D.8] Wind Circulations

1. Low Pressure and High Pressure Systems

1.1 Definition and Characteristics

• High Pressure System (Anticyclone):

  • Air descends from high altitudes.
  • Causes clear skies and calm weather.
  • Common in the Horse Latitudes (30°N and 30°S).
  • Example: Subtropical Highs.

• Low Pressure System (Cyclone):

  • Air rises from the surface, leading to cloud formation and precipitation.
  • Associated with stormy weather.
  • Common in the Doldrums (near the Equator).
  • Example: Monsoon Low Pressure Systems in South Asia.

1.2 Pressure Gradient and Wind Direction

1.3 Impact on Weather

• High Pressure: Generally brings dry, stable weather.
• Low Pressure: Often leads to unsettled, rainy weather.

1.4 Key Facts for Exams

• High pressure systems are associated with the Subtropical Highs.
• Low pressure systems are often linked to monsoon activity in South Asia.
• The pressure gradient force is the primary driver of wind.

2. Trade Winds

2.1 Definition and Origin

• Trade Winds: Steady winds that blow from the subtropical high-pressure zones (30°N and 30°S) towards the equatorial low-pressure zone (Intertropical Convergence Zone or ITCZ).
• Blow from east to west in both hemispheres.

2.2 Characteristics

• Consistent and predictable.
• Blow year-round.
• Influence maritime trade routes and navigation.

2.3 Historical Significance

• Used by sailors for transoceanic voyages.
• Enabled the Age of Exploration in the 15th–17th centuries.
• Example: Columbus’ voyage used the trade winds to cross the Atlantic.

2.4 Key Facts for Exams

• Trade winds are part of the Hadley Cell circulation.
• Blow from subtropical highs to the equator.
• Important for maritime trade and navigation.

3. Planetary Winds (Westerlies, Polar Easterlies)

3.1 Overview

• Planetary winds are large-scale wind systems that dominate global circulation.
• Divided into three main belts: Trade Winds, Westerlies, and Polar Easterlies.

3.2 Westerlies

• Location: 30°–60° latitude in both hemispheres.
• Direction: Blow from west to east.
• Cause: Result of the Ferrel Cell circulation.
• Characteristics: Unpredictable, variable winds with frequent storms.
• Impact: Influence weather patterns in mid-latitudes (e.g., temperate zones).

3.3 Polar Easterlies

• Location: 60°–90° latitude in both hemispheres.
• Direction: Blow from east to west.
• Cause: Result of the Polar Cell circulation.
• Characteristics: Cold, dry winds.
• Impact: Influence polar climates and subpolar regions.

3.4 Key Differences

3.5 Key Facts for Exams

• Westerlies are crucial for mid-latitude weather systems.
• Polar Easterlies are associated with polar climates.
• Planetary winds are driven by global atmospheric circulation cells.

4. Summary of Wind Circulations

4.1 Important Terms

• Hadley Cell: Warm air rises at the equator, descends at 30° latitude.
• Ferrel Cell: Mid-latitude circulation with rising air at 60° and sinking at 30°.
• Polar Cell: Cold air descends at the poles and rises at 60° latitude.

4.2 Frequently Asked Questions (SSC, RRB)

• Q: What are the trade winds?
  A: Steady winds blowing from the subtropical high-pressure zones to the equatorial low-pressure zone.

• Q: Which winds are associated with the Ferrel Cell?
  A: Westerlies (30°–60° latitude).

• Q: What are the Polar Easterlies?
  A: Cold, dry winds blowing from east to west in the polar regions (60°–90° latitude).

• Q: Which wind belt is responsible for most of the world’s weather?
  A: Westerlies (30°–60° latitude).

• Q: What is the main cause of planetary winds?
  A: Differences in solar heating and the Earth’s rotation, leading to global atmospheric circulation cells.