= Solution
Droplets remain <supercooling>[supercooled] because creating an ice embryo costs interfacial free energy: freezing has a <nucleation barrier> even below the equilibrium melting point. Small droplets also have a low probability of containing an effective ice-nucleating inclusion.
Three heterogeneous routes are \b[immersion freezing] around a particle already inside the droplet, \b[contact freezing] following a collision with a suitable mineral particle, and \b[deposition nucleation] from ice-supersaturated vapour onto a suitable surface. At sufficiently low temperature, \b[homogeneous ice nucleation] occurs without a foreign surface. Its onset depends on volume and observation time rather than being a universal exact temperature. No biological ice-nucleating agents are needed for these mechanisms.
Back to article page
