Few of us like to be the first to take the plunge. We hold back at the water’s edge, waiting for others to dive in. Clouds are much the same, as is demonstrated by the phenomenon of a ‘fallstreak hole’.
Layers of high cloud, such as cirrocumulus or the high altocumulus, shown above, are often composed of water that is much colder than 0degC but hasn’t frozen into ice crystals. When water is in the form of tiny droplets suspended in the air, it can behave rather differently from that in an ice tray in the freezer. It can stubbornly refuse to freeze, remaining as ‘supercooled’ liquid at temperatures of –10, –15, –20degC… None of the droplets want to be the first to freeze, and they tentatively wait as liquid, until some brave souls decide to make their move.
For reasons that are none too clear, a particular region of supercooled cloud can throw caution to the wind and decide to freeze into ice crystals that grow and fall below. A hole is left behind, and this spreads outwards as neighbouring droplets are swept up in the excitement and start freezing too.
No sooner have some droplets made the change, than they are all joining in. How appropriate, that the trail of falling crystals can look like a bird taking flight.