Tropical Cyclone Formation and Energy Source
So, where does the energy to produce 170 mph winds come from in a tropical cyclone? The short answer is that warm water contains a lot of heat energy. In the process of extracting a small fraction of the huge amount of heat energy of the top few feet of tropical ocean water, a tropical cyclone uses some of it to move around the air in the storm. The energy embodied in the motion of 3000-pound car that is traveling at 60 miles an hour is roughly the same as the amount of energy it takes to raise the temperature of an inch thick layer of water 10 feet square by 1 degree Fahrenheit!
Image from NOAA.
Tropical oceans are pretty warm year-round, but those in the northern hemisphere are warmest at the end of our summer -- around August-September (summer is 6 months offset in the southern hemisphere, so oceans are warmest around February-March). Temperatures in broad parts of the ocean, as shown above, are above 28 degrees celsius, or 82 Fahrenheit.
Image from Wikipedia.
Above is an image of the tracks of all historically known tropical cyclones. You can clearly see that tropical cyclones can't form or pass too close to the region very near the equator. Much of the equatorial ocean is warm enough to allow tropical cyclones year-round, but they need the earth's rotation to give them spin, and this effect becomes small very close to the equator -- as shown by the figure below. Also, tropical cyclones rarely form in the South Atlantic or Southeast Pacific -- waters are generally too cold.
Image from Lyndon State College.
A tropical cyclone can form from an area of disturbed weather where there is convergence of air, such as in group of thunderstorms, over a tropical or subtropical ocean. Since air can't be compressed too much by normal atmospheric motions, convergence forces air to rise. As a column of air in the disturbance rises, stretching in the vertical direction magnifies the spin the column already has due to being on the rotating earth, as shown in this diagram.
Image from Wikipedia.
Air at the surface moves towards the center of the disturbance because the pressure is lower there, and as it moves over the surface of the ocean, it picks up heat and moisture. Both of these cause the air to become less dense, and start to rise. As the surface area of low pressure becomes stronger, air rushing in at the surface moves more rapidly, and can get heat and moisture from the ocean more effectively. The rotation of the storm contributes to the the surface wind speed, allowing for even faster heat and moisture transfer from the ocean. Higher in the atmosphere, the rising air cools, and as the moisture condenses out of it, heat is released, causing the air to rise even further. Condensation of water vapor releases a lot of energy -- condensing enough water vapor out of the air to make a cup of liquid water would release an amount of energy equal to the energy of our speeding car example above!
Image from University of Illinois.
