Last update: October 17th, 2019 at 9:00 am
In elementary school, students learn that water freezes at 0 degrees Celsius (32 degrees Fahrenheit). That is true most of the time, but there are exceptions to the rule. For instance, water with very few impurities (such as dust or pollution particles, fungal spores, and bacteria) can be chilled to much cooler temperatures and still remain liquid – a process known as supercooling.
Supercooling may sound exotic, but it occurs pretty routinely in Earth’s atmosphere. Altocumulus clouds, a common type of mid-altitude cloud, are mostly composed of water droplets supercooled to a temperature of -15˚ C. Altocumulus clouds with supercooled tops cover about 8 percent of Earth’s surface at any given time.
Supercooled water droplets play a key role in the formation of fallstreak clouds and canal clouds, the distinctive clouds shown in this satellite image. Fallstreak clouds, which are also called hole-punch clouds, usually appear as circular gaps in decks of altocumulus clouds; canal clouds look similar but the gaps are longer and thinner. This true-color image shows fallstreak and canal clouds over Mississippi and Louisiana as observed on December 29, 2015, by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite.
Both types of cloud form when aircraft fly through cloud decks rich with supercooled water droplets and produce aerodynamic contrails. Air expands and cools as it moves around the wings and past the propeller, a process known as adiabatic cooling. Air temperatures over jet wings often cool by as much as 20˚C, pushing supercooled water droplets to the point of freezing.