When the clouds moved like waves

NEED TO WATCH OUT: NIWA forecaster Nava Fedaeff says, 'We get a handful of images shared with us from around the country each year but no doubt they are occurring more than that and we're not aware.' She says she has only seen the cloud formation twice in her life.Picture by EJ Ehau

You don't need a sea to see waves.

Gisborne resident EJ Ehau had just finished work and went to look at the surf when he noticed this rare cloud formation.

“I didn't think it was a big deal and just thought it was ironic that there was good waves and also waves in the clouds,” he said.

Unknown to him, Mr Ehau had seen Kelvin-Helmholtz clouds, says NIWA forecaster Nava Fedaeff.

The spectacular wave shaped cloud is named after Lord Kelvin and Hermann von Helmholtz who studied the physics of instability.

Ms Fedaeff said the concept of instability apllied to all fluids including the atmosphere, “which is a fluid”.

“ When the air at the top of the cloud is moving faster than at the bottom of the cloud and pushes the cloud tops, it creates a wave shape,” she said.

Such a formation could be seen on windy days or when there were differences in air densities — such as near sunrise/sunset when the bottom of the clouds are cooler and air above is warmer.

Ms Fedaeff says while NIWA did not have an official way to keep a track of the occurence, the phenomenon tends to be short lived -with the wave shape breaking down rapidly.

“I would consider them to be uncommon and have only personally seen them twice in my life,” she says.