We shall confine our attention to the atmospheric absorption between 8/i and 14^ , 

 and we note that this is due almost entirely to water vapor. It is, in fact, a function of the 

 amount of water vapor present in the path between the ocean and the Infrared Thermometer, 

 and this is independent of the presence of other gases. We can, therefore, consider the 

 water vapor as a separate "atmosphere" and employ its pressure as a means of stating the 

 quantity present, under the name vapor pressure. Like the pressure in the atmosphere, it 

 can be measured in inches of mercury, millimeters of mercury, or millibars.* The value 

 of the vapor pressure is independent of the presence of other gases; hence, it is incorrect 

 to speak of air saturated with vapor as one might speak of a sponge saturated with water; 

 it is really the space which is saturated with water vapor. Before proceeding further, we 

 should confess that the absorption due to the presence of water vapor is not exactly a 

 function of precipitable water vapor alone. There are other interfering factors, such as 

 pressure broadening: as the pressure of other gases increases, there are slight variations 

 in the absorption of an equivalent total amount of water vapor as the path length changes, 

 but these effects are minor and can be safely ignored for low flying altitudes. 



The entire problem of atmosphere is narrowed down to the following: how much 

 total water vapor is present in the path between the ocean surface and the radiometer? 

 Relative humidity is not the quantity of interest; the amoimt of water vapor in the path is 

 the quantity of interest. Therefore, we must compute the total quantity of precipitable 

 water vapor in the path. This can be done from temperature and relative humidity data. 

 Figure 6 plots the precipitable water vapor versus air temperature for a 1000 foot path 

 and 100% relative humidity.^ For other flying altitudes and other humidities, simply apply 

 linear correction factors. The major forms of spectral atmospheric transmission 



klR TEMPEIATUME IN OCQRCCS CCNTIGHADE 



Figure 6. Precipitable Water versus Air Temperature at 1000 Feet 

 Altitude, lOOVo Relative Humidity 



* 1 bar - 10 dynes per square centimeter, 1 mm mercury = 1.3332 millibars, 760 mm 

 mercury - 1013.2 millibars, 1 millibar = 3/4 mm mercury. 



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