What is global circulation? | Part Three | The Coriolis effect & winds

Global Circulation Cells

  • Three Main Cells per Hemisphere:

    • Hadley Cell: Formed by warm air rising at the equator and falling around 30 degrees latitude.

    • Ferrel Cell: Located between the Hadley cell and polar cell, its dynamics are influenced by the adjacent cells.

    • Polar Cell: Cold air sinks at the poles and flows towards lower latitudes.

Earth's Rotation and Wind Patterns

  • Coriolis Effect:

    • The Earth's rotation causes an apparent deflection of moving air.

    • In the Northern Hemisphere: Winds appear to curve to the right.

    • In the Southern Hemisphere: Winds appear to curve to the left.

  • Speed of Earth's Surface:

    • Faster at the equator than at the poles due to the Earth being wider at the equator, influencing wind movement.

    • Air moving from the equator toward the poles appears to curve due to this differential speed.

Understanding Wind Deflection

  • Air Movement Illustration:

    • An air parcel moving north from the equator appears to curve right (due to slower movement at higher latitudes).

    • Similarly, a parcel moving south from the pole curves right due to the faster-moving equator.

  • Major Result:

    • Winds blow counterclockwise around low pressure and clockwise around high pressure in the Northern Hemisphere; the opposite occurs in the Southern Hemisphere.

    • Deflections lead winds to blow eastward when moving poleward and westward when moving equatorward.

Jet Streams and Prevailing Winds

  • Jet Stream Formation:

    • As warm air moves away from the equator in the Hadley cell, it is deflected, creating an eastward flow.

    • Conservation of Angular Momentum: As air approaches the spin axis, it speeds up, similar to a spinning skater.

    • Subtropical Jet Stream: Located at heights of 12 to 15 kilometers, featuring strong winds above 280 mph. Defined by little associated weather due to its location.

  • Polar Front Jet Stream:

    • Located between the Ferrel and Polar Cells, marking the boundary of cold polar air and warm tropical air.

    • Occurs at 11 to 13 kilometers and is strengthened by temperature contrasts, making it stronger in winter.

  • Jet Stream Influence:

    • Waves in the jet stream can deepen Atlantic depressions, affecting weather patterns.

Surface Wind Patterns

  • Trade Winds:

    • Formed in the Hadley cells, flowing towards the equator and deflected westward.

    • Result in Northeast Trade Winds (Northern Hemisphere) and Southeast Trade Winds (Southern Hemisphere).

    • Enabled historical sailing routes across the Atlantic.

  • Ferrel Cell Winds:

    • Initially flow southerly; Coriolis deflection leads to prevailing westerly and southwesterly winds over regions like the UK.

Comparative Atmospheric Dynamics

  • Jupiter Analogy:

    • Similar circulation patterns exist on Jupiter due to its rapid rotation (one Jupiter day = 9.5 hours).

    • Its size and speed enhance the Coriolis effect, creating numerous alternating bands and circulation cells, resulting in a striped appearance.