1.5 - Geography Notes 23

Elaborate Notes

Development of Pressure Belts

The global pattern of atmospheric pressure belts is a result of the interplay between thermal and dynamic factors. The idealized model of this circulation is known as the Tri-cellular Meridional Circulation Model, which includes the Hadley, Ferrel, and Polar cells.

• Equatorial Low-Pressure Belt (Doldrums):

  • Formation: This belt, located roughly between 5°N and 5°S, is thermally induced. The region receives intense, direct solar radiation throughout the year, causing the surface air to heat up, expand, and become less dense.
  • Mechanism: This warm, light air rises through convection. As it ascends, it cools adiabatically, leading to condensation and the formation of cumulonimbus clouds, resulting in heavy convective rainfall. The upward movement of air creates a zone of low pressure at the surface. This zone of convergence is also known as the Inter-Tropical Convergence Zone (ITCZ).
  • Atmospheric Circulation (Hadley Cell): The air that rises at the equator spreads out poleward at the top of the troposphere (tropopause). This marks the beginning of the Hadley Cell, a concept first proposed by George Hadley in 1735 to explain the trade winds.

• Sub-tropical High-Pressure Belts (Horse Latitudes):

  • Formation: Located around 30°N and 30°S, these belts are dynamically induced, not thermally.
  • Mechanism: The air that rose at the equator and moved poleward begins to cool and accumulate at the tropopause. Due to the Coriolis effect and conservation of angular momentum, this air is deflected eastward. This piling up of air increases its density, causing it to subside (sink) around the 30° latitude.
  • Characteristics: Sinking air is compressed and warmed adiabatically, increasing its moisture-holding capacity and leading to atmospheric stability. This results in clear skies, calm conditions (hence “Horse Latitudes”), and low precipitation. These zones are the source regions for the Trade Winds (blowing equatorward) and the Westerlies (blowing poleward). This subsidence completes the Hadley Cell.

• Sub-polar Low-Pressure Belts:

  • Formation: Located around 60°N and 60°S, these belts are also dynamically induced.
  • Mechanism: Air flowing poleward from the Sub-tropical High (warm Westerlies) converges with cold, dense air flowing equatorward from the Polar High (cold Polar Easterlies). The warmer, less dense air of the Westerlies is forced to rise over the colder, denser polar air. This forced uplift, known as frontal lifting, creates a zone of low pressure at the surface. This convergence zone is often referred to as the Polar Front.
  • Characteristics: The rising air leads to cyclonic storms (temperate cyclones or extra-tropical cyclones), bringing unstable and variable weather with significant precipitation. This zone of rising air forms the boundary between the Ferrel Cell and the Polar Cell.

• Polar High-Pressure Belts:

  • Formation: Situated around the poles (90°N and 90°S), these are thermally induced.
  • Mechanism: Extremely low temperatures at the poles cause the air to become very cold and dense. This cold, heavy air contracts and subsides, creating a high-pressure zone at the surface.
  • Atmospheric Circulation (Polar Cell): From this polar high, cold surface air moves equatorward, forming the Polar Easterlies. It meets the Westerlies at the Polar Front (around 60° latitude), where it is forced to rise. At the tropopause, this air moves back towards the poles, cools, sinks, and completes the Polar Cell.

• The Ferrel Cell: This cell operates between 30° and 60° latitude in both hemispheres. It is a thermally indirect cell, meaning it is driven by the motion of the adjacent Hadley and Polar cells rather than by direct heating. Air sinks at 30° latitude and rises at 60° latitude, with surface winds (Westerlies) flowing poleward and upper-air winds flowing equatorward. It was first theorized by William Ferrel in 1856.

Distribution of Pressure Across the World

The idealized pressure belts are significantly modified by the differential heating of land and water, leading to seasonal shifts and the formation of distinct pressure cells. This is studied using isobars, lines connecting points of equal atmospheric pressure.

