The water droplets and ice crystals that make up clouds don’t usually form spontaneously in the atmosphere - they need a solid or liquid surface to collect on. Tiny particles of dust, soot and airplane exhaust - and even bacteria - are known to provide these surfaces, becoming what atmospheric scientists call cloud condensation nuclei (CCN).
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These microbes could be carried into the atmosphere from an infected plant by winds, strong updrafts or the dust clouds that follow tractors harvesting a field. Christner and others suspect that becoming cloud nuclei is a strategy for the pathogen to get from plant to plant, since it can be carried for long distances in the atmosphere and come down with a cloud’s rain.

The next step in determining how big a role biological particles play in cloud droplet formation is to directly sample the clouds themselves, Christner says.