 over distances greater than the ordinary line of sight. This is as a result of effects within the troposphere. As most of the conditions that govern our weather occur in the troposphere, there are usually many links between the weather and radio propagation conditions at these frequencies. Under normal conditions signals at these frequencies travel more than the line of sight. Prior to the 1940s it was generally thought that communication over distances greater than the line of sight was not possible, but experience soon showed this was not true. As a very rough guide it is usually possible to achieve distances at least a third greater than this. This is possible because of the varying refractive index of the air above the earth's surface. An increase in the pressure and humidity levels close to the earth's surface mean that the refractive index of the air is greater than that of the air higher up. Like light waves radio waves can be refracted, and they bend towards the area of the greater refractive index. This means that the signals tend to follow the curvature of the earth and travel over distances that are greater than just the line of sight. An additional effect is that of diffraction where the signal diffracts around the earth's curvature. Greater distances At times signals can be received over much greater distances than 4/3 of the line of sight. At times like these terrestrial television channels may be subject to interference as may other radio users. There are a variety of mechanisms by which signals can be propagated over these greater distances. Usually it is possible to predict when there is a likelihood of them occurring as there is a strong correlation between them and certain weather conditions. Usually the extended distances result from the normal gradient in refractive index becoming much steeper. In this way the degree of bending is increased, allowing the signals to follow the curvature of the earth for greater distances. 46 Newnes Guide to Radio and Communications Technology A number of weat