• Seasonal Shifting: The entire system of pressure belts shifts north and south following the apparent movement of the sun. During the Northern Hemisphere’s summer (around the June solstice), the belts shift northward. During the winter (around the December solstice), they shift southward.

• Pressure in January (Northern Hemisphere Winter):

  • The ITCZ and the Equatorial Low-Pressure Belt are displaced south of the equator, extending into the Southern Hemisphere over warmer continents like South America, Africa, and Australia.
  • In the Northern Hemisphere, continents cool down much faster than oceans. This intense cooling over large landmasses like Siberia and North America leads to the development of strong continental high-pressure cells (e.g., the Siberian High, Canadian High). These cells are so dominant that they merge with the Sub-tropical High-Pressure Belt, creating a nearly continuous high-pressure system across the mid-latitudes of the hemisphere.
  • In the Southern Hemisphere (experiencing summer), the Sub-tropical High-Pressure Belt is not continuous over land. It exists as distinct cells over the relatively cooler oceans (e.g., South Atlantic High, South Pacific High).

• Pressure in July (Northern Hemisphere Summer):

  • The ITCZ and the Equatorial Low-Pressure Belt shift significantly northward, extending well into the Northern Hemisphere. This is particularly pronounced over the Asian continent, where intense heating of the Tibetan Plateau and the Indian subcontinent creates a vast and deep low-pressure system, which is crucial for the Indian Summer Monsoon.
  • In the Northern Hemisphere, the Sub-tropical High-Pressure Belt is broken over the heated continents. It exists as distinct and strong high-pressure cells over the cooler oceans (e.g., Azores High over the Atlantic, Pacific High).
  • In the Southern Hemisphere (experiencing winter), the Sub-tropical High-Pressure Belt is more continuous and stronger, forming an almost unbroken belt across the oceans and continents.

Winds

Wind is the horizontal movement of air from an area of high pressure to an area of low pressure, driven by a combination of forces.

• Forces Affecting the Motion of Wind:

  1. Pressure Gradient Force (PGF):

     • Definition: The primary force that initiates air movement. It is defined as the rate of change of pressure over a given distance. The steeper the pressure gradient (i.e., the closer the isobars are to each other), the stronger the PGF and the higher the wind speed.
     • Direction: The PGF always acts at a right angle (perpendicular) to the isobars, directed from the high-pressure area to the low-pressure area.

  2. Frictional Force:

     • Definition: This is a drag force that opposes the motion of the wind. It is caused by the wind’s interaction with the Earth’s surface.
     • Influence: Its effect is greatest near the surface (within the lowest 1-2 km of the atmosphere, known as the planetary boundary layer) and diminishes with altitude. The friction is significantly greater over rugged terrain and continents compared to smooth ocean surfaces. This force reduces wind speed and alters its direction.

  3. Coriolis Force:

     • Definition: An apparent or “fictitious” force that arises due to the Earth’s rotation. It was mathematically described by the French scientist Gaspard-Gustave de Coriolis in 1835. It is a consequence of the conservation of angular momentum on a rotating sphere.
     • Mechanism: As air moves from one latitude to another, the rotational speed of the Earth’s surface beneath it changes (fastest at the equator, zero at the poles). This difference in speed causes the moving air parcel to be deflected from its straight path.
     • Direction of Deflection: It deflects moving objects (including winds) to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is known as Ferrel’s Law.
     • Magnitude: The force is directly proportional to the wind’s velocity (stronger winds are deflected more) and the sine of the latitude. It is maximum at the poles and zero at the equator.

Geostrophic Winds

• Concept: At higher altitudes (above the frictional layer, approx. >1 km), the wind’s motion is primarily a balance between the Pressure Gradient Force and the Coriolis Force.
• Mechanism: The PGF initiates air movement from high to low pressure. As the air starts moving, the Coriolis force deflects it. The deflection continues until the Coriolis force is equal in magnitude and opposite in direction to the PGF. At this point of equilibrium, the wind no longer flows across the isobars but flows parallel to them. This balanced wind is called the Geostrophic Wind.
• Cyclonic and Anticyclonic Circulation: When isobars are curved around a pressure center, this geostrophic balance results in circular flow.
  • Cyclone (Low-Pressure Center): PGF is directed inwards. The Coriolis force acts outwards. This balance results in an anticlockwise circulation in the Northern Hemisphere and a clockwise circulation in the Southern Hemisphere.
  • Anticyclone (High-Pressure Center): PGF is directed outwards. The Coriolis force acts inwards. This balance results in a clockwise circulation in the Northern Hemisphere and an anticlockwise circulation in the Southern Hemisphere.

