27. What causes hurricanes and how do they differ from 

 typhoons? 



Hurricanes are great heat engines, much like the gasoline engine in a 

 car. The moisture in the humid air over the sea is analogous to the gaso- 

 line in the gas tank; it contains the potential energy (or fuel) for the 

 hurricane. Once the hurricane is born, it draws moist air up from the sea 

 surface in a counterclockwise spiral to the condensation level. Here 

 cooling of the air, due to reduced pressure, condenses water vapor in 

 the air. This can be equated to the combustion cycle in the gasoline 

 engine; it converts potential energy to kinetic energy. 



The latent heat of condensation (597 calories per gram of water) 

 heats the air, which then accelerates in its upward spiralling journey. It 

 literally goes "up the chimney" formed by the relatively cooler air 

 around it. At the top of the chimney of cooler air, the warm air spreads 

 outward in a clockwise spiral (when viewed from above). As air spirals 

 upward, through and out of the chimney, it draws more warm, moist 

 air into it from below. This self-perpetuating process intensifies the 

 circulation, causing the engine to run faster and causes the hurricane to 

 increase in size. 



The exact mechanism of hurricane formation is still unknown. Sci- 

 entists know that very warm ocean water is required. The warmer the 

 water, the greater will be the volume of moisture (potential energy) 

 carried aloft. A storm must be some distance away from the Equator in 

 order to start spinning, because the spin of an object on the earth varies 

 directly with the sine of the latitude. There must be an outward (diver- 

 gent) flow of air in the high atmosphere; otherwise the chimney would 

 be closed off. 



The origin of a hurricane is associated with an area where air con- 

 verges and showers occur. This may be a remnant of low pressure from a 

 cold front which moved far south; it may be an area of lower pressure 

 moving westward in the Trade Wind Belt (easterly wave); or it may be 

 an area where air from the two hemispheres converges (intertropical 

 convergence zone). The origin could be due to oscillation of the great 

 high pressure system which dominates the ocean. 



Hurricanes and typhoons are alike in origin, structure, and features, 

 their only difference being the area of the world in which they occur. 

 Hurricanes occur in the waters adjacent to North America (North Atlan- 

 tic Ocean, Gulf of Mexico, Caribbean Sea, and Southeastern North 



27 



