Why doesn't water always freeze at zero?
Freezing is a change from a disordered liquid to an ordered crystal. For that change to begin, water molecules must gather into a tiny ice-like cluster, and that first step is easier if a foreign surface—a speck of dust, a mineral grain, or a rough coating—helps organize the molecules. When no such surface is present, the water can cool well below 0°C while staying liquid; this is called supercooled water. The paper on supercooled drops notes that such drops are common in high-altitude clouds and can remain liquid for extended periods if temperatures are above the point where ice forms spontaneously without any help [5].
The same idea appears in the sky over mountains. A 43-year climate reconstruction found that supercooled liquid water below −6°C is most common just upwind of mountains in the Pacific Northwest and is also fairly common over higher, drier mountains inland, while it is far less common over the southwestern United States [2]. That geographic pattern shows that supercooled liquid water is not a rare laboratory curiosity; it is a regular feature of cold clouds, and its presence depends on local conditions rather than on temperature alone.
How do tiny spaces and salt keep water liquid?
When water is squeezed into a space only a few molecules wide—such as the inside of a carbon nanotube—its behavior changes. A simulation study of water in nanotubes explicitly reports liquid water existing below the freezing point, and it also finds ice-like behavior near the boiling point, showing that confinement can shift both ends of water's normal phase diagram [1]. The abstract does not explain the full mechanism, but the result is consistent with the idea that narrow pores disrupt the orderly packing needed for ice.
Salt works differently. Dissolved salt ions interfere with the formation of an ice crystal lattice, so salty water must be cooled further before it freezes. Beneath glaciers and ice sheets, liquid water can exist well below freezing, and some of that water is saline; a geophysical method paper describes subglacial water systems, fresh and saline, that host unique microbial ecosystems and notes that liquid water can persist at temperatures well below freezing in those environments [4]. The paper's focus is on measuring salinity remotely, not on explaining the freezing-point depression itself, but its observation confirms that salty subglacial water is a real example of liquid water below zero.
Where does this matter in the real world?
Supercooled water is not just a curiosity; it affects weather and engineering. In clouds, supercooled liquid water is important for precipitation prediction and can be hazardous to aviation, and it is also the target of glaciogenic cloud seeding, which tries to trigger ice formation to increase snowfall [2]. The climate reconstruction shows that supercooled liquid water is most common in specific mountain regions, which helps narrow uncertainty about where cloud seeding might work [2].
On the ground, supercooled drops hitting a cold surface can freeze on impact and cause ice accretion, a safety hazard. Experiments with icephobic gradient polymer coatings found that coated surfaces can decrease the freezing probability upon impact; rougher, less wettable coatings did not increase freezing and instead caused drops to rebound and roll off, even under high wind velocities [5]. That work is aimed at designing anti-icing materials, and it illustrates that whether a supercooled drop freezes depends on the surface it meets as well as on its temperature.
In pipes, the story is more applied still. A 1997 heat-transfer study used a water-immiscible coolant below zero in direct contact with water in straight and coiled pipes, a setup relevant to making "liquid ice"—a slurry of ice crystals in water that can flow [3]. The abstract does not give a detailed mechanism for how the water stays liquid, but the experiment itself relies on water and coolant interacting at temperatures below zero, showing that engineered systems can manage water in a below-freezing liquid state.
Sources used in this answer
Dynamics of Water in Nanotubes: Liquid Below Freezing Point and Ice-like near Boiling Point
Simulations of water in nanotubes report liquid water existing below the freezing point and ice-like behavior near the boiling point, showing that confinement shifts water's phase behavior.
Climatology of Cold-Season Supercooled Liquid Water and Glaciogenic Cloud Seeding Potential in the Western United States, According to a 4-km Resolution Climate Reconstruction
A 43-year climate reconstruction finds that supercooled liquid water below −6°C is most common just upwind of mountains in the Pacific Northwest and is far less common over the southwestern United States.
Formation behavior of liquid ice in straight and coiled pipes using water-immiscible coolant; Chokkan oyobi dakannai deno hisuiyosei futoeki wo mochiita liquid ice …
A heat-transfer experiment used a water-immiscible coolant below zero in direct contact with water in straight and coiled pipes, a setup relevant to liquid-ice formation.
A quantitative method for deriving salinity of subglacial water using ground-based transient electromagnetics
Liquid water can exist well below freezing beneath glaciers and ice sheets, where subglacial water systems, fresh and saline, host unique microbial ecosystems.
Study of Supercooled Water Drop Impact on Icephobic Gradient Polymer Coatings
Supercooled water drops below freezing are common in high-altitude clouds and can remain liquid for extended periods; icephobic gradient polymer coatings can reduce freezing probability upon impact.