Critical phenomena in atmospheric observations

In a broad sense, the branch of statistical physics called "critical phenomena" deals with scale free systems. In the atmospheric sciences observations of apparently scale-free systems abound: area-perimeter relations in clouds display a robust fractal dimension of 4/3, rain event-size distributions are non-trivially scale-free over 5 orders of magnitude, the variance of precipitation rate obeys finite-size scaling as measurement resolution is changed. From the standpoint of statistical physics I will point out that most of these observations can be related to geometric or dynamical phase transitions. Methodological issues related to causal networks will be discussed.