An ice nucleus is a particle which acts as the nucleus for the formation of an ice crystal in the atmosphere. The presence of ice nuclei increase the temperature that ice will form in the atmosphere from around −42°C to about −10°C. There are many processes that can take place in the atmosphere to form ice particles, the simplest is by water vapor subliming directly onto the solid particle. The presence of an ice nucleus can also cause a previously supercooled water droplet to freeze through contact, immersion or dissolution within the water that would otherwise have stayed in the liquid phase at a given temperature. 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 minerals, soot, organic matter and sulfate. 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 on global climate through ice cloud formation and whether human activity has played a major role in changing these effects. Attempts to measure IN in the lab as recently as June 2007 show that biomass burning-dominated aerosols do not act as ice nuclei.
