A Taranaki tornado, container-toppling winds in Auckland, and now – alien hail.

In the midst of thundery weather that swept across the North Island this week, Aucklanders reported odd-shaped hailstones slamming into their homes yesterday.

Rather than the small polystyrene-like white balls we're used to, some of the hail produced by this week's storm appeared more like spiky, sometimes jagged, globs of ice.

Henderson resident Andy Robinson said his windows were being hit so hard by the giant stones it sounded "like rocks being thrown".

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"I can't see any broken panes but cracks can form a day or so later," he said.

"A lot larger than pea-sized, the largest were approximately golf ball-sized - certainly the largest hailstones I can remember seeing."

Another Henderson resident, Michael Rust, described a quiet, before the hail "pelted down".

"It sounded like rocks hitting my roof."

He hadn't yet noticed any damage yet, except for a small chip in his car windscreen.

WeatherWatch.co.nz reported that some thunderstorms could produce jagged hail as the frozen precipitation bounced around in the towering clouds, joining with other hail which may be melting and refreezing in the chaotic up and downdrafts.

Some of the odd-shaped hailstones that hit Henderson last night. Photo / Michael Rust
Some of the odd-shaped hailstones that hit Henderson last night. Photo / Michael Rust

"It's also a reason why planes do their best to avoid them, due to the mechanics of strong winds inside these clouds."

Niwa meteorologist Ben Noll explained how hail occurred when updrafts in thunderstorms lifted raindrops high up into the atmosphere, where they froze.

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"Hail grows in size when it is continuously lofted up higher into the atmosphere, by way of strong updrafts inside of a thunderstorm, thus colliding with super-cooled water droplets."

The super-cooled water froze when it came into contact with hailstones, increasing their size, and when the thunderstorm's updraft couldn't support the weight of the hailstone any longer, it fell to the ground.

"Depending on the temperature profile between the top of the thunderstorm and the ground, hailstones can begin to melt and re-freeze together - this is what causes the irregularly shaped hail."

Andy Robinson described the size of the hailstones that hit his house as the size of golf balls. Photo / Andy Robinson
Andy Robinson described the size of the hailstones that hit his house as the size of golf balls. Photo / Andy Robinson

It was the latest dramatic feature of an enormous low pressure system that's swept in from the west over past days.

A tornado-like waterspout was reported to have ripped through central Auckland's waterfront last night, toppling at least one shipping container and wreaking havoc on yachts, ships and cars.

Vehicles were swept into the ocean and a yacht was struck by lightning as the storm left a trail of destruction on Monday night.

A shipping container toppled on to a car at Jellicoe Wharf near Tinley St, trapping the driver. They were tended to by emergency services and taken to hospital in a moderate condition.

And earlier yesterday, a tornado blew through Taranaki, ripping windows out of a farmhouse and slamming a woman against a wall, breaking her collarbone.

Noll said some of the most damaging impacts could be put down to straight-line winds, which were associated with lines of thunderstorms.

"The winds in the upper atmosphere have been very strong and vigorous with this low pressure system that's happened in the Tasman Sea," he said.

"And what happens is when you have heavy precipitation and heavy rain in a thunderstorm, it can help transfer the momentum at higher points of the atmosphere down toward the surface."

One of the hailstones that hit Andy Robinson's Henderson, Auckland, home last night. Photo / Andy Robinson
One of the hailstones that hit Andy Robinson's Henderson, Auckland, home last night. Photo / Andy Robinson

He compared yesterday's weather with a similar storm that hit Auckland in April last year, sending 130km/h winds against the SkyTower and causing damage to homes.

But, despite reports, he was unaware of any waterspouts – essentially tornadoes over water – striking the city yesterday.

Auckland was hit by a tornado on average less than once a year, but there was much variation from year to year, with some years seeing no tornadoes at all.