18. Hurricanes are giant heat engines powered by the thermal energy in warm ocea

Question

18. Hurricanes are giant heat engines powered by the thermal energy in warm ocean regions and the order in colder surrounding areas. Why are hurricanes most violent when they form over regions of unusually hot water at the end of summer?

PLEASE CRITIQUE / EXPAND MY ANSWER
The hotter the water, the more thermal energy can be harnessed and the greater the temperature difference between the hot water and the colder surrounding, the more efficient the hurricane is at converting that thermal energy into mechanical work.

Explanation / Answer

On the surface of our rotating earth, the Coriolis effect prevents a surface wind from flowing directly into a region of low pressure. The wind curves as it moves and misses the low. In the northern hemisphere, the wind curves toward the right and ends up circling around the low in a counter-clockwise direction, as viewed from above (Fig. 6.7.8). The pressure imbalance causes the wind to accel- erate leftward, toward the low, but the Coriolis effect acts to make the wind curve rightward, away from the low. These two competing effects balance one another and the wind circles around and around the low. This circulation of air around a region of low surface pressure is called a cyclone. When cold air passes over a warm ocean, the air is heated and expands. High altitude winds begin to blow outward and a region of low surface pressure forms. As more cold air rushes in across the water’s surface to replace the rising air, the Coriolis effect deflects this inbound air and causes it to circulate around the low pressure region as a cyclone. In certain special cases, the circulating winds can reach very high speeds, creating a powerful storm we call a hurricane. Hurricanes only form when the conditions are exactly right, requiring several unusual circumstances to occur simultaneously. First, a hurricane will only form above a very warm ocean, one that is warmer than about 26 or 27 °C (80 °F) to a depth of at least 50 m. The hur- ricane is a giant heat engine and its source of heat is the hot water. For the storm to have a great deal of energy, The hotter the water must be prepared to deliver a very large amount of heat.the more thermal energy can be harnessed and the greater the temperature difference between the hot water and the colder surrounding, That’s why hurricanes only occur in late summer. Second, the air above the warm ocean must be relatively cool so that air that is heated by the water’s surface can rise upward to very great heights. Air that is warm enough to rise upward through colder air above it is called unstable air. As it rises, the air expands and cools, so it must be quite warm in order to continue its ascent. Third, the air rising over the warm ocean must carry a great deal of mois- ture with it., moist air releases heat when its mois- ture condenses to form raindrops and this extra heat helps to propel the air still further upward. Much of the ferocity of hurricanes and thunderstorms comes from energy released by water’s changes in phase, from gaseous water vapor to liquid water or solid ice. Finally,the more efficient the hurricane is at converting that thermal energy into mechanical work. the low and high altitude winds must be favorable or the hurricane will rip itself apart before it starts. At low altitude, the cool winds must converge on the warm ocean water. At high altitude, the winds must help to carry away the air that rises through the center of the storm. When all of these requirements are met, a region of unusually low surface pressure forms, producing a hurricane. Air accelerates to enormous velocities as it approaches the low, but the Coriolis effect causes it to circulate endlessly around the region of lowest pressure. A calm “eye” forms at the center of the hurricane, protected from the circulating winds by the Coriolis effect. The eye is typically about 60 km in diameter. Just outside the eye, the most violent winds in the storm form a narrow ring only a few kilometers thick. The eye itself has little wind, very low air pressure, and may even be dry and only lightly overcast. Near the center of the hurricane but outside the eye, air rises upward and carries heat away from the water. This rising air eventually travels outward as high altitude winds. These outward moving high altitude winds also experience the Coriolis effect and they curve weakly in the opposite direction from the sur- face winds. In the northern hemisphere, surface winds always spiral counter-clockwise in a hurricane. In the southern hemisphere, the spiral is clockwise because the Coriolis effect is reversed. Because of these opposite spirals, a hurricane spawned in one hemisphere can never cross the equator into the other hemisphere.

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