Pressure System	Pressure at Center	Wind Pattern (Northern Hemisphere)	Wind Pattern (Southern Hemisphere)
Cyclone	Low	Anticlockwise	Clockwise
Anticyclone	High	Clockwise	Anticlockwise

Planetary Winds

These are large-scale, persistent wind systems that blow in a general direction throughout the year, driven by the global pressure belts. They are also known as prevailing winds.

• Trade Winds (Tropical Easterlies):
  • Location: Blow from the Sub-tropical High-Pressure Belts (approx. 30°N/S) towards the Equatorial Low-Pressure Belt (ITCZ). They are known as North-East Trades in the Northern Hemisphere and South-East Trades in the Southern Hemisphere due to Coriolis deflection.
  • Etymology: The name “trade” is derived from the Old German word ‘track’, meaning a steady course. Historically, these reliable winds were vital for sailors on trade routes.
  • Characteristics and Climatic Effects:
    • Origin: Originating in the subsiding air of the Sub-tropical Highs, they are initially dry and stable.
    • Eastern Margins of Continents: As they travel over warm tropical oceans, they pick up significant moisture. Upon reaching the eastern margins of continents in the tropics, this moist air is often forced to rise (orographic lift), causing heavy and consistent rainfall (e.g., eastern Brazil, eastern coast of Australia).
    • Western Margins of Continents: By the time they reach the western margins, they are often blowing offshore or parallel to the coast. Coupled with the atmospheric stability from the Sub-tropical High and the presence of cold ocean currents (which further inhibit evaporation and convection), these conditions lead to extreme aridity. This is a primary factor in the formation of the world’s major tropical deserts, such as the Sahara and Kalahari in Africa, the Atacama in South America, and the Great Sandy Desert in Australia.

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Prelims Pointers

• Isobars: Lines on a map connecting points of equal atmospheric pressure.
• Closely spaced isobars indicate a steep pressure gradient and high wind speeds.
• The four major pressure belts are: Equatorial Low, Sub-tropical High, Sub-polar Low, and Polar High.
• Thermally induced pressure belts: Equatorial Low and Polar High.
• Dynamically induced pressure belts: Sub-tropical High and Sub-polar Low.
• ITCZ: Inter-Tropical Convergence Zone, another name for the Equatorial Low-Pressure Belt.
• Horse Latitudes: Common name for the Sub-tropical High-Pressure Belts (around 30° N/S).
• Polar Front: The zone of convergence around 60° N/S where cold polar air meets warmer mid-latitude air.
• The entire system of pressure belts shifts seasonally with the apparent movement of the sun.
• Pressure in January: Strong high-pressure cells develop over continents in the Northern Hemisphere (e.g., Siberian High).
• Pressure in July: Strong low-pressure cells develop over continents in the Northern Hemisphere (e.g., Indian/Tibetan Low).
• Primary forces on wind: Pressure Gradient Force, Coriolis Force, Frictional Force.
• Pressure Gradient Force (PGF): Acts perpendicular to isobars, from high to low pressure.
• Coriolis Force: Deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Coriolis Force is zero at the equator and maximum at the poles.
• Geostrophic Wind: A theoretical wind that blows parallel to straight isobars in the upper atmosphere where friction is negligible. It represents a balance between PGF and Coriolis Force.
• Cyclone (Low Pressure): Anticlockwise circulation in the Northern Hemisphere, Clockwise in the Southern Hemisphere.
• Anticyclone (High Pressure): Clockwise circulation in the Northern Hemisphere, Anticlockwise in the Southern Hemisphere.
• Planetary Winds: Trade Winds (Easterlies), Westerlies, Polar Easterlies.
• Trade Winds: Blow from Sub-tropical Highs to the Equatorial Low. They are North-East Trades in the Northern Hemisphere and South-East Trades in the Southern Hemisphere.
• Trade winds are a primary cause for the formation of tropical deserts on the western margins of continents (e.g., Sahara, Kalahari, Atacama).

