DETERMINING FLUCTUATIONS IN REFRACTIVE INDEX NEAR LAND OR SEA 



91 



26- 

 25 

 24- 

 23- 

 22 - 

 21 - 

 20 

 19 

 18 

 17 

 16 



IS - 

 14 - 

 13 - 

 12 - 

 II - 

 10 - 

 9 - 

 8 - 

 7 - 

 6 - 

 5 



Figure 12. Observations on February 24, 1945 at masthead of ship in Gulf Stream, wind 14 knots. Characteristic 

 diagram. Vapor pressure over water is shown by lower bounding curve, over salt water by upper bounding curve. 

 The family of curves gives modified index for 1,000 mb and zero height; or (jt — 1)10^ for microwaves at 1,000 mb, zero 

 height, where n is refractive index at /t = 0. 



teristic diagram lias the same orientation as the 

 Rossby diagram wliicli is in routine meteorological 

 use but with somewhat different coordinates. 



The ordinate is temperature and the abscissa is 

 vapor pressure. A pair of curves gives the vapor pres- 

 sure over fresh water and over sea water. The family 

 of curves gives refractive index at radio frequencies 

 and at a total pressure of 1,000 mb, or /) = 0. 



On the diagram are plotted a few of a long series 

 of determinations made by reading a sling psychrom- 

 eter at half-minute intervals. These were recently 

 obtained by the Woods Hole Oceanographic Institu- 

 tion at the masthead of a ship crossing the Gulf 

 Stream at a time when the air was much colder than 

 the water. The water temperature was 70 F, fixing 

 the boundary condition. The points lie fairly well 

 on the straight line through the boundary value. They 

 cover a range of 5 X 10"*' in refractive index. Prob- 

 ably a greater range would be indicated by a psy- 

 chrometer having a more rapid response. 



It is seen that, whenever the fluctuation in refrac- 

 tive index at a point in the atmosphere is due to 

 turbulent mixing between a large homogeneous mass 

 of air and air controlled by a fixed boundary condi- 

 tion, the fluctuation may be obtained as follows: 

 Measure the average temperature and humidity at this 

 point and at the boundary, thus determining the rela- 

 tion between refractive index and temperature. Meas- 

 ure the fluctuation of temperature, from which the 

 fluctuation of refractive index may be found from the 

 established relation. 



It may be noted that the water temperature less 

 the average air temperature gives a value for the 

 temperature deficit. In the same way one may arrive 

 at a humidity deficit and an M deficit (one million 

 times the deficit of refractive index). Each of these 

 quantities is represented on the diagram by the 

 change from one end to the other of the line. It follows 

 that the suggested method may be stated in terms of 

 the relation that the ratio of temperature fluctuation 



