Wind System

Winds are essentially the horizontal movement of air from areas of high pressure to areas of low pressure. Though invisible, their presence is constantly felt — as they sway trees, lift dust, cool our skin, and push waves across the oceans. Unlike visible weather phenomena like rain or snow, winds are measured using instruments such as the weather vane (for direction) and anemometer (for speed). Their behavior is influenced by pressure gradients, Coriolis force, friction, and topography. Winds play a fundamental role in weather patterns, ocean currents, and even in shaping landscapes over time.

CLASSIFICATION OF WINDS

1. Primary or Planetary Winds (Permanent Winds)

These winds are part of the Earth’s general circulation system and blow consistently in the same direction throughout the year. Their patterns are dictated by the pressure belts and the Coriolis effect, and they influence the global climate zones.

• Trade Winds:
  • Originate from the subtropical high-pressure belts (around 30° N/S) and blow towards the equatorial low-pressure zone.
  • Due to Earth’s rotation, they are deflected westward: northeast trades in the Northern Hemisphere and southeast trades in the Southern Hemisphere.
  • These winds are steady and moisture-laden, contributing to heavy rainfall in tropical regions.
  • Example: Trade winds bring moisture to the eastern coasts of continents like Brazil and Southeast Asia.
• Westerlies:
  • Blow from subtropical high-pressure belts towards the subpolar low-pressure zones between 30° and 60° latitudes.
  • They are stronger in the Southern Hemisphere due to the absence of large landmasses and less friction.
  • Known for generating cyclones and anti-cyclones in the temperate zones.
  • Example: The strong “Roaring Forties” between 40°–50° S latitude are robust westerlies.
• Polar Easterlies:
  • Originate in the Polar Highs (around 90° N/S) and move toward the Subpolar Lows (around 60° N/S).
  • They are cold, dry, and dense, blowing from east to west, and contribute to the formation of polar fronts.
  • Their interaction with westerlies forms frontal cyclones, especially in winter.

2. Secondary Winds (Seasonal Winds)

These winds change their direction with seasons due to the unequal heating of land and water. They are a modified version of the primary wind systems, influenced by geographical and thermal factors.

• Monsoons:
  • The most prominent example of seasonal winds.
  • They reverse direction seasonally: blowing from sea to land in summer (southwest monsoon) and from land to sea in winter (northeast monsoon).
  • This reversal results in significant seasonal variations in climate, agriculture, and water availability, especially in South Asia.
• Land and Sea Breezes:
  • During the day, land heats up faster than sea → low pressure over land → air moves from sea to land: sea breeze.
  • At night, land cools faster → high pressure over land → air moves toward sea: land breeze.
  • These diurnal winds moderate coastal climates and are most pronounced in tropical and subtropical regions.

3. Tertiary or Local Winds

These are winds created due to local variations in temperature and pressure, often restricted to a small area and short duration.

• Examples:
  • Loo: A hot, dry wind in northern India during summer.
  • Chinook: A warm, dry wind blowing down the eastern slopes of the Rockies in the USA and Canada.
  • Mistral: A cold, dry wind from the north blowing in southern France.
  • Föhn: A warm wind on the leeward side of the Alps in Europe.

MONSOON: THE GIANT BREATH OF THE INDIAN SUBCONTINENT

The term monsoon is derived from the Arabic word ‘mausim’ meaning season. It refers to a seasonal reversal in wind direction, primarily due to the differential heating of land and sea. India’s tropical monsoon climate is largely governed by this phenomenon and has shaped not just its agriculture and biodiversity, but also its culture and economy for centuries.

MECHANISM OF SOUTH-WEST MONSOON

The onset of monsoon is a culmination of multiple interlinked factors:

1. Differential Heating of Land and Sea
   • In summer, land heats up faster → low pressure over Indian subcontinent.
   • Surrounding oceans remain relatively cooler → high pressure.
2. Shifting of ITCZ (Inter Tropical Convergence Zone)
   • Moves northward during summer to over the Ganga plains.
   • ITCZ acts as a trough of low pressure attracting moist winds.
3. Role of Somali Jet and Easterly Jet
   • With the withdrawal of Westerly Jet from north India, the Tropical Easterly Jet (Somali Jet) sets in over the Indian Ocean.
   • It pushes moist air toward India, assisting the monsoon burst.
4. Tibetan Plateau Heating
   • Acts like an elevated heat source.
   • Creates strong vertical air currents, enhancing low pressure over northwest India.
5. Mascarene High (South Indian Ocean High)
   • Guides southeast trade winds across the equator, which then get deflected as southwest monsoon winds.
6. Coriolis Force
   • Causes deflection of southeast trade winds to southwest direction after crossing the equator.

TWO BRANCHES OF MONSOON

1. Arabian Sea Branch
   • Hits the Western Ghats first.
   • Causes heavy orographic rainfall on the windward side (e.g., Kerala, Maharashtra), but dry conditions on leeward side (rain-shadow region like Vidarbha).
2. Bay of Bengal Branch
   • Deflected by Arakan Yoma hills of Myanmar → enters northeast India and Bangladesh.
   • Moves westward and northward along Indo-Gangetic plains.

RETREATING MONSOON (OCT–DEC)

• Begins in October, as the sun moves southward.
• Land cools faster than sea → high pressure over land, reversing wind direction.
• Brings dry conditions in most parts of India, but:
  • Tamil Nadu and southeast coast receive rain due to northeast monsoon and cyclonic depressions.
  • This rain is crucial for Rabi crops and reservoir replenishment in peninsular India.

SOUTHERN OSCILLATION (ENSO LINKAGE)

• A phenomenon where pressure conditions reverse over Pacific and Indian Oceans.
• In El Niño years:
  • Pacific Ocean warms, weakening the Walker circulation.
  • Indian monsoon becomes weaker, often causing drought.
• In La Niña years:
  • Stronger monsoon, increased rainfall over Indian subcontinent.

IMPACT OF MONSOON ON INDIA

Positive Impact

• Supports Agriculture: Over 60% of Indian agriculture is rain-fed.
• Recharges groundwater and reservoirs for drinking water and hydroelectricity.
• Drives biodiversity in forests, wetlands, and rivers.
• Influences cropping patterns, festivals, economy, and rural employment.

Negative Impact

• Unpredictability causes droughts and floods.
• Sudden bursts → soil erosion, landslides in hilly areas.
• Excess rain → damage to crops, transport and infrastructure.
• Cyclonic depressions during retreat → coastal damage, especially in Coromandel coast.

CONCLUSION

India’s wind systems and monsoon mechanism are intricate, deeply interconnected with geographical factors and global pressure patterns. While monsoons are celebrated as the lifeblood of Indian agriculture, their variability continues to challenge our planning and resilience. Understanding these wind systems helps in weather forecasting, disaster management, and building climate-smart agriculture and infrastructure.

---

PDF File:

No PDF attached

---

Subject: Geography

← Back