Superrefraction occurs under strong inversions, which
are typical in subtropical, high-pressure zones.
DUCTING.— In extreme cases, a very rapid
decrease in the N-gradient will cause radar waves to
become trapped in a layer of the atmosphere and travel
abnormally long distances (table 2-l). This
phenomenon is known as ducting and is a frequent
occurrence when strong inversions are present. When
ducting occurs, a returning pulse may display low
targets from hundreds of miles away-targets that are
not normally detected (fig. 2-15, view D). This may
cause an effect similar to range folding. False or
exaggerated echoes are plotted where no
meteorological targets exist. Keep in mind that
ducting is also dependent upon the wavelength of the
radar. The larger the wavelength, the deeper the layer
has to be before ducting can occur.
DIFFRACTION
Electromagnetic waves tend to follow along the
curved surface of an object. Diffraction is the process
that causes waves traveling in a straight path to bend
around an object or obstruction. The direction of
propagating energy is changed so that it spreads into a
shadow zone, as shown in figure 2-16, view (A). ln the
earth-atmosphere system, diffraction occurs where the
straight-line distance between the transmitter and
receiver is just tangent to the earth’s surface as shown
figure 2-16, view (B). Generally, the lower the
Figure 2-16.—(A) Diffraction of a radar wave front around an obstruction. (B) Radar horizon and diffraction region shadow zone.
2-14
