Critical phenomena in atmospheric precipitation

Critical phenomena occur near continuous phase transitions. As a tuning parameter crosses its critical value, an order parameter increases as a power law. At criticality, order-parameter fluctuations diverge and their spatial correlation decays as a power law(1). In systems where the tuning parameter and order parameter are coupled, the critical point can become an attractor, and self-organized criticality ( SOC) results(2,3). Here we argue, using satellite data, that a critical value of water vapour ( the tuning parameter) marks a non-equilibrium continuous phase transition to a regime of strong atmospheric convection and precipitation ( the order parameter) - with correlated regions on scales of tens to hundreds of kilometres. Despite the complexity of atmospheric dynamics, we find that important observables conform to the simple functional forms predicted by the theory of critical phenomena. In meteorology the term 'quasi-equilibrium' refers to a balance between slow large-scale driving processes and rapid release of buoyancy by moist convection(4). Our study indicates that the attractive quasi-equilibrium state, postulated long before SOC ( ref. 5), is the critical point of a continuous phase transition and is thus an instance of SOC.