SLITHERING ISOTHERMS AND THERMAL FRONTS ON THE OCEAN SURFACE 



byG. C. Ewing, Scripps Institution of Oceanography, 

 University of California, San Diego 





Compared to the sensible temperature measured by mercury thermometers in con- 

 ventional bucket samples , the equivalent blackbody temperature of the sea surface measured 

 from a low-flying aircraft is distorted to a variable degree by: (1) a component reflected 

 from the sky, always cold, and (2) the deviations of the thermal boundary layer, usually cool 

 at night and warm in the daytime (References 1,2). 



The distortion due to the reflection depends on two parameters: (a) The coefficient 

 of reflection which varies according to Snell's Law with the angle of incidence at the water 

 surface of the rays reaching the radiometer. At normal incidence the coefficient is roughly 

 .02, increasing to about .06 at 60°. (b) The equivalent blackbody temperature of the sky 

 varies with zenith angle and azimuth. On a clear night the zenith may be 50° Kelvin colder 

 than the horizon and the temperature 30° above the horizon may vary 10° from east to west, 

 being warmest in the west during the hours after sunset. The thermal 'correction" to be 

 applied to the infrared temperature to offset the contribution of the sky depends therefore 

 on the 'look angle' and the prevailing sky conditions. It may vary from under a cloudy sky 

 to 1° Kelvin on a clear day or night. It is always positive, that is, it is to be added to the 

 equivalent temperature measured by the radiometer. 



The correction for the distortion due to thermal boundary layer is sometimes positive 

 and sometimes negative, depending on whether the heat flux is directed upwards out of the 

 water or downwards. During daylight hours the sea is usually being heated from above so 

 that the heat flux is downward. The thermal boundary layer is, therefore, warm and the 

 correction to the infrared apparent temperature is negative. At night the sea is usually 

 cooling and the appropriate correction is positive. In each case the magnitude may be as 

 much as 0.5° Kelvin. 



The foregoing may be summarized as follows: 



Table for Correction of Infrared to Equivalent Bucket Temperature 



Sky correction 

 Boundary layer correction 

 Total Correction 



Day 



to+ 1.0°K 

 to - 0.5°K 

 -0.5°tol.°0K 



Night 



to+ l.°OK 

 to + 0.°5K 

 to+ l.°5K 



As a general rule of thumb, an over-all correction of 1°K may be representative. 



-92- 



