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exceptweath2 - Exceptional Weather II EENS 204 Tulane...

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This document last updated on 24-Mar-2004 EENS 204 Natural Disasters Tulane University Prof. Stephen A. Nelson Exceptional Weather (Part 2) Hurricanes (Tropical Cyclones) Hurricanes are massive tropical cyclonic storm systems with winds exceeding 119 km/hr (74 miles/hour). The same phenomena is given different names in different parts of the world. In the western Pacific they are called typhoons , and in the southern hemisphere they are called cyclones . But, no matter where they occur they represent the same process. Hurricanes are dangerous because of their high winds, the storm surge produced as they approach a coast, and the severe thunderstorms associated with them. Although death due to hurricanes has decreased in recent years due to better methods of forecasting and establishment of early warning systems, the economic damage from hurricanes has increased as more and more development takes place along coastlines. It should be noted that coastal areas are not the only areas subject to hurricane damage. Although hurricanes loose strength as they move over land, they still carry vast amounts of moisture onto the land causing thunderstorms with associated flash floods and mass-wasting hazards. Origin of Hurricanes z When a cold air mass is located above an organized cluster of tropical thunderstorms, an unstable atmosphere results. (This is called a tropical wave ). This instability increases the likelihood of convection, which leads to strong updrafts that lift the air and moisture upwards, creating an environment favorable for the development of high, towering clouds. A tropical disturbance is born when this moving mass of thunderstorms maintains its identity for a period of 24 hours or more. This is the first stage of a developing hurricane. z Surface convergence (indicated by the small horizontal arrows in the diagram below) Exceptional Weather II 3/24/2004 Page 1 of 15
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causes rising motion around a surface cyclone (labeled as "L"). The air cools as it rises (vertical arrows) and condensation occurs. The condensation of water vapor to liquid water releases the latent heat of condensation into the atmosphere. This heating causes the air to expand, forcing the air to diverge at the upper levels (horizontal arrows at cloud tops). z Since pressure is a measure of the weight of the air above an area, removal of air at the upper levels subsequently reduces pressure at the surface. A further reduction in surface pressure leads to increasing convergence (due to an higher pressure gradient), which further intensifies the rising motion, latent heat release, and so on. As long as favorable conditions exist, this process continues to build upon itself. When cyclonic circulation begins around the central low pressure area, and wind speeds reach 62 km/hr (39 mi/hr) the disturbance is considered a tropical storm and is given a name. When wind speeds reach 119 km/hr (74 mi/hr) it becomes a hurricane. Note that all tropical waves, disturbances, or storms do not necessarily develop into hurricanes. To undergo these steps to form a hurricane, several environmental conditions must first be in
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exceptweath2 - Exceptional Weather II EENS 204 Tulane...

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