Calculations show that for spherical hail, a negative value of the range derivative of the logarithm of the ratio of the powers received at 10 and 3 cm wavelengths (P10 and P3, respectively) is indicative of the presence of large hail (diameter 1 cm). The negative derivative can occur at the far edge of a boundary between a region of large hail and rain, as shown by Eccles and Atlas (1973). Calculations with a simple model of a hail region, having monodisperse spherical hail at any range, show that a negative derivative of log (P10/P3) can occur embedded in a region of hail if the diameter of the hail and/or the thickness of water coat on it is a function of the range. The results of the calculations are supported by vertical incidence dual-wavelength Doppler radar observations of a hailstorm. Negative values of the range derivative of log (P10/P3) are inferred from the Doppler observations, to occur within a region of hail in which the mean hail diameter was a function of the range. Accordingly, a negative range derivative of log (P10/P3), although indicative of the presence of large hail, is not useful for delimiting the boundaries of a hail region.
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Jameson et al. (1978) studied this question.