Ice nucleus

In clouds warmer than about −37 °C where liquid water can persist in a supercooled state, ice nuclei can trigger droplets to freeze.

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 homogeneously.

It has become clear that the concentration of ice nucleating particles in shallow clouds is a key factor in cloud-climate feedbacks.

[7][8] Many different types of atmospheric particulate matter can act as ice nuclei, both natural and anthropogenic, including those composed of desert dust, soot, organic matter, bacteria (e.g. Pseudomonas syringae), pollen, fungal spores and volcanic ash amongst others.

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.