, with the reflected path longer than the Away from Tx 6/18 Electromagnetic waves In 'normal' conditions, the refractive index of the atmosphere decreases with height by an amount which slightly bends the wave back towards the earth. However, it is still more convenient to think of the wave travelling in straight lines, which can be done theoreti- cally if the Earth's radius is suitably modified. Under normal atmos- pheric conditions this modification involves multiplying the true Earth radius by 4/3. Changes of meteorological conditions can greatly alter the refrac- tive index from the normal, and hence influence the mode of propagation of signals through the troposphere. If the refractive index decreases from ground level at a sufficiently high rate to bend the wave with a curvature greater than that of the Figure 6.23 The angle of scatter calculations Earth, propagation of high field strengths to great distances can occur via the ground based ducts created by such meteorological conditions. Under these conditions the wave is refracted back to Earth, where it is reflected back towards the troposphere to be likely to be exceeded for small percentages of time and this, there- refracted back again towards the Earth and SO on, to produce a wave fore, requires a knowledge of the cumulative long term statistics for guide type of propagation. Where such a rapid decrease in refractive rainfall in the geographical area concerned. In the absence of such index occurs at a higher level in the atmosphere, it can produce an measured rainfall data for the area concerned, the rainfall data elevated propagation duct, without involving reflection off the presented by the CCIR, for various geographical areas throughout ground. the world, may be used. If the change of refractive index is very rapid over a small height The attenuation, in dB/km, due to a cloud of small droplets of interval, the wave may be effectively reflected from it and again off diameter less than 0.1mm is given by the product of the specific the ground, follow