Supercooled Water Droplets

What Are Supercooled Water Droplets?

Supercooled water droplets are liquid water particles that exist at temperatures below the normal freezing point (0°C) but have not yet turned into ice. This occurs because, in the absence of suitable freezing nuclei, water can remain liquid down to about -40°C. These droplets are common in clouds, fog, drizzle, and even rain at subzero temperatures.

Formation and Atmospheric Role

Supercooled droplets form when water vapor condenses onto microscopic particles called condensation nuclei. If freezing nuclei are scarce, the water remains liquid even as the temperature drops below freezing. The presence of condensation and freezing nuclei in the atmosphere determines whether water vapor forms droplets or ice crystals.

Icing Hazards in Aviation

When an aircraft flies through a cloud or precipitation containing supercooled droplets, these droplets can freeze instantly upon contact with the airframe, creating clear ice. This process is a major cause of in-flight icing, which can degrade lift, increase drag, and add weight. Freezing rain is a classic example—supercooled raindrops freeze on impact, forming a smooth, transparent ice layer.

Temperature Dependence

The amount of supercooled water in the atmosphere depends on temperature. Between 0°C and about -15°C, clouds often contain a high proportion of supercooled droplets. Below -40°C, droplets almost always freeze, so supercooled water is rare at these temperatures.

Key Terms

• Supercooled droplets aviation: Refers to the operational hazard these droplets pose to aircraft.
• Icing cause: Supercooled water is the primary source of in-flight icing.
• Freezing nuclei: Particles that trigger freezing, reducing the amount of supercooled water.
• Condensation nuclei: Particles that allow water vapor to condense into droplets.