Huge hail stones explained after Canterbury storm sees cars dented, windows broken

Thursday, 21 November 2019

Hailstorm that hit Timaru at noon on Wednesday.

EXPLAINER: Lightning, thunder and large hailstorms hit Canterbury 'like a sledgehammer' this week.

Golf ball-sized hail stones smashed windows, damaged roofs, closed businesses and dented vehicles in Timaru on Wednesday.

There's nothing unusual about hail falling in New Zealand, especially at this time year when unsettled weather increases the chance of thunderstorms forming. But Niwa's Ben Noll agrees golf ball-sized stones were a little larger than average.

But what causes hail stones get so darn big?

MetService image showing cumulonimbus clouds forming after being triggered by sea breezes.

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Hail stones the size of golf balls hit Timaru at lunchtime in November 2019.

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THUNDEROUS CONDITIONS

You can spot thunderous-looking clouds from a mile away. Cumulonimbus clouds are huge, tall, puffy clouds that stand out from the rest.

Water droplets can become 'supercooled' and turn into hail.

For the clouds to form - and hail to then fall from them - the conditions have to be just right.

Noll says that the unsettled weather in Canterbury this week provided the perfect conditions for thunderstorms.

At this time of year we're starting to get warmer days, so the ground often heats up. Canterbury also had warm, strong winds in its forecast - and that's really the key ingredient here.

The opposing winds were blowing in different directions, but when the winds hit each other, they were forced upwards and acted as a trigger for thunderstorms to form.

These are not chicken eggs. Hail in Cust, North Canterbury.

Thunderstorm triggers can come from several sources, MetService states. Fronts can create their own updraft, mountains can force air up as it tries to pass over them, and features in the upper atmosphere can create the uplift needed near the ground to generate a thunderstorm.

MetService's Andy Best says the clouds themselves can be up to 12 kilometres in length, possibly even longer, and reach high up into troposphere.

During this cloud formation, warm, lower level winds and moisture are pushed upwards into freezing conditions. It's here that hail can form.

A vehicle with a dented roof at Hollands Motor Cars after a hailstorm.

HOW HAIL FORMS

When warm, moist winds are lifted high up into cold temperatures, water droplets can become 'supercooled', Noll says. This means they're about 0 degrees Celsius but still in liquid form.

Thunder, lightning and hail as severe weather hits parts of Canterbury this week.

As these droplets collide with dust, dirt and other water droplets, and are pushed into freezing conditions, they freeze and can turn into 'hail embryos'.

These small stones then bounce around in the updraft and downdraft winds within the clouds, where they collide with other water droplets, and eventually become bigger and bigger.

The reason why Canterbury's storm produced such large stones was because it had particularly strong updraft and downdraft winds, allowing for the hail to keep growing in size as layer upon frozen layer was added to it while in the clouds.

Hail stones compared to golf balls in Mid-Canterbury.

When the stones get too heavy, gravity forces them to fall to the ground below.

To put it simply, there are lots of different (and rather technical) processes that can cause hail to form, Best says. The shape of the hail stones also depends on the process it goes through.

Some can be nice and round, other stones can have a jagged sort of appearance, he says.

Depending on the size of the stone, they 'can cause quite a bit of damage', Noll says.

In November 2005, a significant hailstorm caused damage to crops in parts of New Zealand, he says.

The golf ball-sized hail stones which struck homes, cars and vehicles in Timaru this week, caused windows to break and dented metal.

Although large hail stones in New Zealand were not unprecedented, the ones which struck Canterbury appeared larger than average, Noll says.

But they definitely look small when compared with some of the hail stones that fall from huge thunderstorm clouds overseas.

Just days ago, hail stones 'the size of cricket ball' pelted Australia's Sunshine Coast. The storm was labelled a 'catastrophe' after causing significant damage to houses, cars, and businesses.

The Insurance Council there estimates there was $40 million worth in damage, ABC reported.

There's also a higher chance for thunderstorms to occur at this time of the year due to the changing seasons, Noll says.

At times during spring, it can seem 'like summer and winter are boxing'.

MetService states thunderstorms occur reasonably often in New Zealand. Severe thunderstorms are much less common, occurring perhaps once a month or less.

Although the next few days are looking to be fine for many with soaring temperatures, there could be a chance of thunderstorms forming on Sunday for Marlborough and eastern parts of the North Island, Noll says.