ocean surface ranges from a minimum of -2°C to a maximum of +35°C. Figure 2 plots 

 Planck's blackbody radiator equation for these two temperatures. Note that 98% of the 

 radiant energy occurs at wavelengths longer than 6 n . From the standpoint of the target 

 radiation, we may confine ourselves to the infrared portion of the electromagnetic spectrum 

 which lies beyond 6 ^l . Other considerations also make this desirable, as will be shown; 

 therefore, our measuring equipment will be designed to operate at wavelengths beyond 6/i . 



Figure 2 plots the radiation of ideal blackbody radiators, that is, targets whose 

 emissivity is 1 at all wavelengths. Unfortunately, the ocean is not a perfect blackbody 

 radiator. To what extent does it deviate from ideal? Figure 3 shows the emissivity of 

 the ocean for wavelengths from 6/1 to 15 ^ • In the region from 8 m to 14^ , the average 

 emissivity of the ocean is .98 or 98% when viewed normally. Now the three properties — 

 emissivity, reflectivity, and trans mis sivity — must equal 1. Since liquid water of 1 milli- 

 meter or greater path length has a trans mis sivity of zero in the infrared, a 98% emissivity 

 indicates that reflectivity is 2%, Figure 3 also shows that the average emissivity of the 

 ocean in this wavelength region reduces to about 96% when viewed at an angle 60° from 

 normal, that is, 4% of energy received by any measuring system is reflected (from the 

 sky). 



Another important characteristic of the ocean surface is the difference between the 

 surface temperature and the sub-surface temperature; this will be discussed later. 



Having established the radiation characteristic of the ocean surface, consider what 

 happens to this radiation on its journey from the ocean to the Infrared Thermometer, The 

 most severe atmospheric attenuation in the infrared results in selective absorption by 

 gases in the atmosphere. In order of decreasing importance, these gases are: water 

 vapor, carbon dioxide, nitrous oxide, ozone, oxygen, methane, carbon monoxide.*^ Of these, 

 oxygen, carbon dioxide, nitrous oxide, and methane are stable and uniform in concentra- 

 tion. Water vapor is highly variable in concentration, and is stratified; this factor can be 

 ignored since accurate measurements require that the pilot fly the measuring aircraft 



ELEMGTM , WiCRONS 



Figure 2. Blackbody Radiation (Planck's Equation) 



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