Supercoola

Meet Supercoola, a drop of water that stays liquid below zero, as long as nobody nudges her.

Water normally freezes at 0 °C, but freezing has to start somewhere. Ice forms by building on a seed: a speck of dust, a scratch, or a tiny crystal already present. Very clean, still water lacks such seeds, so it can cool below its freezing point and stay liquid. It is then supercooled, and unstable: a shake or a single ice crystal sets off rapid freezing.

When supercooled water does freeze, the released latent heat warms it back to 0 °C. Water at −10 °C needs about 4.18 × 10 = 41.8 kJ/kg to warm up, while freezing releases 334 kJ/kg, so only about 41.8 / 334 ≈ 12.5 percent freezes at first, as slush.

Clouds are full of supercooled droplets. When they hit cold surfaces they freeze on contact, causing aircraft icing and freezing rain.

There is a limit: pure water cannot be supercooled indefinitely. Around −40 °C ice forms spontaneously even without seeds.