When Palmerston North resident Steve Wrathall saw some peculiar-looking clouds in the sky, the first thing he thought of was Dr Seuss.
"They looked like a wave, kind of like a tsunami," said Wrathall, who photographed the clouds as they moved across the Tararua Ranges.
As it turns out, the strange formations aren't called Dr Seuss clouds, but something perhaps just as odd-sounding: Kelvin-Helmholtz instability waves.
Named after Scots-Irish scientist Lord Kelvin William Thomson and German physician and physicist Hermann von Helmholtz, the formation was relatively uncommon and there needed to be certain conditions in the atmosphere - and its surrounds - for the breaking waves, or swans, to occur.
Those conditions include moist air in a "stable situation" - no upward movement - and a steady wind source.
