Marsden square quadrants 0012 and 0011 and 

 the northern halves of Marsden square quad- 

 rants 2012, 3011, 3002, and 3001 or about Wr 

 of the wind field's area during 40 ';f of the year. 



In contrast to the small seasonal movement 

 of the oceanic high pressure cells, the strong 

 thermal reaction of the African continent 

 causes its low pressure cell to move from a 

 Northern Summer latitude of lat 25° N to its 

 Southern Summer latitude of 15° S. Through- 

 out the year, this low pressure cell affects sur- 

 face winds of the area just ofl"shore from Cape 

 Palmas to Luanda. Over these coastal waters 

 this strong low pressure cell changes the flow 

 of the South Atlantic High from southeasterly 

 to predominantly southerly and southwesterly. 



The west-east oriented area formed by Mars- 

 den square quadrants 3011, 3002, and 3001 has 

 only a small seasonal change in wind speed. 

 A comparison of the wind fields for February 

 and August shows that this area has only a 

 to 5''r change in frequency of wind speed 

 greater than 10 knots; the ai'ea of zero change 

 is about one-half Marsden square quadrant in 

 size and is centered over Marsden square quad- 

 rant 3002. In contrast, a sharp gradient of 

 seasonal change in wind frequency occurs to 

 the north and south, and (more steeply) to the 

 northeastward towards Port Harcourt. Also 

 centered over Marsden square quadrant 3002 

 is an area of convergence (Trewartha, 1968, 

 p. 100, Fig. 337), which is about the size of 

 one Marsden square quadrant. This area of 

 convergence persists months after the larger 

 ITC, earlier described, has moved northward 

 out of the wind field. Further study could pos- 

 sibly correlate other interface parameters with 

 this coincident area of small change of con- 

 vergence and frequency of wind speed. 



INTERPRETING THE CHARTS 



The wind data are portrayed in the first 

 section by wind rose charts. Wind rose charts 

 are interpreted as follows: 



To determine the percentage of winds in a 

 Marsden square quadrant from any one di- 

 rection, measure the total length of the wind 

 rose arm pointing in that direction and com- 

 pare that length with the scale at the upper 

 left of each chart. The percentage of winds 

 of any force group from any one direction may 



be determined in the same manner. Thus in 

 January in Marsden square quadrant 3011 the 

 wind blew from the southeast 53';^ of the time; 

 and 17 /f of the time this southeast wind was 

 of Beaufort Force 4 (11-16 knots). 



To determine the total frequency of winds 

 falling within a certain force group, measure 

 the length of that force group on the scale lo- 

 cated in the Marsden squai-e quadrant. Thus 

 in January in Marsden square quadrant 3011 

 the wind was of Beaufort Force 2-3 (4-10 

 knots) 61% of the time. 



The percentage frequency of wind speed is 

 contoured in the second and third sections. 

 The second section shows the percentage fre- 

 quency of wind speeds of Beaufort Force 3 

 (10 knots) or less. The third section shows 

 the percentage frequency of wind speeds of 

 Beaufort Force 4 (16 knots) or less. 



ACKNOWLEDGMENTS 



Robert W. Wilder and Dennis C. Yazell, 

 students of St. Petersburg Junior College, as- 

 sisted in plotting wind roses from computer 

 data. 



LITERATURE CITED 



FLOHN, H. 



1969. Climate and weather. (Translated 

 from the German by B. V. de G. Wal- 

 den.) McGraw-Hiil Book Co., New 

 York, 253 p. 

 MAZEIKA, P. A. 



1968. Mean monthly sea surface temper- 

 atures and zonal anomalies of the trop- 

 ical Atlantic. Ser. Atlas Mar. Environ. 

 Am. Geogr. Soc. Folio 16. 

 TREWARTHA, G. T. 



1968. A.n introduction to climate. 4th ed. 



McGraw-Hill Book Co., New York, 408p. 



U.S. NAVAL OCEANOGRAPHIC OFFICE. 



1963. Oceanographic atlas of the North 



Atlantic Ocean. Section IV. Sea and 



swell. U.S. Nav. Oceanogr. Off., Publ. 



700, 227 p. 



U.S. NAVY. 



1955. Marine climatic atlas of the world, 

 Volume 1. North Atlantic Ocean. NA- 

 VAER (Nav. Aeronaut.) 50-1C-528, 

 xviii + 275 charts. 



