SOME FACTORS INFLUENCING THE SKIN-TEMPERATURE OF THE SEA AND ITS 

 MEASUREMENT BY INFRARED THERMOMETER 



by Guy A, Franceschini, Department of Oceanography and Meteorology, 

 Texas A & M University, College Station 



INTRODUCTION 



This is intended to be a preliminary and qualitative review-note on factors influencing 

 IRT readings taken of the sea surface. Related observations made at sea are appended. 



In general, IRT readings may be based on an absolute calibration of the instrument 

 performed under laboratory-control, or they may be based on secondary field calibrations 

 performed repeatedly in situ . In the former case, a blackbody cavity at a knowTi tempera- 

 ture may be employed as the reference, whereas in the latter instance a suitable target, 

 e.g., a well stirred bucket of water at known temperature may be used. There are advan- 

 tages .to each method depending on whether or not: 1) the optical properties of the surface 

 are known, 2) the measuring system exhibits stability. In practice a combination of the two 

 methods may be desirable. 



Evidence obtained within the last twenty years, and to a greater extent in the last five 

 years, definitely indicates that the temperature of a thin layer of water at the surface is usually 

 different from that obtained by standard measuring techniques, i.e., bucket, intake, or towed 

 thermistor. The data tabulations in the Appendix show these differences may be positive or 

 negative depending primarily on meteorological factors. The unusually large positive differ- 

 ences (skin warmer) are found in and near areas of active rainfall. From a theoretical 

 standpoint these differences, both positive and negative, are expected since all energy in- 

 teractions with the atmosphere occur at the surface. Evidence and theory indicate the 

 existence of a thin surface layer which, of necessity, constrains and is influenced by these 

 interactions. Because of this mutual dependence, it is necessary to know the "what, where, 

 how, and why" of skin-surface temperatures. 



Since the size of this surface film is usually rather small, e.g., tenths of millimeters, 

 the near-ideal way to determine its temperature is with a remote sensing radiation instru- 

 ment — the IRT. Advantages of such a device are well known. Further, if the relationship 

 between skin-temperature and sub-surface temperature is known, IRT readings may be 

 used to determine sub-surface temperatures, i.e., temperature of water in the mixed layer 

 immediately below the surface. 



FACTORS INFLUENCING SURFACE TEMPERATURE 



If we neglect chemical and biological reactions, the temperature of the surface film 

 depends on exchar^es of energy with the atmosphere and the underlying water, i.e., trans- 

 fers across the boundaries of the film. The process of concern are: 



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