90 



METEOROLOGY— THEORY 



360 



520 



280 



-. 240 



200 



160 



120 



80 



40 



The other months iUustrate the continental type. Ac- 

 cording to the table a maximum of trapping in India 

 should occur in April just prior to the rainy season. 



12 16 



TIME (1ST) 



20 



24 



Figure 1 1 . Hourly\apor pressure difference, 6 to 46 ft 

 at Calcutta. 



and humidity factors will cause a rise in the vertical 

 M gradient to a maximum shortly after sunset. From 

 that time to sunrise a decrease in the M gradient will 

 occur. However, the height of the inversion continues 

 to grow until sunrise, tending to cause an increase in 

 the heiglit of the duct. Whether a maximum or mini- 

 mum of trapping will occur at sunrise will depend 

 on whether the increase in the height of the inversion 

 balances the decrease in vertical humidity gradient. 

 It is probable that the humidity factor is the more 

 important since the small magnitude of the tempera- 

 ture increase in the upper portions of the inversion 

 will seldom be sufficient to cause a decrease in M with 

 height. After sunrise rising humidity gradients, par- 

 tially balanced by falling temperature gradients, will 

 cause a small maximum of M gradient at midmorning. 

 Thereafter the M gradient will decrease to the after- 

 noon minimum. 



The maximum and minimum decrease of M from 

 6 ft to 46 ft, based on mean temperature and humidity 

 data at Calcutta, India, are given in Table 12. 



In July the morning minimum and afternoon maxi- 

 mum with small amplitude illustrate the oceanic type. 



'•^ DETERMINING FLUCTUATIONS IN RE- 

 FRACTIVE INDEX NEAR LAND OR SEA' 



In connection with the rapid fluctuation or scintil- 

 lation frequently observed in microwave reception, 

 questions arise concerning turlnilent atmospheric 

 fluctuation at fixed |)oints along the transmission 

 path, particularly fluctuations in refractive index. 

 Rapid measurement of both temperature and humidity 

 so as to give a direct determination of fluctuation in 

 refractive index is difficult. The purpose of this paper 

 is to suggest that in certain cases the measurement 

 of temperature fluctuation alone can give a good in- 

 direct estimate of fluctuation in the modified index. 



The basic principle underlying this suggestion is 

 tliat, if two initial kinds of air are mixed in different 

 proportions, for all possible mixtures a fixed relation 

 exists between any two properties cojiservative for 

 adiabatic changes. 



To illustrate this, consider a diagram with poten- 

 tial temperature and specific humidity as coordinates. 

 Two initial kinds of air would be represented by two 

 points on this diagram, and all mixtures of the two 

 kinds woidd be represented by points on the straight 

 line drawn between the two initial points. 



The practical case occurs when one point represents 

 a large homogeneous mass of air, and the other a 

 fixed boundary condition at the ground or water sur- 

 face. The straight line then represents the mixtures 

 that can occur in the vicinity of the boundary. For 

 these the line shows specific humidity as a function 

 of potential temperature. The relation between poten- 

 tial temperature and potential refractive index could 

 be shown by a similar diagram. 



Figure 12 shows some corroboration of this method 

 and also how the method can be applied. This charae- 



'By R. B. Montgomery, Radiation Laboratorj', INIIT. 



