AUSTIN (KXAN) — KXAN viewer Phyllis Miller spotted strange-looking clouds near Luling on Thursday.

This cloud formation is known as a fallstreak hole, or hole punch cloud. It’s a large circular or elliptical gap that appears in cirrocumulus or altocumulus clouds.

How do they form?

Mid and high-level clouds like these are often composed of tiny water droplets that are much colder than freezing but have yet to freeze. These supercooled water droplets need something to cling onto to actually freeze — like dust, pollution particles, or even bacteria.

When an airplane passes through the thin cloud layer, the air around its wings and body expands and cools through adiabatic cooling, triggering the supercooled droplets to flash freeze and turn into tiny ice crystals.

As the newly-formed crystals quickly grow and absorb adjacent water droplets, they become heavier and fall out of the cloud. This leads to an expanding hole in the cloud.