PRELIMINARY RESULTS OF METEOROLOGICAL MEASUREMENTS 



67 



same relative degree. Tlie temperature distribution is 

 modified by radiation also, but for an initial period 

 of a few hours this is unimportant compared with the 

 processes just mentioned. 



AVhen initially homogeneous air flows o\er water of 

 constant temperature, a necessary result is therefore 

 that the cur^'es of temperature and water vapor pres- 

 sure are similar. Furthermore the M curve is similar 

 also, because within the range of any sounding the 

 niodiiied index is approximately a linear function of 

 temperature, vapor pressure, and height. 



The extent of similarity revealed in Figure 2 is 

 unusual. Often the three curves have very different 

 shapes. In the latter case the deviation from simi- 

 larity can be ascribed (1) to lack of homogeneity iu 

 tlie unmodified air, (2) to varying water temperature 

 along the air's trajectory, or (3) to radiation during 

 prolonged over-water modification. 



6^-^ The M Deficit 



The distances on the base line from the straight 

 broken line to the arrow are the temperature excess 

 and humidity deficit respectively. There is obviously 

 a corresponding quantity pertaining to index of re- 

 fraction. The M deficit may be defined as the value 

 of the modified index at the water surface less the 

 representative surface value in the unmodified air. 



Temperature excess, humidity deficit, and M defi.eit 

 are related, so any two fully determine the difference 

 between unmodified air and air at the water surface. 

 The two of most direct significance are M deficit and 

 temperature excess. Forecasting is simplified by their 

 use : Temperature excess is necessary in drawing the 

 temperature curve ; similarity and M deficit then give 

 the M curve directly. Another advantage in using 

 M deficit is that whether it is positive, zero, or nega- 

 tive determines at once whether the modified air is 

 probably characterized by an M inversion (layer 

 where modified index of refraction decreases upward) 

 by standard, or by substandard M curves, respectively. 

 For instance, the positive M deficit in Figure 2 is a 

 condition necessary for the M inversion to occur. 



Specifically, if homogeneous air blows over a water 

 surface of constant temperature and if the M deficit 

 is positive, there is always an M inversion at the 

 water surface. Whether or not this extends sufficiently 

 high to be of importance in the refraction of radio 

 waves depends iu part on the magnitude of the M 

 deficit and on the temperature excess. 



If homogeneous air blows over a water surface of 



constant temperature and if the M tlcticit is zero, the 

 M curve necessarily remains practically standard. 



In the case of a negative M deficit a substandard 

 M curve is developed. It should be noted that in this 

 case (as well as iu the previous one) the air is losing 

 water vapor by condensation on the water surface. 

 This is simiDly the reverse of the process with dry air. 



*-^-^ Neutral Equilibrium 



For simplicity the analysis which follows is limited 

 to cases of positive J\I deficit. There is then a surface 

 M inversion, the height of which is a convenient 

 quantity to study as a dependent variable. The inde- 

 pendent ones are M deficit and temperature excess 

 and, as will be seen, two others. 



The first and least complicated case is the one of 

 neutral equilibrium, which corresponds to a tempera- 

 ture excess close to zero, say within 1 C of zero. Since 

 there is no appreciable temperature gradient, the M 

 curve depends only on the moisture distribution. 

 Furthermore this case practically requires a vapor- 

 pressure lapse at the surface, because in the lower 

 part of a homogeneous layer the vapor cannot be 

 saturated (see Figure 3). Hence there is always an 

 M inversion at the surface. Neutral equilibrium is 

 prevalent far from shore. 



II 13 15 17 19 



TEMPERATURE IN 



DEGREES C 



7 9 

 MIXING RATIO 



Figure 3. Probable course of modification of warm air 

 over water under ideal conditions. A, initial stage. 

 D, final stage. 



With neutral equilibrium frictional turbulence is 

 unhindered. Mixing extends to a height roughly pro- 

 portional to the wind speed; a wind of 20 mph at 

 100 ft gives mixing up to about 2,000 ft. 



The intensity of mixing increases upward rapidly 

 from the surface, so large vertical gradients are con- 

 fined to the region of relatively little mixing close to 

 the surface. The M inversion probably never extends 

 above 100 ft. 



