patrol boat General Greene is employed in constructing current charts 

 based on the dynamic topography of the sea surface calculated from 

 temperature and salinity observations made at several subsurface 

 levels at a number of stations. This vessel is only able to prepare one 

 current map about every 3 weeks. The current maps are of invaluable 

 service to the Ice Patrol but, as the current is subject to rapid changes, 

 the map can only be relied upon for from 1 to 2 weeks after its comple- 

 tion. Therefore, only through the surface temperatures supplied by 

 the passing steamers can the limits of the cold water be followed and a 

 rough idea gained of the changes in the course and position of the 

 Labrador Current. In addition, the approximate drift and an esti- 

 mate of the length of life of a berg can be predicted from these charts. 

 ^'Vlien tropical air masses cover cold water, fog is generally formed and 

 the limits of fog can also be estimated by the use of the surface iso- 

 therm charts. This information is used in conjunction with daily 

 weather maps for forecasting the coming weather in order that effi- 

 cient scouting may be planned. 



Many requests for special information are received in the course of 

 a season. The Ice Patrol force is most anxious to render every possi- 

 ble aid to masters of vessels. In order to furnish complete and intel- 

 ligent information in answer to a special request, it is absolutely neces- 

 sary that the vessel's request be accompanied by its position, course, 

 speed, and destination. 



During the fifth and sixth cruises of the past season rapid changes 

 were taking place in the position of the cold wall, as the Labrador 

 Current water was rapidly receding northward. At the close of the 

 fifth cruise (June 6, 1939), the 50° isotherm extended south to latitude 

 39°33' N. By June 9, following a southerly gale of 24 hours dura- 

 tion, the southern extremity of the 50° isotherm had receded north- 

 ward to latitude 40°53' N., a distance of 80 miles. However, a small 

 pool of water, with temperatures greater than 70° F., existed within 

 the southern projection of the 50° isotherm on June 6. It is not known 

 to what depth the warm water extended. There was no tendency 

 for the 50° isotherm to return southward and after a brief period it 

 continued its northward recession. (See fig. 1 for a comprehensive 

 picture of the change that took place during this period.) 



The scientific program of the 1939 season consisted of daily radio 

 meteorograph observations from April 1 to June 30 and periodic 

 topographic surveys of the area of principal ice activity. Four current 

 maps were constructed during the season. In July the General Greene 

 made an oceanographic cruise from South Wolf Island, Labrador, to 

 Cape Farewell, Greenland. 



Following this section is a discussion of the ice conditions by months 

 with a brief summary. A detailed account of the weather has not 

 been included as it is belie^^ed to be of little significance due to the 



