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The theory of scattering by a turbulent medium is applied to scattering of radio waves in the troposphere. In the region below the horizon of the transmitter, energy is received (1) by diffraction round the curved surface of the earth (modified as appropriate by atmospheric refraction), and (2) by scattering from turbulence in the region of high field strength above the horizon. At distances beyond the horizon that are not too great, we may think of (1) as giving the mean signal received, and (2) as giving the fading. However, contribution (2) usually decreases with distance more slowly than contribution (1). Beyond a certain distance, therefore, contribution (2) becomes predominant and the mean signal is no longer given by (1). (See Fig. 3.) Values of the scale of turbulence and of the departure of refractive index from mean expected on meteorological grounds are fully adequate to explain the scattered field strengths observed experimentally.