Wind change likely to push Australian bushfire smoke away from New Zealand overnight

Auckland's One Tree Hill is back dropped by an orange sky that was caused by smoke. Photo / Greg Bowker Visuals

Smoke from the Australian bushfires - which have created an orange tint over the North Island - will be diverted away from the country overnight.

Scores of people across the North Island were gobsmacked at the sepia-like effect the smoke had on their surroundings.

"It's eerie and scary," one person wrote on Twitter. "There's an orange tinge in the sky and it's heartbreaking," said another.

Most of the North Island was affected, predominantly over the Auckland and Northland region.

A northwest flow had been working as a conveyor belt for the smoke, driving it right over the North Island from Australia, MetService forecaster Sonja Farmer said.

"We have a frontal system going through which has a northwest flow ahead of it, so it's basically drawing that smoke right through from Australia," she said.

The skies above the ASB Tennis Centre as players hit up on Sunday afternoon. Photo / Jason Oxenham

"That weather system is changing overnight and we will be getting a change of winds to southwesterly.

"As those southwesterlies come through, the winds will be diverting the smoke away from New Zealand, probably running parallel to the country heading north."

The phenomenon which created an orange haze was known as cloud condensation nuclei (CCNs).

CCNs were small particles typically 1/100th the size of a cloud droplet which water vapour can condense on.

The smoke particles in the atmosphere above the North Island were acting as this non-gaseous surface to make the transition from vapour to a liquid.

MetService meteorologist Tahlia Crabtree explained the process when smoke altered the atmosphere last week.

"They're little particles in the air and they're big enough that they can interact with the light from the sun which is why it ends up looking quite hazy, scattering the light," Crabtree said.

"The brown or orange tinge is due to the fact it tends to scatter light at the blue end of the visible light spectrum which means eventually the reds and oranges are more significant.

"Especially at sunrise and sunset there is more of the troposphere, the upper atmosphere, that the light has to travel through, and if there's a lot of smoke there's a lot more scattering."

Smoke from the Australian bushfires is making haste across the Tasman, driven by an upper level jet of westerly winds. Haze looks likely colour the skies of the North Island, and parts of the South Island later today. ^Tahlia pic.twitter.com/i7Wb1S4Ejn

— MetService (@MetService) January 4, 2020

#Smoke update for #Auckland. The main plume is now moving over the city & into tonight, also parts of Northland, Coromandel Peninsula & Waikato.

Smoke in NZ's upper North Island peaks across the rest of Sunday & overnight tonight.

- https://t.co/gb7E1qcpZ3

4:40pm NZDT Jan 5 pic.twitter.com/QlPbOewOf2

— WeatherWatch.co.nz (@WeatherWatchNZ) January 5, 2020

A spokeswoman for Auckland Council said air quality data collected from a number of stations across the city found no evidence of unusual spikes at street level, but it was possible there could be spikes in the upper atmosphere.

She said the council would have a better fix on the situation after 24 hours of data is assessed tomorrow.

Meanwhile, a spokesperson from the Auckland District Health Board said no one had presented themselves with effects from the bushfire smoke.

But the effect of the orange tint was felt by the police who were inundated with 111 emergency calls about the unusual change.

A spokesperson said the calls were either to report the haze is present or to ask why the sky has changed colour.

Unique colours could pierce the sky at sunset this evening, Farmer said, given there wasn't dense cloud cover preventing sun to streak through.

This is what the smoke looked like coming into Auckland this afternoon. Video via @MetService. pic.twitter.com/ansb27CPaI

— Isaac Blomfield 🚗 (@NewZulund) January 5, 2020

Auckland weather today #NOFILTER Eerie and scary #prayingforAustralia pic.twitter.com/aIqExWuF4f

— Namita (@Namita33832413) January 5, 2020

In the morning, rain would likely develop over Auckland with the southwest change before clearing in the afternoon and becoming fine.

The weather wasn't so nice in the South Island, with more fronts moving through from the south.

"With the southwest flow, we're getting some colder air coming through so we'll have some below-average temperatures tomorrow," Farmer said.

Persistent showers and unseasonably cool temperatures were expected throughout much of the week, MetService also reported.

Strong southwesterlies were set to drive in the poor weather, with a risk of thunderstorms and hail looming or much of the South Island tomorrow.

However, a brief ridge would make its way on to the country later on Thursday.