tures above absolute zero (— 273.16°C) emit 

 radiation in the infrared range of the electro- 

 magnetic spectrum in quantities and at wave- 

 lengths that depend on the nature of the radi- 

 ating object's surface (its emissivity) and on 

 its temperature. The temperature of the earth's 

 surface normally ranges from about — 50°C 

 (-58°F) to +50°C ( + 122°F), with the major 

 portion of the radiation curve lying within the 

 wavelengths of 5 to 20/x (microns) and the 

 maximum amount of radiant energy between 7 

 and 13/x. This radiation is attenuated by the 

 gases in the atmosphere, such as water vapor, 

 carbon dioxide, nitrous oxide, ozone, oxygen, 

 methane, and carbon monoxide. However, be- 

 tween the wavelengths of 8 and 13/i infrared 

 absorption by these gases is much reduced; 

 this band of low absorption of infrared radi- 

 ation is called an "atmospheric window." 

 Measurement of the earth's radiation reveals 

 that about 30 percent falls within the wave- 

 lengths of 8 to 13jLi and can be measured with 

 the use of sensitive detectors and spectral band- 

 pass filters (Frank, 1964). 



The sea surface has the characteristic of a 

 good radiant energy absorber and, conversely, 

 a good energy radiator when compared to the 

 characteristics of an ideal, optically black, radi- 

 ation source. The average emissivity of the sea 

 surface is 0.98 when compared to the ideal 

 blackbody radiation source which is considered 

 to be 1.0. The difference of 0.02 is accounted 

 for in the reflectivity of radiation from the sun 

 and sky from the sea surface. To measure the 

 sea surface radiation, the infrared detector 

 must respond only at those wavelengths at 

 which the atmosphere is nearly transparent 

 (nonabsorbing) and the ocean is nearly black 

 (nonreflecting) . This condition is largely ful- 

 filled if the optical system of the radiometer 

 contains filters to pass only the spectral region 

 from 8 to 13jn, for in this region the atmo- 

 sphere is relatively transparent and the ocean 

 is nearly nonreflecting. The temperature of the 

 sea surface ranges from a minimum of about 

 -2.2°C (28°F), with a radiant energy flux of 

 0.0304 watts/cm=, to a maximum of +35°C 

 (95°F) with a radiant energy flux of 0.0500 

 watts/cm= (Clark and Frank, 1963). 



The amount of radiant energy flux at the 

 surface, which is relative to surface tempera- 

 ture, can be measured by an infrared detector 



and associated electronic components. To ac- 

 curately determine relatively low temperatures 

 and temperatures having a limited range, such 

 as those of the sea surface, a comparative 

 measurement must be made with a known 

 temperature source. To do this the infrared 

 detector alternately "looks" through the optical 

 filtering system, with the aid of an eight-bladed 

 reflective "chopper" fan revolving in front of 

 the detector, first at the ocean surface and then 

 at a reference blackbody radiation source oper- 

 ating within the instrument at 50°C (122°F). 

 The amplitude of the alternating current wave 

 thus generated represents the temperature dif- 

 ference between the sea surface and the 50°C 

 reference source. The alternating current gen- 

 erated by the detector is amplified, rectified, and 

 sent to a direct current meter and chart rec- 

 order. The instrument is calibrated by exposing 

 the optical detector to an external heat source 

 which can be varied to cover the required range 

 of observed temperatures. 



Airborne infrared radiometry has been used 

 occasionally as a technique for determining sea 

 surface temperature gradients for over a decade. 

 In the early 1950's experiments were conducted 

 on determination of infrared radiation from the 

 sea surface by the Woods Hole Oceanographic 

 Institution under contract with the U.S. Navy, 

 Bureau of Aeronautics. The instrument used 

 was designed to determine the horizontal 

 gradients of sea surface radiation rather than 

 to measure the radiation in terms of degrees 

 F. or C. In 1952 the airborne infrared technique 

 was used to determine surface radiative gra- 

 dients and map the location of the Gulf Stream 

 front (von Arx, Bumpus, and Richardson, 

 1955). Ewing (1952) modified the instrument 

 and used this technique to measure infrared 

 radiation gradients in the eastern tropical 

 Pacific. He reported that the radiation thermo- 

 meter "appeared to be a most useful oceano- 

 graphic tool and holds the promise of making 

 it possible to obtain rapid, nearly synoptic, 

 surveys of ocean surface temperatures." He 

 later concluded (Ewing, 1964) that horizontal 

 temperature gradients are much more precisely 

 defined by the ART than is absolute tempera- 

 ture. He also concluded that thermal maps 

 should be of use to marine biologists and physi- 

 cal oceanographers in defining areas having 

 convergence which may in turn have an effect 



