  Critical phenomena near continuous phase transitions are typically observed
on the scale of wavelengths of visible light[1]. Here we report similar
phenomena for atmospheric precipitation on scales of tens of kilometers. Our
observations have important implications not only for meteorology but also for
the interpretation of self-organized criticality (SOC) in terms of
absorbing-state phase transitions, where feedback mechanisms between order- and
tuning-parameter lead to criticality.[2] While numerically the corresponding
phase transitions have been studied,[3, 4] we characterise for the first time a
physical system believed to display SOC[5] in terms of its underlying phase
transition. In meteorology the term quasi-equilibrium (QE)[6] refers to a state
towards which the atmosphere is driven by slow large-scale processes and rapid
convective buoyancy release. We present evidence here that QE, postulated two
decades earlier than SOC[7], is associated with the critical point of a
continuous phase transition and is thus an instance of SOC.
