120 



METEOROLOGY— FORECASTING 



DRY ADIABATIC 



TEMPERATURE 



Figure 11. Tem])erature-height curves for the cases of 

 (A) unstable air, (B) neutral air, and (C) stable air. The 

 dry adiabatic (lapse rate) is indicated for comparison. 



Ill addition to tlie direct relationship which these 

 staliility conditions bear toward the trapping of 

 microwaves, wliich will lie described presently, there is 

 also an indirect relationship caused by the modifica- 

 tion that air undergoes when it moves over a sea or 

 land surface with properties (temperature and mois- 

 ture) different from those of the air itself. For ex- 

 ample, air moving over land, the temperature of which 

 is higlirr than that of the air, will be heated in its 

 lowest layers by contact with the ground and thus 

 tend to become luistable. This leads to vertical cur- 

 rents which will carry the modifying influences to 

 appreciable heights in the air. On the other hand, air 

 movins' over a surface cool in relation to the air will 

 be cooled by contact with the ground, tend to develop 

 stable characteristics, damp out vertical currents, and 

 so confine the modifying influences to very low layers. 



The distribution of moisture with altitude, in its 

 direct influence on nonstandard proptagation, has an 

 even more pronounced effect than that of temperature. 

 As a means of describing the moisture content of the 

 air, any one of several concepts may be used: dew 

 point, wet bulb temperature, relative humidity, ab- 

 solute humidity, specific humidity, and mixing ratio, 

 all of which are defined in the Appendix. ExceiDt when 

 evaporation or condensation is taking place (as in the 

 case of clouds, rain, dew, etc.), the moisture content 

 of the air has little effect on the temperature structure 

 and therefore is not a major influence on stability 

 conditions in so far as they are connected with the for- 

 mation of ducts. What is of direct importance is the 

 vertical distribution of humidity itself and the man- 

 ner in which this distribution is affected by modifying 

 influences. As an example of the latter, the ease of 



warm and relatively dry air moving over a humid 

 surface, such as dense vegetation or the ocean, might 

 be mentioned. In this case, evaporation of water into 

 the lower layers of the air leads to a greater decrease 

 in moisture with altitude than was originally present 

 in the air. 



Other modifying influences, of course, affect both the 

 vertical distribution of temperature and humidity and 

 the stability conditions of the air. Some of these are 

 sul)sidence (the gradual sinking of large layers of 

 air leading to increased stability and decreased rela- 

 tive moisture content), radiation, and turbulent mix- 

 ing. These are merely mentioned here in view of the 

 fact that their various interactions at times may be- 

 come quite complicated, hence requiring that proper 

 interpretation be made by one trained or experienced 

 in meteorology. 



Eefractive Iiule.v. The manner in wliich pressure, 

 temperature, and humidity directly influence trapping 

 depends on the phenomenon of refraction or the bend- 

 ing of rays as they pass through media with different 

 dielectric properties or through a medium with vari- 

 able dielectric properties. The velocity of electromag- 

 netic waves through any particular medium such as 

 the air depends on a quantity known as the refractive 

 index of that medium. When the refractive index 

 varies throughout the medium, as is usually the case 

 in the atmosphere, the resulting variation in wave 

 velocity leads to a bending of the rays. For example, 

 the refractive index of the atmosphere often decreases 

 with height, in which case rays are bent downward 

 toward the surface of the earth, so that instead of 

 traveling in straight lines they tend to follow to a cer- 

 tain extent the curvature of the earth. The amount 

 of bending depends on the manner in which the refrac- 

 tive index varies with height. Under the proper con- 

 ditions it is possible for rays to be bent to such a de- 

 gree that they are confined to one layer of the atmos- 

 phere. This phenomenon, the trapping of radio waves, 

 is usually associated with only the microwave fre- 

 quencies and is limited to those rays which leave the 

 transmitter at an angle with the horizontal of less than 

 1 degree, and therefore to only the lowest lobe in a 

 radar coverage diagram. 



For the atmospheric refraction to be strong enough 

 to cause trapping of microwaves it is necessary that 

 the refractive index of the atmosphere decrease with 

 altitude at a sufficiently rapid rate. For convenience 

 in dealing with pro1ilems of nonstandard propagation, 

 a quantity known as the modified refractive index has 



