Rainfall is created by the interaction between the earth surface and its atmospheric layer through different scales of time and space. Some suggest that it is better to treat rainfall process in discrete time series，while most researchers prefer continuous probability density functions in describing rainfall data. Both continuous and discrete characters，depending upon the chosen methodology，can be observed. Coding the time stream with rain/non-rain，we have the rhythms of weather spells across space. Assigning every raining unit of time-space the rainfall amount to build up the distribution of rainfall density，we have the spectrum of rainfall across the earth surface. It is，then，lead to Kirkby's statement on the convolution of the rainfall and the drainage network in producing the hydrological response at hydro-gauge.
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