15 HYDKOGRAPHIC MANUAL PaGE 36 



are accompanied by a pearly translucent appearance of the sky to seaward for a few 

 degrees above the horizon. 



When operating in an island area, it is frequently possible to find clear and com- 

 paratively calm weather on one side of an island or group of islands when there is fog or 

 stormy weather on the other side. 



Fog, haze, smoke, and other atmospheric conditions seriously interfere with most 

 survey operations, and when these are prevalent, the few days of good weather which 

 intervene must be taken advantage of to the fullest extent. A shift in the wind, or 

 change in barometric pressure, is often accompanied by local clearing weather which 

 cannot be foreseen from the Government weather broadcast. 



When a survey vessel operates in an area of consistently bad weather, concerning 

 which there is little information, a detailed report of the conditions found, and how 

 best to take advantage of them, shall be made in a special report, in order that the 

 information may be utilized by future survey parties operating in the same locality. 

 (See 8584.) 

 I t i 1 15- MISCELLANEOUS PROJECT OPERATIONS 



W^ n * ?^ l^l- Standard Time 



Standard time shall be used in all hydrographic surveys, the hours being numbered 

 consecutively from (midnight) to 23 (11 p.m.). When applicable, the standard 

 meridian used must always be noted. This is important in all records to be reduced 

 for tide and in records of astronomic observations. 



It is not necessary that the standard meridian used be that of the standard time 

 zone in which the project is located. A party may keep its clocks set to daylight- 

 saving time or any other time, if desired, but the standard meridian corresponding to 

 the clock time must be noted. If the project is located in two standard time zones, 

 one standard time shall be selected for use, and the standard meridian noted in the 

 records. 



152. Tide Predictions 



A knowledge of the approximate tide is necessary in connection with almost all 

 operations of a hydrographic survey party. This is essential for planning work that 

 should be done at a certain stage of the tide. No party should ever be permitted to 

 leave the ship or a shore base without these data. It is frequently sufficient to know 

 the predicted times and heights of the high and low waters, a mental interpolation 

 sufficing for intermediate values. 



For some of the operations more exact values are necessary. When the hydro- 

 graphic survey is of an area with a comparatively large range of tide or where the bottom 

 is even, predicted tides must be used for the preliminary reduction of soundings for 

 boat-sheet plotting. They are necessary also when determining the least depth on 

 a shoal. These predictions may usually be obtained with sufficient accuracy from the 

 Tide Tables (see 1521), but in some instances a predicted tide curve may be needed and 

 will be furnished by the Washington Office, upon request. 



1521. Predictions From Tide Tables 



When the Tide Tables arc used, the procedure is as follows: 



(1) From table 2 find the tide differences applicable to the area being surveyed. Apply these 

 differences to the tide predictions for the reference station and obtain the corresponding times and 

 heights of the high and low waters covering the period of the work. 



