her conditions may cause this increase. An area high of pressure may cause the conditions that can increase the normal propagation distance. A high pressure is normally associated with warm weather, especially in summer. Under these conditions the hot air rises and cold air comes in to replace it. This accentuates the density gradient normally present and the change in refractive index occurring as a result of this can be very sharp. Other weather conditions can also bring about similar increases in the change of refractive index. Cold weather fronts can have the same effect. Here a mass of warm air and a mass of cold air meet. When this occurs the warm air rises above the cold air bringing about similar conditions. Cold weather fronts normally move more quickly than high pressure areas, and as a result increase in propagation distance due to cold fronts is normally more short lived than those caused by high pressure areas. Other local conditions may give rise to increases in propagation distance. Convection in coastal areas in warm weather, the rapid cooling of the earth and the air closest to it after a hot day, or during frosty weather. Subsidence of cool moist air into valleys on calm summer evenings can give rise to these changes. Sometimes the changes in refractive index can trap the signals between two layers forming a type of duct or waveguide. When this happens signals may be carried for several hundreds of kilometres. It is found that tropospheric bending and ducting is experienced more at higher frequencies. Its effects are comparatively small at frequencies at the top of the HF portion of the spectrum, and increases steadily into the VHF and UHF portion of the spectrum. At higher frequencies the effects are still noticed, but other factors start to limit the range. Troposcatter The effects of tropospheric bending are very dependent upon the weather. This is shown by the fact that television signals in the UHF band are only occasionally affected by interference from distant signals. As such it is not p