There are a number of mechanisms of ice nucleation in the atmosphere (see below) through which ice nuclei can catalyse the formation of ice particles. In the upper troposphere, water vapor can deposit directly onto solid particle. In clouds warmer than about −37°C where liquid water can persist in a supercooled state ice nuclei can trigger droplets to freeze. Contact nucleation can occur if an ice nucleus collides with a supercooled droplet, but the more important mechanism of freezing is when an ice nucleus becomes immersed in a supercooled water droplet and then triggers freezing. In the absence of an ice nucleating particle pure water droplets can persist in a supercooled state to temperatures approaching −37°C where they freeze homoegenously. 
Ice particles can have a significant effect on cloud dynamics. They are known to be important in the processes by which clouds can become electrified, which causes lightning. They are also known to be able to form the seeds for rain droplets.
Many different types of particulates in the atmosphere can act as ice nuclei, both natural and anthropogenic, including those composed of desert dust, soot, organic matter, bacteria (eg Pseudomonas syringae), pollen, fungal spores and volcanic ash amongst others. However, the exact nucleation potential of each type varies greatly, depending on the exact atmospheric conditions. Very little is known about the spatial distribution of these particles, their overall importance for global climate through ice cloud formation, and whether human activity has played a major role in changing these effects.
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