Scaling behaviour in daily air humidity fluctuations

We show that the daily average air humidity fluctuations exhibit non-trivial $1/f^{\alpha}$ behaviour which different from the spectral properties of other meteorological quantities. This feature and the fractal spatial strucure found in clouds make it plausible to regard air humidity fluctuations as a manifestation of self-organized criticality. We give arguments why the dynamics in air humidity can be similar to those in sandpile models of SOC.