|MadSci Network: Earth Sciences|
That is a very good question. Unfortunately, science does not have a concrete answer yet. Current theory includes the centrifugal force component you mentioned resulting from wind speeds near the center of the storm that are too fast for the pressure differences near the center. This results in the centrifuging effect about which you spoke. Another component is the ring of thunderstorms surrounding the eye. At the base of the thunderstorms, air is rushing together (a process called convergence). At the top of the thunderstorms, air is rushing apart (called divergence). Since the thunderstorms are arranged in a ring, some of the air rushing away from the tops of the thunderstorms meets in the center of the circle. This air rushing toward the center of the top of a hurricane is then forced down (since the top of the hurricane is at the top of the troposphere...the layer of atmosphere where weather occurs). This sinking air inside the ring of thunderstorms results in decreasing cloudiness over the center and eventually, if it goes on long enough, results in an eye. As for why the eye doesn't get bigger due to centrifugal force in stronger storms, the answer to that may be that the lower air pressure at the hurricane's center results in a stronger force than the centrifugal force. Observations from inside a hurricane indicate that the centrifugal force component may be weak...especially compared to other forces within a hurricane. In any hurricane, air rotates around the storm toward the center. It may be that the momentum of the air rotating around the storm inwardly toward the center of the hurricane becomes balanced by centrifugal force the storm exerts near its center...resulting in a certain eye size. As the air pressure at center of a hurricane decreases, the air rotates in toward the center at a faster rate...yielding higher momentum air. Since the centrifugal force produced by a hurricane at its center seems to be weak, the higher momentum air would reach balance with the centrifugal force closer to the center of the storm. To get a better idea of how this might work, picture a tetherball. It is on a rope attached to a pole. As the ball rotates around the pole, centrifugal force holds it at the end of the rope. However, as the rope shortens, the seemingly suspended ball gets closer to the pole. In this analogy, the pole is the center of the hurricane and the rope represents the increasing force generated by the falling pressure at the center of the storm. In the case of this tetherball though, the centrifugal force on the ball keeps it from reaching the pole because the rope called pressure can only shorten so much. Hope this isn't too confusing. Someday, science will have a firm answer to this question. For detailed and somewhat complicated information about eye formation, see: http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqA.html#A11 and for some general information about hurricanes see: http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqA.html
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