Attenuation caused by hydrometeors, along the propagation path from the radar to the resolution volume of interest, has been studied since the beginnings of radar history (Ryde 1946; see also, the historical review by Atlas and Ulbrich 1990). The expression for specific attenuation was derived in (1.133d), using fairly simple concepts. The specific attenuation is, in essence, the extinction cross section of the particles weighted by the size distribution. In the microwave frequency band (<35 GHz), absorption by water along the propagation path (cloud water, raindrops, or melting hydrometeors) is the principal cause for attenuation (absorption by gaseous constituents in the atmosphere is excluded). In particular, attenuation caused by rain along the propagation path must be estimated so that the intrinsic reflectivity from a given resolution volume can be retrieved. Even at a relatively low radar frequency of 3 GHz (S-band range), attenuation effects can be significant when the propagation path passes through multiple rain cells of high intensity. With a single polarized radar, it is not possible to measure the cumulative attenuation along a propagation path since no reference signal is generally available against which to compare the attenuated signal (an exception is satellite-based radar for which the ocean surface forms a reference, e.g. the Tropical Rain Measurement Mission (TRMM) precipitation radar; see Meneghini and Kozu 1990).