4.5 Global Wind Patterns

Solar Radiation

Due to the fact that the earth’s axis is tilted, heat and solar radiation is unevenly distributed. Because of our unique circumstances, heat accumulates at the equator naturally, thus leaving the poles without heat. The Earth uses various processes to circulate warm air towards the poles and move cooler air towards the equator.

Image Courtesy of Wikimedia

There are different kinds of convection cells found in the atmosphere that move air from the equator to the poles. These convection cells are polar cells, Ferrel cells, and Hadley cells. They are distinguished by where they are found.

• Hadley cells occur between 0° and 30° latitudes (directly north and directly south of the equator). At the equator, these cells start with warm, rising air. Then, as the air moves away from the equator, the air falls as cooler air.
• Ferrel cells occur between 30° and 60° latitudes. Around the 30° latitude line, the cold, dry air of a Hadley cell falls, pushing warm air up.
• Polar cells occur at latitudes greater than 60°. Polar cells start around the 60° latitude line where warm air from the Ferrel cells is pushed up. At higher latitudes, this air cools and falls as dry air on the poles.

Pressure and Wind Direction

Pressure in our atmosphere has a lot of effect on wind, which travels best from a high-pressure to a low-pressure environment. Think about a hill. One will go faster rolling downwards than attempting to roll up.

Looking at the image above, we can see the pressure created at a boundary between two convection currents. For example, between a Hadley and Ferrel cell, there is high pressure, but between two Hadley cells, there is low pressure. Thus, the wind will blow from the Ferrel-Hadley boundary (30° latitude) to the Hadley-Hadley boundary (0° latitude). This helps in keeping the convection cells separate, with different wind direction that allows Earth to redistribute its received heat energy.

Coriolis Effect

Imagine you are standing on a merry-go-round at the park. If you throw a ball straight ahead while the merry-go-round is spinning, the ball will appear to curve to the right (if you are in the Northern Hemisphere) or to the left (if you are in the Southern Hemisphere). This is because the ball is moving in a straight line relative to the ground, but the ground is moving in a circular path around the center of the merry-go-round.

The Coriolis effect works in a similar way. When an object is in motion relative to a rotating frame of reference, it appears to curve in a certain direction. This effect is most noticeable at long distances and at high latitudes, where the rotation of the Earth has the greatest influence.

It plays a role in the way that winds and ocean currents behave. As we established above, winds will go from the Ferrel-Hadley boundary (30° latitude) to the Hadley-Hadley boundary (0° latitude), or high to low pressure. These winds are called trade winds. If the earth wasn’t spinning, the winds would travel in a straight line; however, since the earth rotates, these winds do as well. If you look at the global circulation image, you will see that the lines representing wind currents are curved.

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