It was the height of summer in the UK and the country found itself in the grip of a heatwave. In Leicestershire, in the midlands of England, children on their school holidays played in paddling pools to stay cool. Then the sky darkened.

In the early evening of 21 July 2021, hailstones the size of golf balls pelted suddenly from the sky, smashing windows and battering cars. Gardens that were a few moments earlier filled with people soaking up the evening sun, were left badly damaged by the downpour of ice.

While the hailstorm – caused by strong updrafts of cloud high in the atmosphere – was unusual in its severity, it was mild compared to a hailstorm that struck Calgary in Canada in June 2020. Hailstones the size of tennis balls caused damage to at least 70,000 homes and vehicles, destroyed crops and left the area facing a C$1.2bn (US$940m/£720m) repair bill. The 20-minute hailstorm was one of the country's most costly weather events.

And climate change is altering the pattern of hailstorms. In Texas, Colorado and Alabama the records for largest hailstone have been broken in the last three years, reaching sizes of up to 16cm (6.2 inches) in diameter. In 2020, Tripoli, the capital of Libya, was struck by hailstones nearly 18cm (7.1in) across.

While giant hailstones – classed as those with a diameter greater than 10cm (3.9in) – are extremely rare, they are an indicator and hail damage in the US now averages more than $10bn (£7.6bn) a year.

But why might global warming be causing an increase in the amount of ice falling from the sky? And are their limits to just how big a hailstone can grow?

Hail forms as droplets of water are carried upward into a thunderstorm. Updraughts carry them into parts of the atmosphere where the air is cold enough to freeze the droplets. Moisture from the air accumulates on the outside of the drops of ice as it moves through the air, causing the hailstone to grow in onion-like layers.

How fast a hailstone grows depends on the amount of moisture in the air. It will continue to grow until the updraught is no longer strong enough to keep it aloft. A 103km/h (64mph) updraft supports hail the size of a golf ball, while one 27% faster can create hailstones the size of baseballs, according to the US National Oceanic and Atmospheric Administration (although as we will see in a moment, the size of a hailstone doesn't always directly relate to its weight). More humid air and more powerful updraughts will bring bigger hailstones. Often larger hailstones will fall closer to the updraught while smaller hailstones will fall further away, often blown there by cross winds.

Destructive storms that produce hailstones more than 25mm (1in) in diameter require a specific set of conditions, says Julian Brimelow, a physical sciences specialist at Environment and Climate Change Canada, a department of the Canadian government, who has studied how climate change affects hail formation. They require enough moisture, powerful updraughts, and a "trigger factor", typically a weather front. This is why serious hailstorms are usually confined to particular regions such as the Great Plains in the US and Australia’s Gold Coast. Typically such regions have cool, dry air in the upper atmosphere above warm, humid surface air. This unstable situation leads to strong updraughts and the formation of thunderstorms.

Such locations are particularly prone to a type of thunderstorm known as supercells, which can produce very large hail due to the powerful rotating updraughts they create.

But as climate change alters the temperature of the Earth's atmosphere, so too is the amount of moisture in the air. Warmer air can hold more water vapour while higher temperatures also mean more water is evaporated from the Earth's surface. This is predicted to lead to heavier rainfall and more extreme storms in parts of the world.

"As the planet continues to warm, areas where hailstorms are favoured are likely to shift," says Brimelow. "An area now where sufficient moisture is a limiting factor may become more moist and consequently, hailstorm frequency may increase."

A combination of observations of changes already taking place and climate modelling has led researchers to conclude that hailstorms will become more frequent in Australia and Europe, but there will be a decrease in East Asia and North America. But they also found that hailstorms will become generally more intense.

And while hailstorms might become less frequent in North America, hailstones when they fall are also likely to get larger, according to a separate study by Brimelow and his colleagues that looked at how hail conditions in North America might change in a warmer world.

One of the reasons for this is because the height at which hailstones start to melt as they fall will be raised, so small hailstones will melt into rain before they hit the ground, but larger stones pass too rapidly through the warm zone for melting to have much effect on them.

"We have in fact already seen evidence of this, with hail pad data in France indicating a shift in the size distribution of hail," says Brimelow. Hail pads are blocks of soft material that are left out in storms and deform when impacted by hail to give a record of the size and number of hailstones in the area. "Fewer days with small hail have been observed with warming, but there have been more days with larger hail."

It could mean that annual damage caused by hail might also increase. But pinning down exactly which areas will see increased damage from hail is difficult, Brimelow says.

