Tropospheric V/UHF DX Modes..
U = instabile Signalbereiche
Line-of-Sight (GW)...is normal continuous reception where the receiving and transmitting antennas can see each other..taking into account the 4/3 Earth curvature of radio waves.
Tropospheric Scatter (TrS)...is ever-present under normal conditions. That's the mode that produces the distant fluttery signals that randomly fade in and out. These are your most distant regular stations that barely make it in. Depending on your location and equipment..tropo scatter can extend to 300..500..or even 700 km. The theoretical maximum limit for most TV/radio DXers is 800 km (500 mi) (Some semi-professional setups can extend furthur). Scatter is caused by small particles/droplets in the air such as haze, dust, volcanic ash, clouds, etc.
Tropospheric Enhancement (TrE)..(akaTropospheric Refraction) is common under normal conditions. On most clear nights with calm or light winds..the ground radiates and the air near the ground cools. Eventually an inversion is formed and signals begin to refract off the inversion. Stations that normally fade in and out via tropo scatter come in continuously..with increasing strength. Also..weaker tropo scatter stations that are normally not heard (because their signal strengths never cross the background noise threshold signal level) also begin to appear. When the sun comes up..the ground & air heats up..the inversion breaks down..and the enhancement disappears. The enhancement is subtle on some nights..and very obvious on other nights. Distances are no different than your tropo scatter catches..it's just that the signals are stronger. Tropo enhancement is greatly influenced by terrain..with valley and coastal paths favoured. ("Fog-prone" areas are also "DX-prone" areas!!). From a DXers point of view, multiple directions usually are enhanced at the same time.
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. Distances are theoretically unlimited. One large area can have multiple ducts going on simutaneously..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 make DXers' mouths water. They are most common over the oceanic areas in the tropics and sub-tropics.