246 RADIO WAVE PROPAGATION EXPERIMFNTS 
In general the lapse rate of temperature is im- 
portant in meteorology because of its relationship to 
the vertical stability of the atmosphere, that ig, to the 
feasibility with which vertical air currents can de- 
velop. It turns out that, except within clouds or regions 
of active precipitation, the stability conditions can be 
closely evaluated from a knowledge of the actual lapse 
rate relative to the “dry adiabatic” lapse rate (approxi- 
mately 5.5 F per 1,000 ft). If the actual lapse rate is 
larger than the dry adiabatic, i.e., if the temperature 
decreases at a rate greater than 5.5 F per 1,000 ft in 
elevation, any vertical currents which develop will 
tend to exaggerate in intensity, and a condition of 
unstable equilibrium (Figure 11A) will exist. Con- 
versely, if the actual lapse rate is less than the dry 
adiabatic or, especially, if a temperature inversion is 
present, the development of vertical currents will be 
hindered and the air will tend to become horizontally 
stratified. This is the case of stable equilibrium (Fig- 
ure 110). The in-between case, in which the actual 
temperature lapse rate is the same as the dry adiabatic, 
is that of neutral equilibrium (Figure 11B). 
DRY ADIABATIC 
TEMPERATURE ——— 
Ficure 11. Temperature-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. 
In addition to the direct relationship which these 
stability conditions bear toward the trapping of 
microwaves, which will be 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 higher than that of the air, will be heated in its 
lowest layers by contact with the ground and thus 
tend to become unstable. This leads to vertical cur- 
rents which will carry the modifying influences to 
appreciable heights in the air. On the other hand, air 
moving 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 propagation, 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 hnmidity, and mixing ratio, 
all of which are defined on pp 258-259. Except 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 case 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 
subsidence (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. 
Refractive Index. The manner in which 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 fo those rays which leave the 
