20 Expedition of 1906 



during the month of April; during May it increases in force, 

 reaching its maximum from June to September. The direction of 

 the wind does not remain constant during this period but oscillates 

 principally between south-east and east. The high steep coasts of 

 Java, which extend in a north-westerly direction close to Sumatra 

 and considerably reduce the breadth of the strait, frequently screen 

 the strait and Sumatra from the south-east monsoon and a westerly or 

 south-westerly wind takes its place. The south-east monsoon brings a 

 fine season with prevailing dry weather and comparatively light 

 winds. At the period of its maximum force, especially in July and 

 August, the average strength of the wind reaches 3 degrees Beaufort 

 (5'5 met. per second) ; stronger winds of 6 to 7 degrees Beaufort (13*5 

 to 16*5 met. per second) are rare in the Sunda Strait area and in the 

 more northerly region to which the data given in the following tables 1 

 refer. 



The change of monsoon occurs between September and November ; 

 it is introduced by the occurrence of southerly and westerly winds. 

 The season of the typical north-west monsoon (which is the north- 

 east monsoon deflected by the earth's rotation after crossing the 

 equator) begins in November and lasts until March. This brings bad 

 weather ; the rainfall increases in amount and reaches its maximum 

 in January and February. The direction of the wind (table, p. 22) 

 oscillates from west to north-north-west : its average force being 

 approximately that of the south-east monsoon, about 3 degrees 

 Beaufort. Tn the ship's log an intensity of 1 3 occurs frequently ; 

 more rarely, for a period of several days, 5 6 and exceptionally 

 an intensity of 7 is recorded (see the accompanying table). At the 

 end of March or in April there is another change of monsoon. There 

 is no prevailing direction for the winds of this period (table, p. 22) : 

 winds blowing irregularly from all points of the compass alternate 

 with calms, sudden gusts of wind and cyclones. At all seasons the 



1 Seyelhandbuch fur den indischen Ozean, p. 56. The scales for wind-velocities 

 most commonly used are the Beaufort scale of twelve divisions (0 = calm, 12=hurri- 

 cane) and the six- and ten-division scales derived from it. According to the most 

 recent researches (Hann, J., Lehrbueh dor Meteorologie, Leipzig, 1901, pp. 376, 377) 

 the following wind-velocities correspond, on an average, to the following values for 

 wind-intensity according to the twelve-division Beaufort scale : 

 Beaufort intensities 



(estimated): 1 2345 6 7 8 9 10 11 12 



Average velocity in 

 metres per second 

 (after Koppen): 1'7 31 4'8 67 3*8 10*7 129 154 180 210 26 4050. 



On reduction to the 10-di vision scale used above: 

 Estimated intensity (010 Beaufort): 1 2 3 4 5 6 7 8 9 10 

 metres per second : 2 3"5 5\5 8 105 135 16'5 225 28 30 



