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Earth's Rotation
We have seen that the tropical regions receive more energy per unit time
than the polar regions and that this energy is redistributed around the planet by
the movement of the atmosphere and ocean. To understand how this happens,
we need to review the effect of the earth's rotation on air and ocean movement.
If we forgot for a moment that the earth rotates on its axis, like Figure 8, then the excess energy in tropical areas would cause the warm, less dense air to rise. As it
moves away from the earth's surface, it would be replaced by air coming from
either of the poles. Over North America, a persistent wind from the north at
the surface would carry air to the tropics where it would rise and drift
northward at high levels to the North Pole where it would sink to the surface
and then return to the south. A symmetrically similar pattern would develop in
the Southern Hemisphere.
In reality, however, since we observe motions relative to fixed locations on the earth, the rotation of the earth creates an "apparent" force on any moving object or fluid. We call this the Coriolis force. In the Northern Hemisphere it exerts a force to the right (and to the left in the Southern Hemisphere) so a parcel of air moving toward the North Pole will be deflected toward the right. Air moving toward the equator is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, resulting in a persistent surface wind from the northeast on the north side of the Equator and from the southeast on the south side of the Equator. This creates what we call a sub-tropical high-pressure region at about 30° north and south of the Equator where air moving poleward at high levels does not go directly to the polar region but, rather, is deflected eastward and subsides back to the earth's surface (see Figure 9).
On the poleward side of this circulation cell in each hemisphere is another cell that rotates in the opposite direction: air at the surface moves toward the pole, and air at high levels moves toward the tropics. This leads to a generally westerly wind in the middle latitudes (30° to 60°) north and south of the Equator. In the United States, we have a generally west-to-east movement of weather systems. At latitudes higher than 60o, a third circulation cell exists with surface flow away from the pole and poleward flow aloft. The Coriolis force creates winds generally from the east at the surface at these high latitudes.
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