What are supercooled water droplets?

• A. Water droplets that have frozen into ice crystals.
• B. Water droplets that have been cooled below the freezing point, but are still in a liquid state.
• C. Water vapor droplets formed at high altitude.
• D. Liquid water droplets formed by condensation on the aircraft surface.

Answer: (B)

The idea being tested is that water can stay liquid even when its temperature is below 0 °C if there’s no trigger to start freezing. These are supercooled water droplets. In cold clouds, pure water droplets can exist in a metastable liquid state because there aren’t enough ice-nucleating particles to kick freezing into gear. They remain liquid until something—like a collision with an ice crystal or a rough aircraft surface—provides a site for rapid ice formation. That’s why supercooled droplets are important in aviation icing: they can freeze instantly on contact with an aircraft, forming icing quickly.

This differs from the other options because:

• Droplets that have frozen into ice crystals are already solid ice, not supercooled liquid.

• Water vapor droplets formed at high altitude aren’t droplets of liquid water at subfreezing temperatures.

• Condensation on a surface produces liquid droplets, but the term supercooled specifically refers to droplets that are liquid despite being below the freezing point, not simply condensation at the surface.