upper air circulation. They usually pass an area within a day or two, although 

 sometimes they remain stationary over an area for several days. 



Tropical cyclones, in contrast, are believed to originate from pre- 

 existing disturbances called easterly waves, which are found in the tropical 

 oceans in the 8° to 15° latitude band. The conditions that promote their for- 

 mation include low level cyclonic vorticity, a Coriolis force, minimal vertical 

 changes in velocity, sea surface temperatures warmer than 27 °C, unstable 

 lapse rates, and high humidities at mid-tropospheric heights (Riehl 1979). 

 Such disturbances do not form in oceans such as the southern Atlantic, which 

 do not meet these conditions. 



If the storm intensifies and average wind speed increases, tropical 

 cyclonic disturbances become tropical depressions, tropical storms, and finally 

 hurricanes, with average wind speeds in excess of 75 mph. Increasing wind 

 speeds are caused by an increased pressure gradient, where central pressures 

 in severe hurricanes can fall below 900 mb. The great majority of tropical 

 cyclones develop in the late summer and early fall in both hemispheres. As 

 with other cyclones, meteorologic events follow a characteristic sequence 

 (Figure 1) that affects coastal geomorphic features, depending upon the rela- 

 tionship of the storm orientation and track to the coast (i.e., Penland and Suter 

 1984). 



The geomorphic activity of tornadoes and waterspouts, because they are 

 small in size and develop and die rather quickly, is not on the same scale as 

 other cyclones. Tornadoes are associated with cumulonimbus or thunderstorm 

 clouds, and are most common in zones where air masses of contrasting 

 temperature and moisture meet, such as the interior of the United States. 

 Occasionally, tornadoes and waterspouts affect coastlines, with most damage 

 being caused by high wind speeds; wind duration and fetch are typically insuf- 

 ficient to develop large waves. 



The relative influence of extratropical cyclones compared to tropical 

 cyclones depends largely on latitude. Closer to the tropics (e.g., the 

 U.S. Gulf coast and southern U.S. Atlantic coast) tropical cyclones have a 

 higher frequency of occurrence than farther poleward. Farther poleward, 

 (e.g., in the northern U.S. Atlantic coast) extratropical cyclones and 

 associated fronts occur more frequently and typically exhibit greater 

 magnitudes. In the lower mid-latitude climates where both may occur, an 

 issue of some debate concerns the relative importance of these storms, as it is 

 unknown whether the higher frequency-lower magnitude extratropical cyclones 

 have a greater geomorphic impact than the lower frequency-higher magnitude 

 tropical cyclones. 



Chapter 2 Relevant Processes and Factors 



