A bizarre 'bone' cloud seen above Whangarei last weekend has meteorologists baffled.
One photo, taken by Sharron Cleary at 5:30pm on Sunday and sent to WeatherWatch, shows the unusual formation sitting high in the sky above the usual cumulus clouds below.
"Weirdest clouds I've ever seen. Bone structures?" she asked.
"What causes these clouds? Seen over Ngararatunua, Whangarei," wrote Les Gates, who snapped a photo from a different angle.
Meteorologist Bob McDavitt told WeatherWatch it was an altocumulus cloud, normally seen "downstream from a mountain range" - but couldn't explain the strange gaps that made it look like bones in an X-ray.
"The 'ribs' are likely formed by vertical turning over within the cloud itself. Maybe they are spaced so far apart by the cloud moving over something fixed [like ranges] that is affecting it," he suggested.
Another meteorologist suggested they could have been made by planes flying through.
"Looks like contrails from planes through a cirrus layer, or some sort of wave through a cirrus layer," said Dave Hennan of CNN.
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But WeatherWatch is sceptical, saying there was little air traffic in the region at the time.
Another suggestion WeatherWatch shot down right away.
"People [asked] us if these are chemical trails sprayed from planes," WeatherWatch wrote on its site.
"However there is zero evidence of any chemtrails in New Zealand, and the main issue with this is that it's not leaving a trail at all - it's simply clear lines through very high cloud."
WeatherWatch head forecaster Philip Duncan said it was a "cool cloud" created by the local conditions.
"When you look down in the water you'll see some really interesting patterns in the sand caused by currents and the rocking motion of the ocean itself. High clouds and air movements can cause similar patterns above us in the sky and sometimes they do stand out from all the other usual patterns."
A strange cloud formation recently formed over the South American nation of Brazil, some locals comparing it to the scene in 1996's hit film Independence Day when the alien ships arrive.