TECHNICAL SURVEY 153 
existence of a duct presupposes that the refractive 
index n decreases with height over at least a limited 
range of altitudes at a rate more rapid than 0.157 
MU per meter. Such a deerease can be produced by 
two different meteorological conditions. 
1. A rapid increase of temperature with height. 
This temperature inversion must be very pronounced 
in order, by itself, to produce a duct. In practice, a 
temperature inversion contributes to duct formation 
when accompanied by a sufficiently strong moisture 
lapse. 
2. A rapid decrease of humidity with height desig- 
nated as a “steep moisture lapse.” 
When ducts are produced by only one of these 
causes, they may be designated as “dry ducts” and 
22 
IN DEGREES C PER 100 METERS 
o 
aT 
dh 
“wet ducts,” respectively. In the general case a 
temperature inversion and a muisture lapse cooperate 
in producing a duct, but one of the two factors will 
be preponderant, thus facilitating the analysis of the 
meteorological problem. 
Whether or not a duct occurs under given meteoro- 
logical conditions and what the-rate of change of M@ 
is inside the duct may be determined by means of 
the diagram, Figure 27. (This discussion is presented 
for the purpose of illustrating the importance of 
temperature and moisture gradients. The technique 
more readily usable in practice is to compute the 
values of M at various altitudes directly from tem- 
perature and relative humidity data with the aid of 
Figure 19.) The abscissa in Figure 27 is the rate of 
WA 
TINS 
PZZAA 
SOILZA 
OL/A 
ae 
| rec | (Fit 
eS 
4.878 
4.706 
100% RH 
50 % RH 
0 % RH 
4,389 
2 
- Sn MB PER 100 METERS 
Figure 27. Temperature and humidity gradients. 
