Actually all three of these conditions can and do exist. Where exceptionally 

 careful personnel are employed and exceptionally good secondary measuring in- 

 struments are used, individual correction should be applied, since this corresponds 

 to condition (1). Where Bathythermographs have been taken using injection ther- 

 mometers for corrections, it has been shown that these correspond more closely to 

 condition (2) and average corrections are in order or even no corrections at all. 

 All Bathythermograph data should be studied to determine whether they correspond 

 to condition (3), and if so which would be the better method to employ, individual 

 correction or average corrections. Failure to use the best method for correcting the 

 data can inject greater sources of error than not employing any correction to the 

 Bathythermograph slide. 



8.3.3. Factors Affecting Zero Depth Reading of the Bathythermograph 



It has been pointed out previously that the zero depth value of the Bathythermo- 

 graph may in actuality not be a zero depth value at all but is the value at whatever 

 depth the pressure element of the Bathythermograph responded. Should, for in- 

 stance, the Bathythermograph pressure element stick until the depth of five feet is 

 reached before it suddenly moves, the line it forms from to 5 feet will register 

 the temperature at five feet. Furthermore, it is at the surface where considerably 

 high gradients as well as thermal microstructure exist. Failure of the Bathythermo- 

 graph to respond either in temperature or pressure in the initial depth of even one 

 foot will produce erroneous results when attempting to apply correction factor at 

 that point. 



8.3.4. Reliability of Surface Bucket Samples 



Not only does the extreme temperature gradient in the few inches of the surface 

 have an effect upon the actual value recorded by the Bathythermograph, it also has 

 a considerable effect upon the reliability of the sample which is taken by the bucket. 

 Thermal gradients exist not only vertically, but also horizontally. The surface bucket 

 scoops up a sample of the first few inches of the surface of the water. The temperature 

 that this represents is being compared to the zero depth reading of the Bathyther- 

 mograph which is in reality some average of the first few feet due to the inherent lag 

 of the instrument in the initial descent. Should there be any time lag between the 

 taking of the bucket sample and the lowering of the Bathythermograph, it is quite 

 possible that the bucket will scoop up a sample which is separated from the point 

 at which the Bathythermograph was lowered by an extreme horizontal gradient. In 

 other words, the bucket sample is a poor representation of the surface conditions of 

 the ocean since it samples such a small amount of the surface, and this amount is 

 subject to random surface variations which will not be reflected in the Bathyther- 

 mograph record. This is particularly true on the edges of currents. 



Where injection temperatures are read, the injection intake is considerably be- 

 low the surface of the water. Thus the injection temperature does not represent a 

 surface reading, but a reading in the subsurface, the distance below the surface 

 being a function of the pitch and roll of the ship. The injection thermometer cannot 

 be regarded, therefore, as a reliable indication of the surface conditions at the point 

 where the Bathythermograph was lowered. 



The attempt to compare Bathythermograph surface value with an independent 

 surface temperature measurement is, in theory at least, a poor technique because of 

 the fact the sampling is being done in the most unstable portion of the ocean. Com- 

 parisons would be much better made at some considerable depth where the ocean is 

 more stable and where conditions do not vary so radically in their spacial relation- 



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