A bacteria called Pseudomonas syringae has the ability to use proteins in their outer membranes to help ice crystals form at warmer than normal temperatures which they, then, use to form frost and invade damaged plant tissue. After these bacteria die, many of these proteins move into the atmosphere where they can alter the weather by seeding clouds and precipitation.

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http://www.eurekalert.org/pub_releases/2013-10/aiop-utt102313.php