Weather: Aucklanders fascinated by gravity wave cloud formations after week of rain

Gravity waves appear due to density differences in the atmosphere, NIWA says. Photo / Johann Schoonees

Aucklanders were fascinated by cloud formations called gravity waves among the blue afternoon sky today, after a week of cloud and rain that is set to return tomorrow.

In a Twitter post, Niwa stated that gravity waves appear because of density differences in the atmosphere.

"For instance, dry air is more dense than moist air. A wave or disturbance can cause air to be pushed into a less or more dense region, but it will 'bounce back' due to gravity. This causes oscillations, or waves."

While these cloudy ripples provided a reward-like break from week-long rain, MetService has flagged heavy wind and rain warnings for the upper North Island as a front is set to slowly head southwards across the upper North Island from later tomorrow into early Tuesday.

MetService says this front is expected to bring heavy rain, with the largest accumulations likely in the north and east of Northland, northern Auckland and the Coromandel Peninsula.

Northeast winds are also expected to approach severe gale strength over Northland and Auckland during this time.

Auckland will have a high of 15C tomorrow with showers mainly north of the city, turning to widespread rain from the afternoon. Northeasterlies will become strong by evening.

Another northerly rainmaker next week - what's behind the wet? 💧

The combination of a strengthening negative Indian Ocean Dipole + restrengthening La Niña = more moisture for lows to harness as they track toward New Zealand.

The pattern may continue into August. pic.twitter.com/lPc8vPqZJH

— NIWA Weather (@NiwaWeather) July 22, 2022

Northland can expect 100 to 140mm of rain to accumulate in northeastern areas from Whangarei to Kaitaia, and 60 to 90mm of rain elsewhere.

Heavy rain may cause streams and rivers to rise rapidly in these areas.

Gravity waves appear due to density differences in the atmosphere. Photo / Johann Schoonees

Surface flooding and slips are also possible and driving conditions may be hazardous.

Rain will pour down throughout the week, with the lower North Island seeing spots of scattered sun on Monday and Friday.

Wellington will get high clouds and light winds today with a high of 12C.

The South Island won't catch much of an escape from the north's rain and its average high of 13C next week with Nelson, Blenheim, Christchurch, Dunedin, Queenstown and Invercargill all seeing showers from Sunday through to Monday.