Planetary Circulation and Wind Patterns

Chapter Overview

• Transitioning from forces to planetary circulation.

• Focus on large scale wind patterns and their implications for climate.

Key Concepts in Planetary Circulation

• Planetary Circulation: General atmospheric wind patterns, major regions of high and low pressure.

• Regions of stable high and low pressure exist (though not permanent).

Historical Knowledge of Wind Patterns

• Historical insights from European mariners in the 1400s-1500s:

  • Navigate to the West Indies using trade winds (Easterlies) and adjust routes to avoid headwinds (Westerlies) when returning.

• Latitude Knowledge: Specific latitudes are critical for understanding and using trade winds (bands around 23.5°N and 23.5°S known as the Tropics).

• Significance in navigation for Polynesians, who settled in the Pacific using wind patterns for extensive ocean voyages.

Understanding Trade Winds and Westerlies

• Trade Winds: Consistent Easterlies found in tropics, crucial for westward navigation.

• Westerlies: Predominantly found between 30° and 60° latitude, coming from the west, aiding return trips to Europe.

• The difference in wind patterns has been used for thousands of years in navigation.

Intertropical Convergence Zone (ITCZ)

• Location: Near the equator, experiences rising warm air that leads to precipitation.

• Average location shifts seasonally due to solar heating, moving north during the Northern Hemisphere summer and south during winter.

• Cloud Formation: The ITCZ leads to heavy cloud cover and thunderstorms due to rising air and moisture.

Wind Gradients and Their Causes

• Pressure Gradients (PGF): Result from temperature differentials; drive high to low-pressure air movement.

• Temperature Effects: More heat at equator causes warm air to rise, creating low pressure; cooler air sinks at poles, causing high pressure.

• Coriolis Effect: Earth's rotation leads to wind patterns deviating right in the Northern Hemisphere and left in the Southern Hemisphere.

Global Wind Circulation Cells

• Hadley Cell: Strongest and most prominent, driven by equatorial heating, causes low pressure near the equator and diverges around 30° latitude creating deserts.

• Ferrell Cell and Polar Cell: Contribute additional complexity to the circulation pattern; involved with weather systems and climatic variations.

Impact on Climate Zones

• Subtropical Highs (30°N/S): Clear skies and deserts (e.g., Sahara, Mojave) due to descending dry air from Hadley cells.

• Weather in Mid-latitudes: Varied due to jet streams resulting from interactions between cells, affects seasonal weather events, storms, and regular climatic conditions.

Jet Streams and Atmospheric Dynamics

• Polarlatent Stream: Strong winds occur between cells, influences movements of weather systems.

• Seasonal Variability: Jet stream positions change with seasons, impacting North American winter weather, especially frontal systems.

Convergence and Divergence in Weather

• Low pressure (rising air) leads to precipitation; high pressure (sinking air) results in clear skies.

• This dynamic is responsible for creating distinct climatic regions across the globe.

Summary of Seasonal Shifts

• ITCZ Seasonal Migration: Significant shifts account for dry to wet seasons in regions like North Africa, owing to solar input and land-water contrasts.

• Real-World Complexity: Effects of continental land mass versus ocean dynamics affect regions differently, particularly in how air masses interact seasonally.

Conclusion and Applications

• Understanding these patterns is key for predicting weather, explaining climatic differences across regions, and comprehending global climate variances.

Important Note: Focus on reasoning over memorization—understanding solar input and atmospheric behavior is crucial in climate science.