Hadley Cell (0° to 30°): Warm air rises at the equator (ITCZ), travels poleward at high altitudes, cools, and sinks at approximately latitude. This sinking air creates high-pressure belts and dry conditions, often associated with the world's major deserts.
Ferrel Cell (30° to 60°): This is a 'secondary' cell driven by the movement of the other two. It involves air flowing poleward and eastward near the surface and equatorward and westward at higher altitudes.
Polar Cell (60° to 90°): Cold, dense air sinks at the poles, creating high pressure. This air flows toward the lower latitudes, where it meets warmer air from the Ferrel cell at the Polar Front, causing it to rise.
Intertropical Convergence Zone (ITCZ): Located near the equator, this is a permanent low-pressure belt where the trade winds meet. The rising air leads to high humidity, frequent clouds, and heavy precipitation.
Subtropical Highs: Found at N and S, these are regions of sinking air and high pressure. They are characterized by calm winds and very low rainfall, forming the 'Horse Latitudes'.
Surface Wind Belts: The interaction of pressure and Coriolis creates the Trade Winds (blowing toward the equator), the Westerlies (blowing toward the poles in mid-latitudes), and the Polar Easterlies (blowing away from the poles).
| Feature | Low Pressure Zone (e.g., ITCZ) | High Pressure Zone (e.g., Subtropical High) |
|---|---|---|
| Air Movement | Rising air | Sinking air |
| Cloud Cover | High (Condensation occurs as air cools) | Low (Air warms as it sinks, evaporating moisture) |
| Precipitation | Frequent and heavy | Rare and sparse |
| Surface Winds | Converging (Trade winds meet) | Diverging (Winds move away) |
Wind Naming Convention: Always remember that winds are named after the direction from which they blow. A 'Westerly' wind comes from the West and moves toward the East.
Latitude Check: When asked to describe weather at a specific latitude, first identify the pressure belt. and are Low Pressure (wet/unsettled), while and are High Pressure (dry/settled).
Coriolis Direction: Use the 'Right-Hand Rule' for the Northern Hemisphere—if you stand with your back to the wind, the deflection is to your right. Reverse this for the Southern Hemisphere.
The '8' Pattern: Visualize the Ferrel cell as a gear being turned by the Hadley and Polar cells; it rotates in the opposite direction to its neighbors.
Equatorial Wind Strength: A common mistake is assuming winds are strongest at the equator. In reality, the ITCZ is often a zone of calm winds (the Doldrums) because the primary air movement is vertical (rising) rather than horizontal.
Coriolis at the Equator: Students often forget that the Coriolis effect is zero at the equator. This is why tropical cyclones cannot form exactly on the equator; they require the 'spin' provided by Coriolis at higher latitudes.
Cell Boundaries: Do not treat the and boundaries as fixed lines. These pressure belts shift north and south throughout the year following the seasonal position of the sun.