In areas where hail damage is expected, structures may be rated for hail resistance. The current method uses steel balls which may be dropped or fired from a pneumatic launcher to simulate impacts, but increasing the size of hailstones does not scale up the damage as simply as you might expect. A 2020 study by Texas Tech University explored why prediction is so difficult and why hailstorms can be far more damaging than expected.

The temperature and the level of moisture in the air a hailstone forms in can influence how dense it is. In very cold air, water freezes as soon as it collides with the hailstone, but this can lead to a lot of air and being mixed with the ice. If the water freezes more slowly, perhaps because the air is warmer or the amount of moisture in the air is high, meaning not all of it freezes instantly, the air bubbles have time to escape. This leads to clear ice that tends to be denser. Small hailstones are only half as dense as pure ice, as they have a lot of air mixed in as they tend to move rapidly up through the atmosphere before falling again.

The largest hailstones are often composed of a complex mixture of ice layers that form as they move around in the air column. Looking at a cross section of ice can reveal a great deal about how it formed while lopes and icicle-like structures on the outside of the hailstone also provide hints at how it might have been rotating as it was tossed about in the storm.

One large hailstone measuring 17cm (7in) across that was examined after it fell during a storm in Aurora, Nebraska in 2003, for example, was found to have a type of "spongy" air-filled ice at its core and dense clear ice on the outer layers. If it had been made of pure ice, scientists who studied it say the volley-ball sized hailstone should have weighed about 2.5kg (5.5lbs), but it in fact weighed only 500g (1.1lbs) due to the lower density core. They concluded that the hailstone had initially formed as it quickly rose through the clouds, before being tossed out of the updraught by sidewinds before falling back into it again, and this time rising more slowly due to its larger size, and so growing bigger with denser ice.

The density of the hailstone also effects how large it can grow. The heavier it is, the more likely it will fall out of an updraught. And it will also fall faster too, because the bigger a hailstone, the less drag it experiences per unit weight. Hailstones of less than 25mm (1in) diameter typically fall at 11 to 22 m/s (25 – 49 mph), while those of 25-45mm (1-1.7in) fall at 22 to 29 m/s (49 - 65 mph), according to Brimelow. The heaviest hailstone ever recorded fell in Gopalganj district of Bangladesh in 1986, weighing 1.02kg (2.25lbs). The hailstorm killed 40 people and injured 400 others, according to reports at the time, but later reports suggest as many as 92 people may have lost their lives.

But the speed at which a hailstone falls is far from simple. Researchers have in the past assumed that hailstones are approximately spherical, whereas recent research has shown they are more like flattened rugby balls, which can lead to more air resistance as they fall. They also become more uneven as they get larger, with nodules and lobes forming. Both of these factors affect their aerodynamics and how fast they fall, and so how much damage they cause when they finally hit the ground.

Finally, the speed at which a hailstone hits is not the same as its falling speed. For one thing, there may be a horizontal component – side winds can increase the impact speed of a hailstone compared to if it had hit in freefall. The most damaging hail events are downbursts, driven by powerful downdraughts – where air rapidly descends from storms and spread outwards when they hit the ground, producing very high wind speeds. Downbursts are typically only a few kilometres or miles across and last a matter of minutes, but can feature vertical windspeeds of 70-80m/s (156-179mph) with correspondingly destructive hail.

Large hailstones travelling at these sorts of speeds have the power to punch through roof tiles, smash car windows and tear off cladding on buildings. They can devastate crops, injure people and animals. They pose a particular threat to aircraft.

(I once witnessed a downburst in Mendoza, Argentina some years ago – it brought down trees, and the hail was piled up in drifts afterwards, even though it was a warm day).

All of these factors put together mean that scaled-up hailstones can cause significantly more damage than expected.

In 2018, the town of Villa Carlos Paz in Argentina was hit by stones of unprecedented size, with some measuring 18cm (7.1in) across but there could have been some hailstones possibly even as large as 23.7cm (9.3in) in size. Although such dimensions are thought to be close to the world record in size for a hailstone, it is difficult to be certain. For one, giant hailstones are rarely recovered intact, as they tend to strike with shattering force.

Meteorologist Matthew Kumjian of Pennsylvania State University came up with the estimate of the hailstones that fell on Villa Carlos Paz after analysing the many images posted on social media after the storm. He then visited the site and measured lamp posts, awnings and other background objects to get an exact scale, as well as interviewing witnesses. He also managed to inspect one stone preserved in a freezer that measured 11.4cm (4in).

Kumijan notes that reports of giant hail have become more common in recent years.

"In the last two decades, there's been about 10 reports of hail about six inches (15cm) in maximum dimension or greater in the US," says Kumjian. "Those are exceptionally rare."