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Mains Insights

The Tri-cellular Model: An Idealized Framework

• Concept: The global atmospheric circulation, including pressure belts and planetary winds, is best understood through the Tri-cellular Meridional Circulation Model (Hadley, Ferrel, and Polar cells). This model provides a foundational cause-effect framework: solar insolation differences (cause) lead to temperature gradients, which in turn create pressure gradients (effect), driving the wind systems.
• Limitations & Reality: While essential for understanding the basics, this model is a simplification. In reality, the circulation is not a simple set of uniform belts but a complex system of semi-permanent high and low-pressure cells that shift, intensify, and weaken seasonally due to the land-sea contrast and topography. This is evident in the stark difference between January and July pressure maps.

Interplay of Forces and Resultant Wind Patterns

• Cause-Effect Analysis: The motion of wind is a classic example of multiple forces acting on a body.
  1. Initiation (PGF): The fundamental driver is the Pressure Gradient Force, created by differential heating. Without PGF, there would be no wind.
  2. Deflection (Coriolis): Earth’s rotation introduces the Coriolis Force, which prevents wind from flowing directly from high to low pressure, instead deflecting it and establishing large-scale rotational systems (cyclones, anticyclones).
  3. Modification (Friction): Near the surface, friction slows the wind down. This reduction in speed weakens the Coriolis Force, allowing the PGF to have a greater influence. Consequently, surface winds do not blow perfectly parallel to isobars but cross them at a slight angle, spiraling into lows and out of highs.
• Significance: Understanding this interplay is crucial for weather forecasting, predicting storm tracks, and explaining regional climate patterns.

Seasonal Pressure Distribution and its Climatic Impact

• Land-Sea Differential: The specific heat capacity of water is much higher than that of land. This means land heats up and cools down much faster than oceans. This is the primary driver of seasonal pressure variations.
• Monsoons as a Manifestation: The most dramatic example of this seasonal reversal is the monsoon system. The intense low-pressure cell that develops over the Tibetan Plateau and the Indian subcontinent in summer (July) is a key engine that pulls in moist air from the Indian Ocean, causing the summer monsoon. In winter (January), the development of the powerful Siberian High reverses this flow.
• Global Teleconnections: These seasonal pressure cells are not isolated phenomena. Changes in the intensity of the Siberian High or the Pacific High can have far-reaching effects on weather patterns elsewhere, a concept known as teleconnections (e.g., influencing the path of winter storms in North America).

Trade Winds and the Pattern of Global Aridity

• Historiographical Perspective: Early explorers mapped these winds for navigation, but modern geography explains their profound climatic significance.
• Mechanism of Desertification: The formation of tropical deserts on the western margins of continents is a multi-causal phenomenon where Trade Winds play a key role. The causal chain is:
  1. Subsidence: Air sinks in the Sub-tropical High-Pressure belt, leading to compressional warming and atmospheric stability, which discourages cloud formation and rain.
  2. Offshore Winds: The Trade Winds in these regions often blow from land to sea (offshore) or parallel to the coast, carrying dry, continental air.
  3. Cold Ocean Currents: The presence of cold ocean currents (like the Canary, Benguela, and Peru currents) along these western coasts cools the surface air, increasing stability and creating temperature inversions that further prevent moist air from rising and condensing.
• Analytical Insight: This demonstrates that a single geographical feature (a desert) is rarely the result of a single cause, but rather the convergence of multiple atmospheric and oceanic processes. Understanding this complex interaction is vital for studying climate change and desertification.