White swans at dawn make majestic sight

Waves were breaking in the Palmerston North sky yesterday morning in a relatively uncommon phenomenon.

Unsurprisingly, the formation is dubbed “breaking wave clouds” but its official title is Kelvin-Helmholtz instability, named after ScotsIrish scientist Lord Kelvin William Thomson and German physician and physicist Hermann von Helmholtz.

MetService meteorologist Tuporo Marsters said 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. The photo appeared to be taken early in the morning, when temperatures were still cooler.

“It’s pretty impressive, all right. It’s probably one of the best Kelvin-Helmholtz waves I have ever seen,” he said.

In this case, there was a steady easterly flowing, likely funnelling through the Manawatu Gorge, he said. The gorge is also needed to create the waves, as it acts like a barrier.

With the waves, the moist air is rolling over the cloud creating a wave-like formation.

He said the clouds appeared to be quite low and have formed from right to left.

“The first one is the most perfect and then it breaks up on the third and fourth . . . it’s low cloud and they’ve got the prevailing wind which is going across it and it’s enough to make these swan, or breaking wave, clouds.”

Palmerston North, like much of the North Island, had warm, muggy conditions overnight Saturday. It woke to 18C with a 10-knot, or 19km/h prevailing easterly, before climbing to 24C at lunchtime.

The city hadn’t been too wet over the weekend, receiving just 30.8mm of rain between early Friday morning and 10am yesterday.

Marsters said most places were forecast for showers today. However, Taranaki would be the wettest with heavy showers and thunderstorms likely.