Records have been tumbling in recent years. A hailstone measuring 16cm (6.4in) across and weighing 590g (1.3lbs), for example, was collected after a storm near Hondo, Texas in April last year. The hailstone was preserved in a freezer and later confirmed as a new record in the state.

Gargantuan hailstones are often outliers in storms that contain far more abundant, smaller hailstones that are still capable of causing widespread damage
But just how big can a hailstone get? Kumjian estimates the largest possible hailstone at 27cm (10.6in) across or "bowling ball sized", based on data from modelling simulations, the maximum mass of a hailstone to be reported (around 1kg/2.2lbs) and research on shape. However, nothing quite this large has yet to be recorded and he says he is working with some colleagues to refine the estimate. While 27cm (10.6in) is at the upper end of the estimates, a hailstone of those proportions would be highly irregular in shape. But he says the ingredients needed to create such a large hailstone – strong updrafts, plenty of supercooled liquid water and plenty time spent travelling around in the cold air – exist today.

"The strong 'supercell' thunderstorms that produce the world's largest hailstones have many of these ingredients coming together already, so the strongest of these storms today is probably capable of producing a supergiant stone," he says.

Gargantuan hailstones, however, are often outliers in storms that contain far more abundant, smaller hailstones that are still capable of causing widespread damage. However, because of their potential to kill livestock and people and severely damage property, giant hailstones are significant even though they are rare.

On 9 June 2006, an Airbus 321 airliner in South Korea, encountered a powerful hailstorm which ripped off the radome (the structure on the nose that protects the radar) and destroyed the radar. Hail battered the wing edges and stabilisers, and parts of the radome were ingested by an engine, damaging it. The crew had to deal with a barrage of automated warning messages triggered by all the damage. They eventually managed to land safely, but only after two missed approaches due to poor visibility.

Aircraft have always been at risk from hail, with 20 incidents recorded from 2017-2019. Their windscreens are strong enough to resist bird strikes so hail does not usually damage them, but hail damage can obscure the windscreen making landing more difficult, as in the South Korean incident.

Weather radar normally allows aircraft to avoid hailstorms, but hail at high altitude – seven of the recorded incidents between 2017-2019 took place above 30,000ft (9,144m) – tends to be dry because the extremely cold temperatures means all moisture is frozen. This means it reflects radar faintly and is difficult to spot. And, as you might expect, larger hailstones are more dangerous than small ones.

On the ground, two new and increasingly common structures are particularly at risk: solar panels and wind turbines.

A 2019 study by the Institute for Environmental Studies in Amsterdam showed that more solar panels means more hail damage. An EU initiative is aiming to have a million zero-carbon homes by 2023 and solar is becoming much more common, but the researchers noted there is a lack of rules and standards to ensure panels are hail-resistant. Destructive hail triggered by climate change may destroy solar panels meant to counter climate change.

Hail damage also erodes wind turbine blades, pushing up maintenance costs and increasing energy losses from wind farms. This is because the leading edge of the wind turbine has to be highly aerodynamic, slicing through the air with minimal resistance.

The edge is typically a curved glass-fibre-reinforced polymer laminate with a brittle polyurethane-based coating. Even rain wears away at this edge, but hail has literally more impact, and repeated strikes will crack it. Any damage to the blade affects airflow and increases drag, making the turbine less efficient. A 2017 Danish study suggests hail damage can be reduced simply by stopping the turbine blades during extreme weather events to reduce the speed of impact.

While more big hailstones may be coming our way, damage is not necessarily inevitable. One option is issuing hail warnings to affected areas. In South Africa insurance companies already send text alerts warning of hail, giving people a chance to get their cars or other property under cover.

Hail netting made from monofilament polyethylene can protect vulnerable fruit such as apples and grapes, catching all but the largest hailstones. Similar netting is now also installed at some car dealerships in the US – a sector which, Brimelow notes, accounts for a significant proportion of hail insurance claims.

A 2021 study led by Leila Tolderlund at the University of Colorado also highlighted the potential for green roofing as hail protection. This consists of a waterproof membrane with a thick layer of soil planted with vegetation. Green roofs provide insulation, reduce heat in summer and absorb CO2, but they also turn out to be excellent hail armour. The study found that in a simulated severe hailstorm, all the non-protected roof surfaces were damaged, while those with green roofing remained unharmed.

There have also been attempts to predict the size of hailstones that might be generated by particular storms, but many of these lack accuracy. As Brimelow notes, it is too early to tell exactly where hail damage will occur in future. But it's clear from his work and others that the really big stuff is likely to still keep hurling down at us. All we can do is prepare, and find a decent shelter.

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