Residents of Wonthaggi, near Melbourne, Australia, were taken off guard by the appearance of a hole punch cloud earlier this week.

A hole punch cloud forms over southeastern Australia on Monday.

PHOTOGRAPH BY LEESA WILLMOTT, AP

Residents of Wonthaggi, Australia (map) snapped pictures of a rare, rainbow-filled “hole punch” cloud on Monday. By the next day, the photos had gone viral with speculation about the unusual phenomenon overhead.

Clouds are made of water droplets, and hole punch clouds—also known as fallstreak hole clouds—occur when part of that cloud falls out, leaving behind a hole. That opening in the cloud is the result of an extremely localized snowfall.

Usually, atmospheric water droplets latch on to particles in order to form ice crystals, or snow. This happens on a massive scale during snowstorms. The only way water droplets can spontaneously form ice crystals without those particles is if temperatures fall to roughly -40°F (-40°C). (Learn more about these giant cloud holes.)

In a hole punch cloud, temperatures fall in only a small portion of the cloud, forming a localized snowstorm. When that snow falls, it leaves behind a hole. Refraction of sunlight by the ice crystals results in the rainbow, while the arrangement of those crystals gives us a bright patch of light in the middle called a sun dog. (See pictures of sun dogs and halos.)

The expansion of air as an airplane passes can also produce hole punch clouds by cooling water droplets enough for them to form ice crystals.

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