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