30 PHYSICAL PROPERTIES OF SEA WATER 



the maximum values in the oceanic water approach the minimum values 

 in the coastal water. In any given locality the great variations that 

 also occur in a vertical direction further complicate the actual conditions. 



Similar results have been obtained by other investigators from such 

 widely different areas as the English Channel, the waters off the east 

 coast of the United States, and those off southern California. In all 

 instances it has been found that the absorption is less in oceanic than in 

 coastal water, but varies within wide limits both locally and with depth. 

 Where examination of absorption in different parts of the spectrum has 

 been conducted, it has been found that the absorption is much less in the 

 blue than in the red end of the spectrum, and that the blue light penetrates 

 to the greatest depths in clear water, whereas the green or yellow light 

 reaches farther down in turbid water. 



Influence of the Altitude of the Sun upon the Extinction 

 Coefficients. The extinction coefficient is a measure of the reduction of 

 intensity in a vertical distance and therefore depends upon the obliquity 

 of the rays. The obliquity of the incident rays is reduced, however, by 

 refraction when they are entering the water from the air and by the effect 

 of scattering. When the sun's rays pass the water surface, the angle of 

 refraction increases from zero with the sun at zenith to 48.5 degrees with 

 the sun at the horizon, and the most oblique rays penetrating into the 

 water therefore form an angle of less than 48 degrees with the vertical. 

 Owing to the scattering and the sifting out by absorption of the most 

 oblique rays, the measured extinction coefficients will be independent 

 within wide limits of the altitude of the sun. The reduction of the 

 obliquity of the incident radiation has been directly demonstrated by 

 Johnson and Liljequist. Conditions at very low sun have not been 

 examined, but it is probable that the extinction coefficients are then 

 increased, and this may have bearing upon the diurnal variation of the 

 incoming energy at greater depths. 



Cause of the Large Extinction Coefficients in the Sea. The 

 fact that extinction coefficients in the sea are large in comparison to those 

 of absolutely pure water is as a rule ascribed to the presence of minute 

 particles which cause scattering and reflection of the radiation and which 

 themselves absorb radiation. According to Lord Ra3deigh, if such 

 particles are small in comparison to the wave length, X, of the radiation, 

 the scattering will be proportional to X""*, and the effect therefore at wave 

 length, say, .46 will be 2.86 times greater than at wave length .60 n. This 

 selective effect leads to a shift toward longer wave lengths of the region 

 of minimum absorption. 



Clarke and James found that the increased absorption in oceanic 

 water was chiefly caused by suspensoids which could be removed by 

 means of a "fine" Berkefeld filter and that these suspensoids were largely 

 nonselective in their effect. Utterback's data indicated, on the other 



