strong, consistent winds in regions like the “Roaring Forties” offer immense potential for offshore wind farms.
    • Health and Infrastructure: Winds like Harmattan (“The Doctor”) improve health conditions by reducing humidity but also carry dust that can cause respiratory issues (Meningitis Belt). Sandstorms like the Haboob and dust-laden winds like Karaburan can disrupt transport, communication, and daily life, requiring resilient infrastructure.

  2. Winds as Natural Hazards:

    • Disaster Management: Extreme wind events like Blizzards, Pamperos, and dust storms (Haboob) are significant natural hazards. Effective disaster management requires robust early warning systems, infrastructure designed to withstand high winds and temperature extremes, and public awareness campaigns.
    • Climate Change Impact: There is an ongoing debate and scientific investigation into how climate change might alter the intensity, frequency, and tracks of these global and local wind patterns, potentially exacerbating their hazardous impacts and affecting regional climates and economies. For instance, a weaker monsoon or more frequent heatwaves (Loo) could have devastating consequences.

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