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Turbulence makes such cavities by clearing away material during convective motions.
We propose that most of the low density intercloud medium is the result of turbulence and not overlapping supernova remnants. This model has important implications for diffuse H alpha and ionization of the halo. Fractal clouds have a gradually decreasing average density with increasing distance, so ionization zones around these clouds can be very extensive. Fractal clouds are also highly clustered, making the mean free path for ionizing photons at least twice as large as in the ''standard cloud'' model. This long mean free path allows stray ionizing photons from midplane HII regions to reach and ionize the halo.