systems to extend into the gulf area setting up the prevailing northerlies, 

 which on the average are the highest winds, from November through February 

 (Corps of Engineers 1973). 



Annual wind roses for three stations in coastal Alabama for which data is 

 available are shown on the atlas maps; Mobile, Batesfield (Mobile quadran- 

 gle), Dauphin Island (Biloxi quadrangle) and Foley, Alabama (Pensacola 

 quadrangle). The wind roses show the percentage of the time the wind blows 

 at each velocity and direction. 



Extratropical Cyclones 



Some 15 to 20 significant frontal systems, or winter storms and their 

 associated polar air masses penetrate the Gulf of Mexico each year, bringing 

 cool air and strong northerly winds. From October to March these systems 

 produce rapid temperature drops and the highest frequency of winds above 

 33 kn. The extratropical cyclones (low pressure systems with counter- 

 clockwise winds) are usually followed by associated migratory anticyclones 

 (high pressure systems with clockwise winds). Continental systems continue 

 to affect the gulf until the Bermuda High strengthens its influence in the 

 early spring (U.S. Army Corps of Engineers 1973). 



Tropical Cyclones and Hurricanes 



The majority of tropical cyclones (low pressure systems with counterclock- 

 wise winds originating in tropical regions) affecting Alabama originate as 

 cyclonic disturbances from the Abyssinian Plateau of eastern Africa. These 

 circular winds move westward across the southern Sahara desert and begin to 

 pick up energy and moisture as they move across the eastern Atlantic Ocean. 

 It is not until these storms gather size, strength, and a characteristic 

 cloud pattern that they can be observed and monitored via satellite (National 

 Science Board 1972). 



About 100 of these cyclonic disturbances form every year, but 75% dissi- 

 pate and never develop low-pressure centers with well-defined circulating 

 wind curents. About 10 per year develop sufficient strength (wind velocities 

 of 63 to 117 kmph [39 to 73 mph]) to be classified as tropical storms. 

 Approximately six of these tropical storms will strengthen into hurricanes 

 (wind velocities greater than 119 kmph (74 mph) , and an average of two of 

 these per year will impact the coast of the United States (Chermock 1976). 



Not all Atlantic hurricanes originate near the coast of Africa. A few 

 each year originate as a result of cold fronts degenerating and encountering 

 warm moist tropical conditions. Most of these hurricanes develop between 10° 

 and 20° longitude in an area called the doldrums, and none have developed 

 within 9° of the equator. The paths of hurricanes that have influenced 

 Alabama since 1887 are shown in Figure 12. Hurricanes impact Alabama from 

 June to October, most commonly in September (Figure 13). Hurricanes 

 impacting the gulf coast early in the season (June or July) generally 

 originate in the western Atlantic or western Caribbean and are usually weak. 

 Hurricanes later in the season (August and September) generally develop into 

 recognizable storms in the eastern Atlantic near the Cape Verde Islands. 



134 



