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Tropospheric Ducting (TrD)
...is an abnormal condition. An inversion has formed at a much higher level above the ground...the vast majority of duct-producing inversions lie between 450 and 1500 m (1500 to 5000 ft)..with a few between 1500 and 3000 m (5000 to 10,000 ft). These inversions are not formed due to nighttime radiation/cooling..but rather because of some other weather phenomenon (high pressure subsidence aloft, warm frontal boundary, cold frontal boundary, oceanic or lake inversion, Chinooks, etc.). Because of this..ducting can occur day or night (though it strengthens at night)..is not usually influenced by terrain (the exception being large mountain chains like the Rockies)..and from a DXers point of view is usually either uni- or bi-directional. In fact..typical ducts are sharply directional. Signals refract off of and also travel along the inversion..thus the analogy of a duct. Strong ducting can result in super-refraction where signals are bent so far in a downwards direction that they actually hit the ground and reflect off it, only to bounce of the top of the inversion again and so on. Distances are theoretically unlimited. One large area can have multiple ducts going on simultaneously..but they are usually parallel paths. It is possible in a very strong high pressure system to have large areas of ducting creating multi-directional openings. These are the rare "blockbuster" openings that bring signals great distances and cause havoc with interference. They are most common over the oceanic areas in the tropics and sub-tropics.