862 



BULLETIN OF THE BUREAU OF FISHERIES 



It is natural to think of the wind as partly responsible for these variations in 

 the direction and velocity of the drift, and this is borne out by the following table 

 giving the wind movements and directions at Portland, Me., for each month, and 

 the resultants calculated therefrom by traverse tables. ^° 



Month 



Wind movement, miles 



N. NE. 



BE. 



SW. W. NW 



Resultant 



October, 1913-- 

 November, 1913, 

 December, 1913. 



June, 1919. 



July, 1919 



August, 1919 



September, 1919 

 October, 1919... 



2,471 

 933 



I, SIS 

 362 

 382 

 382 

 690 

 695 



449 

 132 

 443 

 464 

 186 

 382 

 675 

 407 



597 

 425 

 235 

 836 

 551 

 623 

 485 

 449 



813 

 442 

 232 

 400 

 411 

 605 

 462 

 679 



667 

 916 

 208 

 1,904 

 2,094 

 1,465 

 2,088 

 1,116 



574 

 1,738 

 1,422 

 584 

 826 

 863 

 638 

 870 



264 

 664 

 942 

 348 

 1,013 

 536 

 504 

 758 



1,247 



1,701 



2,265 



875 



624 



983 



1,097 



1,020 



N. 2° W., 2,030. 

 S.84° W., 2,274. 

 N. 50° W., 3,697. 

 S. 3° E., 1,290. 

 S. 28° W., 2. 279. 

 S. 33° W., 1,247. 

 S. 27° W., 1,118. 

 S. 73° W., 1,073. 



When the directions and velocities of winds and currents are compared for the 

 individual months it becomes clear that the drift is not purely a wind current, though 

 considerably affected by the wind. With winds prevaihng from anywhere between 

 north and west, the drift has a southerly component, driven eastward and offshore 

 by strong west winds (as in November, 1913), but setting toward the southwest, 

 when the average wind direction is between north and west. It is when drifting 

 southward (whether with an easterly or a westerly component) during periods when 

 winds prevail between west and north that the surface set attains its greatest daily 

 velocities of 9 to 11 miles per day. By common knowledge this applies also during 

 northeast winds. During the one month (June, 1919) when south winds prevailed, 

 the current ran, none the less, toward the southwest, though held back by the head 

 wind to an average rate of only about 4 miles per day. The dominant drift was also 

 very slow (0.5 to 2.4 miles per day) during the three months when southwesterly 

 winds prevailed, setting against the wind (WSW.) for one month, but with the wind 

 (between north and east) during the other three. 



According to this correlation between current and wind, the direction of the 

 nontidal current at this station is between WSW. and SE. and reaches a considerable 

 velocity when westerly or northerly winds prevail; but its inherent stength is so 

 small that southerly winds greatly reduce its velocity, or may even reverse it and 

 produce a slow sui'face drift toward the northeast. 



The wind table for Portland (p. 965) shows that the average direction of the wind 

 there, from early autumn until April, is between northwest and a few degrees south 

 of west." Consequently we may assume that the dominant sets recorded at the 

 lightship for the months of October, November, and December are representative 

 for autumn, winter, and for the first two months of spring. These combined (by 

 the traverse tables) give a resultant movement toward the south and west (S. 15° 

 W.) at an average rate of about 8 miles per day. In spite of the prevalence of south- 

 west winds in summer the resultant of the combined drifts for June, July, August, 



w From data supplied by the United States Weather Bureau, 

 every-day parlance. 



" Calculated on a time-percentage basis. 



The directions are those from which the wind blows, as in 



