Supercooled water droplets are liquid water that remains in a fluid state even when its temperature falls below the standard freezing point of \(0^\circ\text{C}\) (\(32^\circ\text{F}\)). This temporary condition is known as a metastable state. The water molecules possess enough kinetic energy to flow past one another, but they are not cold enough to initiate the rigid, crystalline structure of ice on their own.
How Droplets Remain Liquid Below Freezing
Water’s transformation into ice requires a seed, or a “nucleation site,” to begin crystallization. These sites are typically microscopic impurities, such as dust particles, mineral fragments, or bacteria, suspended within the water. Liquid water molecules easily align and lock into the solid ice lattice around these structures.
In the atmosphere, cloud droplets are often extremely pure and small, meaning they lack these necessary foreign particles. Without a solid template to build upon, the water molecules cannot spontaneously organize into the ice structure, even at sub-zero temperatures. This absence of a trigger is why water can remain liquid down to temperatures as low as approximately \(-40^\circ\text{C}\) before the molecules are forced to align by homogeneous nucleation.
The supercooled liquid state is temporary and highly unstable, making it sensitive to external forces. Any sudden disturbance, such as a physical shock, a vibration, or contact with a surface, can provide the necessary energy or alignment to initiate the freezing process.
The Sudden Transition to Ice
When a supercooled water droplet encounters a solid object, the energy transfer or the surface itself acts as the missing nucleation site, triggering freezing. As the liquid water freezes, it releases latent heat, which is the energy absorbed when ice melts. This heat release slows the freezing rate or can warm the immediate area, influencing the type of ice that forms.
The nature of the resulting ice accretion is determined by droplet size and temperature. Rime ice forms when small supercooled droplets freeze almost instantly upon impact in colder temperatures, typically between \(-10^\circ\text{C}\) and \(-20^\circ\text{C}\). Rapid freezing traps tiny air pockets, giving the deposit a rough, opaque, and milky-white appearance.
Glaze ice, also known as clear ice, forms in warmer supercooled conditions, usually between \(0^\circ\text{C}\) and \(-10^\circ\text{C}\), and is caused by larger droplets. These droplets freeze more slowly, allowing the water to spread out before solidifying and for air bubbles to escape. The result is a smooth, transparent, dense, and highly adhesive layer of ice.
Supercooled Droplets in Weather and Aviation
Supercooled water droplets are a common feature of the atmosphere. They are prevalent in cold clouds, particularly in layers where temperatures range from \(0^\circ\text{C}\) down to about \(-15^\circ\text{C}\). These droplets are the source of freezing fog, which occurs when liquid fog droplets remain suspended in air below freezing and freeze immediately upon touching surfaces.
Freezing rain occurs when supercooled water droplets, often categorized as Supercooled Large Droplets (SLD), fall to the ground as a liquid. They freeze only upon contact with objects whose surface temperature is at or below freezing, creating glaze ice on roads, trees, and power lines. This process can cause widespread power outages and treacherous travel conditions.
In aviation, supercooled water leads to airframe icing. Ice accretion on the wings and control surfaces alters the airfoil’s shape, which reduces lift and increases aerodynamic drag and weight. Supercooled Large Droplets (SLD), like those found in freezing rain, are dangerous because they can flow or “bleed back” over the wing before freezing, forming ice ridges behind protected areas. Aircraft utilize anti-icing systems to prevent ice from forming and de-icing systems to remove existing ice, often by heating the leading edges of the wings.