PROCESSING OCEANOGRAPHIC DATA 



Pm = niean density of the water column above the 



level of reversal, 

 C = pressure coefficient of the unprotected ther- 

 mometer, expressed in degrees centigrade in- 

 crease in the reading per 0.1 kg. /cm.' increa.se 

 in pressure. As so defined, Q has a magnitude 

 of roughly 0.01. Q is given on the thermom- 

 eter calibration certificate, 

 AD = meters depth bv which D differs from 100 



(7'„-rj. 



In evaluating the equation for depth, all 

 quantities have been determined except the 

 mean density of the water column, p„. This 

 may be obtained by plotting the densitj- in situ, 

 Ps. t. p, against depth and averaging the curve by 

 numerical integration from the surface to each 

 required depth. Density in situ determination 

 is given in section D. 3, page 15. 



For work in a limited area it is sufficient to 

 establish a mean densitj', p„, for use at each 

 level. In the mid-latitude area near 30° N. and 

 120° W., p„ has the following values: 



Figure 8. — Mean density of water columns above indi- 

 cated depths in the sea (30° N., 120° W.). 



Depth of reversal of the thermometers may 

 be found by substituting directly in the formula 



D= 



T„ 



nQ 



or by prepared tables or graphs for use with each 

 unprotected thermometer in limited areas where 

 Pm remains relatively constant for the same 

 depths throughout the area. To calculate such 

 a table, as shown in figure 9, Q is constant for an 

 individual thermometer (e. g., 0.00920) and 

 Tu—T^ varies by 0.01° C. steps; however, p„ 

 must be increased with increasing depth (or 



Tu—Tu,), depending on the established vertical 

 density of the area. Depth is obtained directly ; 

 for example, when T,,— 7'„=6.73 the depth is 

 711 meters. 



Figure 9. — Portion of table for determining thermometric 

 depths from corrected thermometer readings Tu — Tw 'or 

 an individual unprotected thermometer. 



A graph corresponding to figure 9 is some- 

 times used; however, both table and graph 

 must be large to obtain the necessary accuracy. 

 For this reason a graph of depth anomalies is 

 recommended. 



The depth anomaly graph used to facilitate 

 the calculations of depth achieves accuracy by 

 giving only the deviations from an easily 

 determined standard. Since p„ is nearly unity 

 and Q is roughly 0.01, depth of reversal is 

 appro.ximately 100(r„— T„). Any deviation is 

 expressed by an anomaly, I)=\^{){T^—T^ 

 + Ai9. As shown in figure 10 for a thermom- 

 eter of Q = 0.00920, the anomaly is plotted 

 against values of T^—T^. To find depth of 

 reversal of the thermometers, AD is read from 

 the graph and added algebraically to 

 100(r„— r,f) corresponding to the given 

 T„—Tu,- To prepare the graph, D, Dp„Q, 

 and D— 100 (Dp„Q) are tabulated as follows: 



D 



Dp^Q D-100{Dp^Q) 



The last two columns, which may equally well 

 be labeled T^—Ty, and AD, are plotted in 

 figure 10. 



Recent investigations indicate that Q may 

 not be constant, but instead may decrease very 



