= Solution
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.
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