Global Air Circulation!

Miss Tang taught us more on Global Air Circulation for today's lesson. :)

In the previous lesson, we covered the distribution of the earth's climate. After learning more about the distribution of the earth's climate, some of you might have been wondering why desserts are not found at the equator since that is where the earth receives the greatest amount of solar isolation per year.

The answer to why desserts are not found at the equator is actually because of the way air moves around earth.

Miss Tang first explained how the one cell model accounted for why desserts are not found at the equator. The one cell model is based on three assumptions.

1. The Earth does not rotate.
2. The Earth is made of similar materials.
3. Wind blows form a higher pressure to low pressure.

Based on these assumptions, air circulation on the Earth should approximate the patterns on the above one cell model.

Air flows from the poles to the equator. When the air reaches the equator, it is lifted vertically by the processes of convection and convergence. Air then flows horizontally from the equator to the poles. At the poles, the air in the upper atmosphere then descends to the Earth's surface to complete the cycle of flow.

However, the one cell model does not take into consideration planetary rotation. Planetary rotation would cause the development of three circulation cells in each hemisphere rather than one and these three circulation cells are known as the: Hadley cell; Ferrel cell; and Polar cell.

Both the polar cell and the Hadley cell are driven by heat. However the Ferrel cell is induced. Unlike the other two cells, where the upper and low-level flows are reversed, a generally westerly flow dominates the Ferrel cell at the surface and aloft. The stronger downward vertical motion and surface convergence at 30°N coupled with surface convergence and net upward vertical motion at 60°N induces the circulation of the Ferrel cell.

This three cell model can explain why desserts are not at the equator. From the pervious lesson on rain formation, we know that there must be adiabatic cooling for rain to form. Air must rise, cool and condense to form clouds. However, as observed from the diagram above, there is only sinking air at 30◦ N. Hence, clouds cannot form and thus there is no rain.

The lack of rain at 30◦ N explains why desserts are found there instead of the equator which is where the earth receives the greatest amount of solar isolation per year. :)

Lastly, remember

Good Attitude
Engagement
Ownership
Growth

Is important during Geog lessons :)

Love,
Jeanette.