From the highest of highs to the lowest of lows: Here’s why it’s so windy this week

Wednesday, 22 October 2025

Weather-wise, Tuesday was a really bad day.

Thursday is forecast to be even worse, with red wind warnings in place for Canterbury and the lower North Island.

According to Earth Sciences NZ principal scientist Chris Brandolino, Thursday’s wind could be “really impactful”.

“There could be wind like we haven’t seen in quite some time. From a science perspective, it will be quite interesting to watch,” he said.

Which begs the question: From a science perspective, why is this happening?

Would you like to see something explained or fact checked? Email me at emma.ricketts@stuff.co.nz.

Think of New Zealand as a tall mountain

This week’s blustery conditions can all be attributed to pressure gradient, Brandolino says, which basically refers to the rate at which pressure changes over distance.

Brandolino thinks of pressure gradient as an elevation gradient: “If you make high pressure the top of a mountain, and low pressure the bottom, the speed at which any water flows down will depend on the gradient of the elevation.

“If the two systems are close together, the gradient will be really steep and the water will flow really fast. It’s the same thing in the atmosphere - only the fluid isn’t water, the fluid is the atmosphere.”

On Thursday, forecasts show a high pressure system to the north and northeast of Auckland. These bring nice weather - which is why Auckland has had such nice days this week.

But - also on Thursday - an intense area of low pressure will pass the south of New Zealand, near Invercargill and Stewart Island. Lows are basically big holes in the atmosphere which air moves to fill, Brandolino said.

“The difference in pressure between Invercargill and Auckland will be quite significant. The air is going to have to move really fast and try to fill that hole at the bottom of the South Island - hence the wind.”

The reason the wind will be worse down south is because of the high pressure near Auckland.

“Even when it gets bad further south, we’re still close to that high pressure up here in Auckland,” he said.

What about Wellington?

I can hear what you’re thinking - as a creature of the North Island, shouldn’t Wellington be better off than down south?

In short, no. Wellington is never better off when it comes to wind.

“In Wellington, your wind is always accentuated because of Cook Strait,” Brandolino said. “It basically acts like when you put your thumb over a garden hose.”

We’ve all done that. You cover the hole and the water streams forcefully out the side instead. What fun.

When you apply this to the atmosphere, the same sort of thing happens.

“Whenever you get wind coming into Wellington, the wind has to squeeze from the big space over the sea through the Cook Strait. When that happens, it’s just like putting your thumb over half the garden hose - the water has to come out faster to compensate for the smaller area, so it accelerates. But again, it’s not water - it’s air. And the air has to speed up.”

This analogy explains why Wellington is a windy place in general - but in a situation like Tuesday or Thursday, it’s accentuated.

“Because of that pressure gradient, there will be strong wind in Canterbury and strong wind in Southland. But that wind gets squeezed even more in Wellington because of Cook Strait.”

What about Tuesday?

In short, Tuesday’s wild weather was caused by the same sort thing, just on a lesser scale.

“A low pressure [system] moved to our south and produced a steep pressure gradient, which meant it was windy. But that pressure difference was not as substantial as it is going to be on Thursday,” Brandolino said.

“If you think of the mountain again, on Tuesday we had a mountain and a valley pretty close to each other, which led to water (the air) moving down the hill quite fast.

“On Thursday, the mountain will still be pretty tall but the valley will be even deeper. That means the gradient will be steeper and the water (the air) will rush even faster.”

Is this normal for this time of year?

Spring tends to be windy - any good Wellingtonian will tell you that. But this spring has been “next level”, Brandolino said.

To make something complicated very simple, this is because there have been lots of low pressure systems over the Southern Ocean through September and into October.

Low pressure means a significant pressure gradient, remember? These systems mean lots of air has been rushing south.

So, we could think of this spring as repeatedly rolling down a steep mountain.