INTRODUCTION TO SONAR 



Figure 5-17. — Effect 



71.90 

 of daily heating. 



and usually disappears at night as the surface 

 cools off and water continues to mix. Figure 

 5-17 is a simplified illustration showing how 

 traces on a series of slides may indicate daily 

 heating or cooling near the surface. 



An isothei-mal layer is seen at 0600 (local 

 time) on a calm, clear day. From 0600 to 1600 

 the surface is shown gaining heat from the sun 

 faster than it can dissipate the heat back into 

 the atmosphere. The surface temperature in- 

 creases, and a negative gradient layer, repre- 

 sented by the crossed lines, is established. 

 This layer deepens as the heat penetrates, and 

 diffuses downward by conduction during the time 

 the surface is gaining heat. 



After 1600 the surface cools off by evapora- 

 tion faster than it receives heat from the declining 

 sun. The cooler and denser surface water mixes 

 and may destroy the negative gradient layer. 

 Sufficient cooling may occur during the night to 

 reestablish a single isothermal layer, as shown 

 at 0400. 



MARKING BT SLIDE 



Immediately after the slide is removed from 

 the BT, it must be marked as outlined herein. 

 Use a sharp pencil, and be careful that you 

 don't obscure or touch the temperature trace. 

 Information on the slide and BT logsheet must 

 agree. Figure 5-18 illustrates the information 

 to be marked on the slide. 



1. Line 1, Consecutive slide number and 

 time gi "A' Num'"^r the slides consecutively 

 between ^ cs. L JOO slides are used, they 

 are numbered from 1 to 300. Use Greenwich 



mean time (0000 to 2359), giving the hour and 

 minute at which the bathythermograph entered 

 the water. Draw a dash between the slide num- 

 ber and the time. Slide number 5, taken at 2240, 

 for example, is marked 5-2240. 



2. Line 2, Date: Day, month, and year are 

 entered as numerals. Use Roman numerals for 

 the month. For example, 29 November 1950 is 

 written as 29-XI-50. 



3. Line 3, BT instrument serial number; The 

 serial number of the BT is stamped near the 

 nose of the instrument. It is an important number, 

 because the data center that processes your 

 slides has over ten thousand grids, only one of 

 which is a duplicate of yours. Always include 

 any letter that precedes or follows the serial 

 numbers; for example, BT 1216A orBT AA-1257. 

 Without the proper instrument serial numbers, 

 the information on your slide is worthless. 



FURTHER USE OF BT SLIDES 



In addition to providing immediately useful 

 data on the temperature conditions of the sea, 

 BT slides are used in compiling charts that 

 give the ocean's average thermal conditions for 

 each month of the year. These charts are pub- 

 lished by the Navy Oceanographic Office in the 

 form of an atlas. Atlases give the average 

 thermal conditions and probable detection ranges 

 for various types of operational search sonar 

 equipments. The charts are used by planning 



Figure 5-18, 



71.91 

 - Marking the BT slide. 



76 



