Question

Explain the Hadley model and the three-cell model for global circulation. Discuss limitations of each.

Short answer

The Hadley model describes tropical circulation, while the three-cell model explains global circulation in three zones. Both models are limited by simplicity and idealized assumptions.

Step-by-step solution

Introduction to the Hadley Model

The Hadley model, proposed by George Hadley in 1735, is a simplified atmospheric circulation model for the tropics. It describes how heat from the sun causes the air near the equator to rise and move toward the poles. This model assumes a single, large convection cell in each hemisphere, where air rises at the equator, moves poleward at high altitudes, sinks at the subtropics, and returns to the equator at the surface.

Introduction to the Three-Cell Model

The three-cell model extends the Hadley model by dividing each hemisphere into three distinct circulation cells: the Hadley cell, the Ferrel cell, and the Polar cell. The Hadley cell operates between the equator and approximately 30° latitude, where air rises due to heating, moves poleward, sinks around 30° and returns to the equator. The Ferrel cell sits between 30° and 60° latitude, with opposite circulation, where subtropical air sinks, moves poleward, and rises near 60° latitudes. Finally, the Polar cell circulates from 60° to the poles, where air rises, moves towards the poles at high altitudes, sinks at the poles, and flows back to 60°.

Limitations of the Hadley Model

The Hadley model was initially thought to work for the entire hemisphere, but it only adequately describes tropical atmospheric circulation. Its assumption of a single convection cell does not account for observed mid-latitude and polar air movements and oversimplifies the complexities of global wind patterns.

Limitations of the Three-Cell Model

While the three-cell model provides a better representation of atmospheric circulation compared to the Hadley model, it still has limitations. It assumes a constant Earth surface and symmetrical cells, which do not account for irregularities such as landmasses, ocean currents, and seasonal changes. Additionally, interactions among cells, eddies, and the jet stream are simplified, leading to discrepancies when applied to real-world atmospheric conditions.

Key concepts

Hadley cell

The Hadley cell is a crucial component of Earth's atmospheric circulation. It operates predominantly in the tropical regions, spanning from the equator to around 30° latitude in each hemisphere. This system is driven by solar heating, which causes warm air at the equator to rise. As the air rises, it cools and moves poleward at higher altitudes. The air eventually sinks at around 30° latitude, where it becomes dry and forms regions of high pressure often linked with deserts. After sinking, the air travels back towards the equator near the Earth's surface, completing the cycle.

Its functionality is essential for distributing heat from the equator towards the poles, influencing global climate patterns and trade winds. Despite being called a single convection cell, its real-world manifestations are affected by numerous factors, including Earth's rotation and various geographical features.
Although insightful, the Hadley cell model's main limitation is its simplicity. It omits important atmospheric dynamics at higher latitudes, especially in the mid-latitudes and polar areas, where more complex systems of circulation are in play.

Three-cell model

The three-cell model is an evolution of the Hadley model designed to extra refine and extend our understanding of global atmospheric circulation. Each hemisphere is divided into three distinct cells: Hadley, Ferrel, and Polar, which together cover different latitudes and their specific circulation characteristics.

• **Hadley cell**: Spans from the equator to 30° latitude. Warm air rises at the equator, moves toward the poles, sinks at 30°, then returns to the equator.
• **Ferrel cell**: Exists between 30° and 60° latitudes. Air sinks at 30°, flows poleward near the surface, and rises around 60° latitude. This cell exhibits reverse circulation compared to Hadley.
• **Polar cell**: Found from 60° latitude to the poles, where air rises at 60°, travels to the poles, descends at the poles, then returns to 60°.

The three-cell model provides a more thorough structure for understanding atmospheric circulation. However, it is limited by its assumptions of constant surfaces and symmetrical cells, which don't fully reflect the complexities of Earth's atmospheric movements. Factors like uneven land masses, ocean currents, and temporal changes cause deviations from the model's predictions.

Ferrel cell

The Ferrel cell is the second key circulation cell in the three-cell model, spanning between 30° and 60° latitude. Unlike the adjacent Hadley and Polar cells, the Ferrel cell operates with a more complex and opposite circulation pattern:

• Air sinks at around 30° latitude, creating regions of high pressure.
• It then moves poleward, converging with polar air masses around 60° latitude.
• As air travels poleward, it rises around 60°, driven by differentials in pressure and temperature.

This cell plays a significant role in the creation of the westerly winds, which are prevalent in the mid-latitudes. The Ferrel cell also contributes to variance in weather patterns due to its interaction with other atmospheric flows. Despite its importance, the Ferrel cell's operations are approximations, and its boundaries are fluid, affected by jet streams, pressure systems, and seasonal changes.

Polar cell

The Polar cell is the final component of the three-cell model. It exists from 60° latitude to the poles and represents atmospheric circulation in the high latitudes of each hemisphere. Here's how it functions:

• Air rises at approximately 60° latitude due to lower pressure regions and cooling temperatures.
• It subsequently moves towards the poles in the upper atmosphere.
• Upon reaching the poles, the air cools further, descends, and forms areas of high pressure at the surface.
• The cycle completes as this air flows back towards 60° latitude near the ground.

The Polar cell plays a crucial role in transferring cold air from the poles towards lower latitudes and has a direct impact on polar weather systems. However, like the other cells, it is an idealized representation. The Polar cell's reality is affected by seasonal changes, ocean influences, and the dynamic interactions it has with the Ferrel cell and other environmental features.