CHAPTER 15 (Moran and Morgan, 1997) One of the potentially destructive types of weather systems originating in the tropics is the synoptic (macro-) scale low pressure system that could become a hurricane. These systems, representing an organized collection of thunderstorms are formed over the tropical and subtropical oceans. Hurricanes have been classified on the Saffir-Simpson scale. A description of the hurricane threat to the Southeastern states is provided. STUDY NOTES CHAPTER 15 Figure 15.1 -- Spend a moment looking at the maps showing the region where the greatest damage occurred as Hurricane Andrew made landfall south of Miami, FL on 24 August 1992. Note that in the left panel of A, this region was roughly 15 miles across. Figure 15.2 -- Make a comparison on this surface weather map between Hurricane Allen, a tropical cyclone found over the southern tip of Texas (along the Rio Grande River near Brownsville) and an extratropical cyclone forming over the western Plains. Specifically, note the smaller and more compact size of the hurricane, with more concentric and circular isobars, than the larger midlatitude storm system. Furthermore, no fronts emanate from a hurricane since the tropical system is surrounded by a tropical air mass containing relatively uniform air temperatures. Make a comparison with the satellite image that shows Allen in Figure 15.5. Figure 15.3 -- Study the graph showing the near-surface wind speed increasing rapidly from the center of the hurricane eye (side of diagram) outward to approximately 30 to 40 km from the center, then decreasing rapidly outward from this region. Also note that the pressure decreases from the center of the eye. Figure 15.4 -- Compare the artist's schematic of a hurricane with a picture taken from a low-altitude earth orbit. As you look at this cut-away diagram, note the organization of large cumulonimbus clouds surrounding the well-defined central eye, which is relatively clear. The height of the hurricane is on the order of 10 km. Observe that a counterclockwise inflow into the hurricane near the surface, while aloft a clockwise outflow is found. Part C is essentially over the eye. Figure 15.5 -- Inspect the visible satellite image that includes Hurricane Allen on the same day as the map in Figure 15.2. Figure 15.6 -- Inspect this world map depicting the breeding grounds of hurricanes and the trajectories that these systems take as they move across the various ocean basins. Note that the hurricanes are carried first westward and then move poleward around the western flank of the subtropical high pressure cells. Also note the names of the most intense tropical cyclones (such as hurricanes and typhoons) used in the various basins to describe these most intense tropical cyclones. Figure 15.8 -- Inspect this depiction of how a storm surge produced by a hurricane can cause problems to coastal communities. Table 15.1 -- Read through the table listing the criteria for classification of a hurricane in terms of the Saffir-Simpson Hurricane Intensity Scale. You will not need to memorize the specific numbers. However, you should realize that this classification scheme is based upon the sustained near-surface winds, central pressure and the storm surge. Tables 15.2 and 15.3 -- Take a moment to look at these two tables, noting that essentially all of the 10 deadliest hurricanes occurred before 1957, at a time when advance warnings of impending hurricanes were considerably more limited than at present. On the other hand, all but 4 of the 10 costliest hurricanes have taken place in the last 30 years, reflecting the increased populations and more expensive developments that have occurred along the shorelines in regions that are susceptible to damage by hurricanes. Figure 15.9 -- Take a look at the overall structure of an easterly wave, one of the tropical weather features that can evolve into a hurricane. Note that this system moves from east to west in the trade wind belt. The red arrows signify the overall airflow (also known as streamlines). The easterly wave is a wavelike disturbance in this airflow, which produces a leading zone of divergence with clear skies, followed by a band of convergence accompanied by rain. Figure 15.10 -- Look at this diagram, noting that the apparent erratic track of hurricanes may present a forecasting dilemma. After moving from east to west across the tropical Atlantic Ocean, many hurricanes may continue into the Caribbean, while others develop in the Gulf of Mexico. Depending upon the position and the intensity of the large subtropical "Bermuda high", these systems may move northward along the western flank of the high. Figure 15.14 -- Note the example of how a hurricane may change path. The legends for Figures 15.11, 15.12, 15.13 and 15.15 are sufficiently explanatory. Read Weather Fact (Naming hurricanes) page 356. Skim Special Topic (Arctic hurricanes) page 361. CHAPTER 15 (Moran and Morgan, 1997) HURRICANES In this final chapter on weather systems, we examine hurricanes and tropical storms. Hurricanes are ocean storms that have sustained winds greater than 119 km (74 mi) per hour. We describe the characteristics of hurricanes, the geographical and seasonal distribution of hurricanes, hazards associated with hurricanes, and the life cycle of tropical storms. Hurricanes originate over certain portions of tropical seas and develop principally in late summer and autumn. Hazards of hurricanes are strong winds, associated tornadoes, heavy rains, and storm surge. Based on intensity, hurricanes are rated from 1 to 5 on the Saffir-Simpson Scale. A hurricane originates from a tropical disturbance that strengthens into a tropical depression, then a tropical storm, and finally, a hurricane. Trade winds steer hurricanes and their precursors westward until eventually they are caught in the midlatitude westerlies. Then the system moves north and northeastward. In the final sections of this chapter, we describe the hurricane threat to the southeast United States and efforts to modify hurricanes. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: describe the basic structure and characteristics of a hurricane. identify the conditions required for hurricane development. locate the major hurricane breeding grounds. explain why hurricanes are rare along the west coast of North America. explain why hurricanes are most common in late summer and autumn. identify and describe the hazards of hurricanes. describe the basis of the Saffir-Simpson Hurricane Intensity Scale. present the life cycle of a hurricane. explain why the southeastern United States is particularly vulnerable to a destructive hurricane. discuss efforts to modify hurricanes. 15 Hurricanes 354 Hurricane Characteristics 355 Distribution of Hurricanes 359 Hazards of Hurricanes 361 Life Cycle of Tropical Storms 365 Hurricane Threat to the Southeast 367 Hurricane Modification 369 Conclusions 371 Weather Fact: Naming Hurricanes 356 Special Topic: Arctic Hurricanes 361 Special Topic: Atlantic Hurricanes and West African Rainfall 370 Key Terms 372 Summary Statements 372 Review Questions 372 Questions for Critical Thinking 372 Selected Readings 373