

TEMPERATURE DIFFERENCE, C 



Figure 12. Ogives of positive and negative differences between 153 XBT 

 profile surface temperatures and simultaneous thermistor chain temper- 

 atures made during SUDS I. 



were positive, 33.3% were negative, and 3.3% were zero. This suggests that, on the average, 

 the XBT measures a surface temperature slightly higher than the thermistor chain. The 

 differences varied from -0.36°C to 0.45°C. 



An examination of the raw data suggests that the larger differences were observed in 

 the afternoon. Figure 1 3 was prepared to examine whether this was true. Figure 1 3 is a 

 plot of the differences versus time of day. Figure 13 shows that the 12 largest absolute 

 differences (7 .8% of the 1 5 3 comparisons) were observed between 1 1 00 LST and 1705 LST. 

 Of these 12 differences, eight were positive and four were negative. Figure 14 contains 

 copies of the XBT profiles associated with the largest positive (XBT 253L) and the largest 

 negative (XBT 190L) differences. It is interesting that both these profiles have a large 

 negative temperature gradient from the surface to about 3 m. Such a large negative gradient 

 may be the result of solar heating of the near-surface waters under conditions of clear skies 

 and calm to near-calm winds. When such near-surface gradients are present, it is difficult to 

 obtain accurate simultaneous measurements of surface temperature since a small difference 

 in the depth of a sensor results in a relatively large change in temperature, and hence a large 

 difference in temperatures. It is concluded that the larger absolute differences are the result 

 of not being able to measure the surface temperature accurately. Omitting the 12 largest 

 absolute differences, more of the differences are still positive than negative, with the average 

 difference being 0.03°C and the standard deviation 0.09°C. 



38 



