Radar echoes from clear air in relation to refractive-index variations in the troposphere

From results obtained, both in the UK and the USA, it is shown that the measured intensities of layer-type radar echoes, observed at vertical and oblique incidence at centimetric wavelengths, agree with values calculated from measurements of the spectral density of refractivity variations in the pulse volume. The radar soundings sometimes show as many as five or six horizontal layers in the first 5 km above the ground. Some layer-type echoes could be explained at decimetric wavelengths in terms of reflection from smooth `disc-type¿ irregularities but this model is an inadequate explanation of other clear-air echoes.