Water can stay liquid below 0°C if it has not yet formed the first stable ice crystal. This metastable state is called supercooling. In everyday water, dust, scratches, bubbles or contact with ice can provide nucleation sites that start freezing near the usual freezing point. If a supercooled sample is disturbed or seeded with ice, crystallization can spread quickly through it.
Below zero is not always instantly frozen
The freezing point is the temperature at which ice is thermodynamically favoured at ordinary pressure, but liquid water can persist briefly below it. To freeze, molecules need to assemble into a stable crystal nucleus. If a clean sample lacks a good starting site, the liquid can remain supercooled.
Pure water droplets can reach much lower temperatures before freezing spontaneously than ordinary water in a scratched container. The exact threshold depends on sample size, surfaces, impurities and how it is cooled. [1]
A seed crystal can start the change
Ice crystals, dust and some surfaces can provide a template that helps water molecules organize into a solid. Once nucleation begins, crystal growth can spread through the liquid, making a bottle seem to freeze suddenly when tapped or opened.
Shaking is not magic: it can bring the liquid into contact with a nucleation site or create conditions that initiate crystallization. A carefully controlled experiment is needed to distinguish a true supercooled sample from water that already contains tiny ice crystals. [2]
Supercooling matters in clouds
Cloud droplets can remain liquid below zero and then freeze when they encounter an ice-nucleating particle. That process is important for ice formation in clouds and precipitation. [3]
Supercooled water is a temporary state, not a different kind of water. Given a suitable nucleation event or enough cooling, it will freeze.
Sources and evidence
- University of Utah — Why does ice form at a range of temperatures? ↗
Nucleation sites and supercooling in water.
- Harvard Natural Sciences — Supercooling of water demonstration ↗
Demonstration of liquid water below 0°C freezing after an ice seed is introduced.
- PNAS — Surface crystallization of supercooled water in clouds ↗
Supercooled cloud droplets and ice nucleation.
Sources consulted: 11 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Original explanatory article prepared from linked scientific and institutional sources; scope and uncertainty are stated in the text.