Weather, Climate, Atmospheric Pressure, and Indian Monsoons

Quick Summary

• Atmospheric pressure belts influence global wind systems. Winds are affected by the pressure gradient force, the Coriolis effect, and friction.
• General atmospheric circulation is commonly explained through three major cells: the Hadley Cell in the tropics, the Ferrel Cell in the middle latitudes, and the Polar Cell.
• The Indian Monsoon is a seasonal reversal of winds influenced by differential heating of land and sea, the seasonal movement of the ITCZ, jet streams, and ocean-atmosphere interactions such as ENSO and the Indian Ocean Dipole (IOD).

Why This Topic Matters

This is an important topic for UPSC CSE and State PSC examinations, including BPSC. Common areas include ITCZ movements, monsoon onset and retreat, the effects of El Niño and La Niña on Indian rainfall, and Köppen climate classifications such as Amw, Aw, BWhw, and Cwg.

1. Insolation, Heat Budget and Atmospheric Pressure

• Insolation: Solar radiation received by Earth. In a simplified textbook heat-budget model, incoming solar radiation is taken as 100 percent. Approximately 35 percent is reflected back into space, 14 percent is absorbed by the atmosphere, and 51 percent is absorbed by Earth's surface. These are approximate textbook values, and the reflected portion contributes to Earth's planetary albedo.
• Atmospheric Pressure Belts:
  1. Equatorial Low Pressure Belt (approximately 0 to 5 degrees north and south): Strong heating causes air to rise, creating low pressure. The generally calm region near the equator is known as the Doldrums.
  2. Subtropical High Pressure Belts (approximately 30 to 35 degrees north and south): Descending air creates high pressure. These regions are also known as the Horse Latitudes.
  3. Subpolar Low Pressure Belts (approximately 60 to 65 degrees north and south): The convergence of air masses and rising air contribute to low pressure.
  4. Polar High Pressure Belts (near 90 degrees north and south): Very cold, dense air descends and produces high pressure.

2. Wind Systems and Planetary Circulation

• Forces Influencing Winds:

  • Pressure Gradient Force: Drives air from higher-pressure areas toward lower-pressure areas.
  • Friction: Slows winds near Earth's surface and influences their direction.
  • Coriolis Effect: Earth's rotation causes moving air to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The effect is zero at the equator and strongest near the poles.

• Tricellular Atmospheric Circulation:

  • Hadley Cell: Air rises near the equator, moves poleward at higher altitudes, and descends near 30 degrees north and south. Near the surface, it returns toward the equator as the Trade Winds.
  • Ferrel Cell: In the middle latitudes, surface winds generally flow poleward from the subtropical high-pressure belts as the Westerlies. This circulation is strongly influenced by the Hadley and Polar cells and by mid-latitude weather systems.
  • Polar Cell: Cold air descends over the poles and flows equatorward near the surface as the Polar Easterlies. It rises again in the subpolar low-pressure belt.

3. Indian Monsoon Mechanism and Teleconnections

• Core Drivers:

  • Differential Heating: Land heats and cools more rapidly than the ocean. Seasonal differences in heating contribute to pressure gradients that influence monsoon circulation.
  • Shift of the ITCZ: The Inter-Tropical Convergence Zone moves northward during the Northern Hemisphere summer. Its seasonal movement influences the monsoon trough over the Indian subcontinent.
  • Jet Streams: The seasonal northward displacement of the Subtropical Westerly Jet Stream from south of the Himalayas and the development of the Tropical Easterly Jet are associated with the summer monsoon circulation. These jet streams are important components of the larger atmospheric circulation, rather than the sole causes of the monsoon.
  • Retreating Monsoon: During the post-monsoon season, especially October and November, winds change direction over parts of India. Moisture-bearing northeasterly winds can bring significant rainfall to the Coromandel Coast of Tamil Nadu.

• Teleconnections:

  • El Niño and ENSO: El Niño involves unusually warm sea-surface temperatures in the central and eastern equatorial Pacific. It can alter Walker circulation and is often associated with weaker Indian summer monsoon rainfall, although the effect varies from year to year.
  • La Niña: La Niña involves unusually cool sea-surface temperatures in the central and eastern equatorial Pacific. It is often associated with normal or above-normal Indian monsoon rainfall, but this is not guaranteed.
  • Positive IOD (Indian Ocean Dipole): A positive IOD involves relatively warmer sea-surface temperatures in the western Indian Ocean compared with the eastern Indian Ocean. It can support Indian monsoon rainfall and may partly offset the influence of El Niño in some years.

4. Köppen Climate Classification: India

• Amw: Tropical monsoon climate with a short dry season, associated with parts of India's western coastal region in some textbook classifications.
• Cwg: A subtropical monsoon climate with a dry winter, associated with parts of northern India in some textbook classifications.
• Aw: Tropical savanna climate with a dry winter, found in several parts of the interior peninsular region.
• BWhw: Hot desert climate with a dry winter, associated with the Thar Desert and western Rajasthan in some classifications.

Exam note: Köppen climate codes and their regional distribution may vary by classification map or textbook edition. Verify the exact codes and locations against the NCERT edition prescribed for your examination.

Key Terms and Definitions

Exam Insights and Memory Aids

• Memory Aid for Coriolis Deflection: North = Right; South = Left.
• Common Confusion: El Niño is often associated with weaker Indian monsoon rainfall, while a positive IOD can support rainfall in some years. Neither relationship guarantees a particular seasonal outcome.
• Tropical vs. Extratropical Cyclones: Tropical cyclones generally develop over sufficiently warm tropical oceans and have a warm core. Mature systems may develop an eye. Extratropical cyclones generally develop in association with fronts and contrasting air masses in the middle latitudes.
• Important Distinction: Weather describes short-term atmospheric conditions, while climate describes long-term patterns and variability in weather.

Quick Revision Checklist

• Can you identify the four global pressure belts and the three circulation cells: Hadley, Ferrel, and Polar?
• Can you explain the roles of the pressure gradient force, friction, and the Coriolis effect?
• Can you describe how the ITCZ and jet streams relate to the onset and retreat of the Indian Monsoon?
• Can you distinguish El Niño, La Niña, and the positive Indian Ocean Dipole?
• Have you revised the main Köppen climate codes and their approximate distribution in India?