Figure B-SO. The Secchi disc. 



attached to the Secchi disc should be marked off 

 in 1 -meter intervals to at least 50 meters. It is 

 recommended that 14-inch tiller line with a 

 phosphor bronze core, which minimizes stretch- 

 ing, be used. 



The Secchi disc is designed to measure trans- 

 parency and is dependent upon the available 

 illumination which varies with the time of day, 

 cloud formation, and amount of cloud cover. The 

 Secchi disc observations are recorded at the top 

 of the A-Sheet and must be taken at the same 

 time the associated meteorological data is taken 

 for that sheet. 



To obtain Secchi disc observations, the Secchi 

 disc with the white side up should be lowered 

 into the water from the shaded side of the ship 

 until the disc is just perceptible, and the depth 

 in meters noted. The lowermg is then continued 

 until the disc is no longer visible. The disc is 

 then slowly raised until it is again barely visible. 

 The depth reading of this point is then averaged 

 with the reading obtained on lowering and is 

 recorded as given above. This same procedure is 

 repeated using the black side of the disc. Wliite 

 and black Secchi disc observations for 15 and 10 

 meters are recorded as Wliite 15 Black 10. 



B-24 Determining Water Color with the 

 Forel Scale. — The standard Forel scale consists 

 of a series of 11 small vials containing ammonia- 

 cal copper sulphate and neutral potassium 

 chromate in such proportions tliat a different 

 graduation of color is imparted to each vial. 

 These vials are numerically designated and are 

 compared directly with the water in the manner 

 described below. 



The water color is most easily determined in 

 conjunction with the Secchi disc (fig. B-31). 

 After completion of the transparency measure- 

 ment described in B-23, raise the white Secchi 

 disc until it lies approximately 1 meter below 

 the surface. The number of the vial that blends 

 most closely with the water color against the 

 Secchi disc is recorded on the A-Sheet. The 

 whiteness of the disc provides the background 

 to which the color is referred ; this color may not 

 be the color of the sea surface visible away from 

 the ship. The vials must be shaded from open 

 sunlight when the determination is being made. 



Figure B-31. 



Obtaining water color with the Forel 

 scale. 



B-25 Ice Observations. — Ice observations are 

 made from aboard ship, from land stations, and 

 from aircraft, with each type obseiration offer- 

 ing specified information. 



Shipboard observers are in a position to ex- 

 amine closely the ice immediately around their 

 vessel. From this vantage, they can determine 

 the texture and solidity of the ice, variations in 

 thickness, state of deterioration, and other fea- 

 tures requiring actual contact with the ice. 



Estimated ice coverage, or concentration, in 

 the immediate vicinity of the ship is recorded 

 to the nearest tenth on the A-Sheet ; however, for 

 comprehensive ice observations, H.O. Pub. No. 

 606-Hd, Ice Ohservations., should be used as a 

 reference, and observations should be recorded 

 on the Ship Ice Log. 



B-15 



