Rain viewed as relaxational events

By following an analogy to earthquakes, we demonstrate how, from the point of view of energy flow through an open system, rain is analogous to many other relaxational processes in Nature. Defining rain events as the basic entities of the phenomenon, we show that the number density of rain events per year is inversely proportional to the released water column raised to the power 1.4. This is the rain-equivalent of the Gutenberg–Richter law for earthquakes. The event durations and the waiting times between events are also characterized by scaling regions, where no typical time scale exists. The Hurst exponent of the rain intensity signal H = 0.76 > 0.5. It is valid in the temporal range from minutes up to the full duration of the signal of half a year. All of our findings are consistent with the concept of self-organized criticality, which refers to the tendency of slowly driven non-equilibrium systems to evolve into a state of scale free behaviour. We note that self-organized criticality may offer an alternative to the chaos theoretic perspective on the subject of rain.