On his morning walk near Victoria, British Columbia, Canada, James Gardner (Member 43,543) watched a layer of Altocumulus stratiformis undulatus clouds gather over the Strait of Juan de Fuca. After a short stop in the grocery store, he emerged to see a cavum had formed in the cloud layer.

Also known as a fallstreak hole, a cavum is a distinctive gap in a cloud layer, often with a cloudy trail hanging from it. It can develop in a cloud layer made of supercooled droplets, which start to freeze in one region – triggered, perhaps, by an aircraft flying through the cloud. Once ice crystals form in one part, they encourage the droplets around them to freeze too. It’s like a chain reaction of freezing. The droplets in the cloud are so cold, they’ll freeze as soon as there’s something suitable for them to get started on – like nearby ice crystals. In this way, the freezing spreads in an ever-widening circle. The ice crystals grow large enough to fall below in a streak, leaving behind a hole in the sky.

A lot can happen up there while you’re getting your groceries.