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Dust particles and other fine particulate act as condensation nuclei (where water in the gas stage can condensate onto something). Dust is a good example of something that acts as a condensation nuclei. One thing that works well is "Silver Iodide", which is something scientists use to cloud seed to make rain. But it is somewhat expensive to drop into the clouds. At the Salt Lake Airport in the winter time when fog forms, they take Frozen Carbon Dioxide (dry ice) and grind it up and drop it out of the back of an airplane to cause the fog droplets to freeze and turn into snow and fall the the ground.
You can learn more about this subject here: http://en.wikipedia.org/wiki/Cloud\_condensation\_nuclei
