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Joined 2026-09-21 Message
Incoming links: Cloud ice nucleation versus ice growth
Past exam of the atmospheric physics course of the University of Cambridge 2012 Paper 1 2 c Solution 2026-10-20
A droplet can remain liquid below its equilibrium melting temperature through supercooling: a microscopic ice embryo must first pay an interfacial free-energy cost. The nucleation barrier can therefore delay freezing even when bulk ice is thermodynamically favoured. Solutes, available surfaces, temperature, droplet volume and exposure time affect the probability; zero degrees Celsius is not an automatic freezing threshold for every atmospheric droplet.
At least four physical routes are relevant:
- Homogeneous ice nucleation: an ice embryo forms within deeply supercooled liquid without a foreign surface.
- Immersion freezing: a suitable particle already inside a liquid droplet catalyses an ice embryo.
- Contact freezing: a suitable particle contacting the droplet surface triggers heterogeneous freezing.
- Deposition nucleation: ice appears from vapour on a suitable solid surface under ice-supersaturated conditions; pore condensation or a thin liquid film can participate, so the observed mode need not imply a single direct molecular pathway.
Mineral particles provide nonbiological examples of heterogeneous nuclei. A particle's presence alone does not guarantee activity at a given temperature. The cloud ice nucleation versus ice growth distinction matters: the processes listed concern the formation of new ice; subsequent depositional growth, riming and aggregation are separate from initiating that first crystal.