10 



In spite of the established value of the curient maps they do have 

 their limitations. It is impossible at present to foretell how long 

 the picture of the current system portrayed by the map will be a true 

 one. Attempts are being made to forecast from one map what the 

 situation will be on the next, much as is now done with weather 

 charts. More research is necessary, however, before the feasibility 

 of the scheme can be established. For purposes of the Patrol the 

 maps are not depended on more than T to 10 days after completion. 

 The average time consumed in collecting the data is 7 or 8 daj-s so 

 that any single oceanographic station has been observed to be good 

 for 15 days. The present monthly charts show a complete change of 

 conditions between charts. Bergs observed in this interim period 

 are apparently carried by currents which indicate conditions inter- 

 mediate between the two maps. In other words semimonthly maps 

 seem to be necessary in order to catch these changes. It has been 

 thought that several days of gales would materially distort the cur- 

 rent picture as represented on the map. Within the period of relia- 

 bility (15 days) of the map this has not been found true. While 

 wind may be responsible for the final arrangement of the current 

 system, 1 or 2 clays of high wind does not immediately change an 

 established system. Much more missleading is an extended period 

 of calm clear weather with its attendant intense solar warming. 

 From surface-temperature records one is in danger of concluding 

 the cold stream has entirely ceased or the warm water has suddenly 

 pushed northward. A few meters below the surface, however, are 

 found low temperatures indicating that the cold current exists, much 

 as before. A few hours of wind stirring will restore the surface 

 thermal gradient almost unchanged. As these cold subsurface cur- 

 rents are the controlling influence in the drift of bergs, the' patrol 

 cannot be guided by surface temperatures under such conditions. 

 The maps are further limited in that each map covers only a small 

 portion of the berg-infested area. 



Because of the small areas covered bj^ the Patrol vessel and the 

 current surveys the Patrol is dependent on the many merchant 

 vessels passing through this region for a general picture of ice 

 conditions at any particular time. These ships are requested, in 

 each broadcast, to report, besides all ice sighted, their position, course, 

 speed, and weather every 4 hours, while in the ice-patrol area. Not 

 only do these reports give the Patrol excellent information about 

 the quantity and distribution of ice over the entire North Atlantic, 

 but enables the Patrol to plot the track of every vessel and to send out 

 appropriate warnings and advice to those who are standing into dan- 

 ger. Thus the cooperating ships are of immeasurable help to the 

 Ice Patrol and, at the same time, contribute greatly to their own 



