DIFFRACTION

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