A cavum, also known as a fallstreak hole, is a gap in a cloud layer that can form when its droplets in one region all start to freeze and fall below. For this to happen, the cloud’s droplets first need to be supercooled. This means they’re in liquid form even though temperatures in the cloud are below the 0ºC (32ºF) that we’d expect water to freeze at near the ground. Strange as it sounds, the droplets in clouds are often like this, remaining in liquid form at temperatures like –20ºC (–4ºF). But it is an unstable state of affairs, and all they need to start freezing is a little encouragement.

An aircraft flying through the cloud layer or some falling ice crystals from higher clouds is enough to set off a chain reaction of freezing. Once the supercooled droplets start to freeze in one region, the developing ice crystals encourage other droplets nearby to freeze too and soon grow large enough to start falling. They typically dissipate away in the drier, warmer air below and leave a hole behind that gradually expands as the freezing spreads. Steven Kondrak (Member 62,929) spotted this cavum in a layer of Altocumulus stratiformis above Elma, New York, US. The falling ice crystals in the middle of the hole are refracting the sunlight shining through them to form an optical effect called a parhelion. This is a bright patch of light off to one or other side of the Sun that’s known more colloquially as a sun dog. It’s a dog that likes to jump in the icy puddles of the sky.