The tri-cellular model, the global winds, and where it breaks down

Overview

Hadley cell, ferrel cell and polar cell.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Describe the Hadley cell.

   Model answer

   The Hadley cell operates between the equator and about 30 degrees in each hemisphere and is driven directly by heating. Intense insolation at the Inter-Tropical Convergence Zone makes air rise to the tropopause, where it spreads polewards. As it travels it cools and loses moisture, and by about 30 degrees it has become dense enough to subside, creating the subtropical high pressure belt. Some of this descending air then flows back along the surface towards the equator to complete the loop, and that return flow is the trade winds.

2. 2

   Describe the Ferrel cell.

   Model answer

   The Ferrel cell operates between about 30 and 60 degrees in each hemisphere. At the surface, air diverging polewards from the subtropical high travels towards the sub-polar low. At about 60 degrees it meets cold air from the polar cell at the polar front and is forced to rise. Aloft, part of that risen air returns equatorwards and subsides again at 30 degrees, completing the loop. The surface flow in this cell is what gives the mid-latitudes their prevailing westerly winds.

3. 3

   Describe the Polar cell.

   Model answer

   The Polar cell operates between about 60 and 90 degrees in each hemisphere. Very cold dense air subsides over the pole, giving the polar high, and then flows outwards along the surface towards lower latitudes. At about 60 degrees it meets warmer air arriving from the Ferrel cell at the polar front, and undercuts it, forcing the warmer air to rise. Air returns polewards aloft to complete the loop. It is the weakest of the three cells.

4. 4

   Why is the Ferrel cell described as an indirect cell while the Hadley and Polar cells are direct?

   Model answer

   The Hadley and Polar cells are thermally direct: in each, air rises where it is warm and sinks where it is cold, so they are driven by heating and cooling. The Ferrel cell does the reverse, since air rises at 60 degrees where it is relatively cool and sinks at 30 degrees where it is relatively warm. It is not driven by its own heating but is dragged round mechanically between the two neighbouring cells, so it is called indirect and is far less regular than a textbook diagram suggests.

5. 5

   What are the trade winds, and from which direction do they blow in each hemisphere?

   Model answer

   The trade winds are the surface flow of the Hadley cell, running from the subtropical high at about 30 degrees back towards the equatorial low. Air heading equatorwards is deflected to the right in the Northern Hemisphere, so it arrives as the north-east trades; in the Southern Hemisphere it is deflected to the left, so it arrives as the south-east trades. Winds are named by the direction they come from, so a north-east trade blows out of the north-east towards the south-west. They are steady and reliable, which is how they earned their name from sailing ships.

6. 6

   What are the westerlies, and from which direction do they blow in each hemisphere?

   Model answer

   The westerlies are the surface flow of the Ferrel cell, running polewards from the subtropical high at about 30 degrees towards the sub-polar low at about 60 degrees. In the Northern Hemisphere the poleward-moving air is deflected to the right and so arrives out of the south-west; in the Southern Hemisphere it is deflected to the left and so arrives out of the north-west. In both hemispheres the flow is therefore from west to east, which is why they are called westerlies. In the Southern Hemisphere, with little land to slow them, they are famously strong and are known as the roaring forties.

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