A rare cloud formation appeared over Melbourne on Thursday morning, stunning onlookers across the city.

Image: Asperitas cloud over Melbourne on Thursday morning. Source: Ben Layton

Image: Asperitas cloud over Melbourne on Thursday morning. Source: Anthony Ferrell

The unusual wave-like clouds were called asperitas clouds, which in this case formed in a layer of stratocumulus cloud that was passing over Melbourne. This means the more technical name for Thursday morning's weird and wavy clouds was 'stratocumulus asperitas'. There's one to keep in mind for the next trivia night!

Image: Asperitas cloud over Brunswick on Thursday morning. Source: @pfctdayelise/Twitter 

Image: Asperitas cloud over Melbourne on Thursday morning. Source: @swa11ace/Twitter

Asperitas clouds often look like a rough sea that's formed on the underside of a uniform cloud bank. They can happen at varying heights in the atmosphere and are thought to be caused by a combination of sinking air and contrasting wind in the atmosphere.

Image: Asperitas cloud over Melbourne on Thursday morning. Source: @zehrafahd/Twitter

Image: Asperitas cloud over Melbourne on Thursday morning. Source: @HRShahrokh/Twitter

These clouds were only formally recognised in the 2017 edition of the International Cloud Atlas, which was first published in 1896.