y reflect the signals by almost 100 %. Microwave Propagation Warm air Cool air Warm air Cool air Ducting Layering Figure 3.53 Ducting and layering phenomenon Microwave engineers should note that the vertically polarised signals are less susceptible to the destructive interference from ground signals. Layering Layering is a part of the multipath phenomenon. As discussed at the beginning of this chapter that atmosphere consists of layers and it is these layers that help in the signal transmission from the transmitting to the receiving antennas. However, phenomena such as advection, frontal systems, radiation nights and subsidence can affect the refractivity lapse rate, resulting in (usually) degradation of the signal level. Advection This effect of temperature inversions is caused by the movement of the land based dry air over moist air over the sea by high pressure systems. Frontal Systems These systems are caused by changes in the signal levels observed due to the passage of warm (increase in signal level) or cold fronts (decrease in signal level). Radiation Nights When a cold night follows a warm day, the earth surface cools faster. This results in the air near the surface cooling faster than that above it, thereby causing temperature inversion. Subsidence In a high pressure system, dry air that descends becomes heated by compression and spreads over cool moist air, causing a temperature inversion. The layering phenomenon is usually observed in an area where links are either near the large water bodies, e.g. the sea, and/or in desert areas. In the valley regions, the drainage of the cold air from the higher slopes can create multiple layers, and in the coastal areas, land sea breezes and trade winds can cause layers. Sometimes, this layer phenomenon also leads to ducting, wherein the signal becomes trapped between two layers (shown in Figure 3.53). Ducting occurs when the refractivity gradient of the atmosphere becomes less than - 157 N units/km. When the refractivity gradient of the atmosphere is equal to that 