Due to the fact that the earth’s axis is tilted, heat and solar radiation is unevenly distributed. The equator gets more solar radiation than the poles do. Thus, the equator is warmer than the poles. This imbalance creates convection cells in our atmosphere that work to move warm air toward the poles and colder down toward 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.
Hadley cells occur between 0° and 30° latitudes. 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.
Atmospheric pressure plays a big role in wind direction. Wind travels from high pressure to low pressure. The best way to remember this is to think about a ball rolling down a hill going from the higher side to the lower side.
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).
The Coriolis effect is the reason why winds don’t move in a straight line but rather curve. This is caused by the earth's rotation. Sometimes, this can be a confusing topic. The best way to explain this is by trying to draw a straight line on a spinning cup. The line will be curved because the cup is spinning.
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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