Radiation pressure on dust grains can raise small dusty clouds above the main gaseous disk to high Galactic latitudes. This photolevitation effect drives neutral gas and dust into a soft Galactic fountain and can maintain a column density of the order of 10 to the 20th/sq cm above the main gaseous disk. This value is defined by dust opacity and corresponds to a normal dust-to-gas ratio with cosmic abundances. The maximum height reached by the photolevitated clouds depends on the radiation field and dust-to-gas ratios. Clouds located above luminous stellar clusters or near spiral arms with intense star formation can reach several hundred parsecs in height.
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Franco et al. (1991) studied this question.