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Ice nuclei (mineral dust, bacteria, pollution particles) catalyze freezing of supercooled droplets (liquid below 0°C), enabling ice crystal formation. Freezing temperatures range from −5°C to −40°C depending on ice nuclei type, with heterogeneous nucleation on particles dominating over homogeneous freezing in clouds. Ice formation initiates the Bergeron process, key to precipitation in mid-latitude clouds and colder regions.
Study cloud chamber experiments showing ice nucleation at different temperatures. Examine relationships between cloud temperature and ice fraction.
From your study of cloud condensation nuclei, you know that liquid cloud droplets need particles to form on. Ice formation in clouds faces an even higher barrier. Water does not freeze at 0°C in the atmosphere — in fact, cloud droplets routinely remain liquid at temperatures well below freezing, a state called supercooling. Pure water droplets can persist as liquid down to about −40°C before freezing spontaneously. The reason is that forming an ice crystal requires water molecules to arrange themselves into an ordered lattice, and the energy cost of creating the surface of a tiny ice embryo is enormous relative to the energy gained from freezing at temperatures only slightly below 0°C. This is the same surface-energy barrier you encountered with liquid droplet formation, but it is even more severe for ice.
Ice nucleating particles (INPs) solve this problem the same way CCN solve the condensation problem — by providing a surface that lowers the energy barrier. Certain particles with crystal structures resembling ice, particularly mineral dust (especially clay minerals like kaolinite and feldspar), some biological particles (certain bacteria like *Pseudomonas syringae*), and volcanic ash, can template ice formation at much warmer temperatures than homogeneous freezing. This process is called heterogeneous nucleation, and it can occur through several mechanisms: deposition nucleation (vapor deposits directly as ice on the particle), immersion freezing (a particle already inside a supercooled droplet triggers freezing), contact freezing (a particle collides with a supercooled droplet's surface and initiates freezing), and condensation freezing (water condenses on the particle and immediately freezes).
The temperature at which freezing occurs depends on the type of INP. The most effective biological INPs can nucleate ice near −2°C, while typical mineral dust operates between −10°C and −20°C, and less effective particles require temperatures below −25°C. This is critically important because the mixed-phase zone of a cloud — the layer between about −10°C and −40°C where both supercooled liquid droplets and ice crystals coexist — is where most mid-latitude precipitation originates. Ice crystals in this zone grow rapidly at the expense of surrounding liquid droplets through the Bergeron process, because the saturation vapor pressure over ice is lower than over liquid water at the same temperature. The ice crystals quickly gain mass, aggregate into snowflakes, and fall — melting into rain if they pass through warm air below. Without ice nucleation, clouds in the −10°C to −40°C range would remain entirely liquid, drastically altering global precipitation patterns. Understanding which particles nucleate ice, at what temperatures, and through which mechanisms is therefore essential to both weather prediction and climate modeling.