Do tornadoes or hailstorms do more damage?

Both tornadoes and hail have claims to being the most destructive climate events in the US.

Which is more dangerous: tornadoes or hail? It may seem like an abstract comparison, along the lines of the recent movie Godzilla x Kong. But comparing the impact of extreme weather events can reveal crucial lessons that help us to better understand and prepare for climate disasters.

Tornadoes loom large in the American public's perception of weather and land-based risks – and rightly so. An average of around 1,200 hit the US each year. Those that plough through urban areas often prove fatal: in 2011, a devastating series of tornadoes killed over 300 people in a single day.

Yet just as gender, optimism bias, and past experiences of weak tornadoes can all lead individuals to under-estimate their threat, so too can other climate events be under-played in comparison to tornadoes' swirling power.

In 2023, heat claimed more than double the fatalities caused by tornadoes. Meanwhile, in any given year in the US, hail accounts for 60-80% of the insured losses from severe thunderstorms, according to the US catastrophe modelling industry. (Extreme winds – including tornadoes and derechos, long-lived winds that often drive bands of heavy rain and thunderstorms – account for up to a third, and lightning for the rest.)

To those who have only experienced mild hail, the icy sprinkles may seem harmless by comparison. But hail can unleash a torrent of chaos. In June, a plane travelling from Spain to Austria had its nose cone ripped away and its cockpit windows smashed after flying into a hail storm. In 2018, softball-sized ice pummelled a Colorado zoo, killing two birds and injuring 14 people. And, although rare, hail can be fatal to humans too, with eight deaths recorded in the last 70 years. (Read about how climate change is leading to bigger hailstones.)

Harold Brooks is a senior research scientist at the US National Oceanic and Atmospheric Administration's severe storms laboratory, and also a resident of Norman city in Oklahoma. In April 2021, his home was hit by one of three hail storms that caused billions of dollars in property damage across three large cities in the Southern Plains. He recalls windows being trashed, roofs damaged and having to replace the rear window on one of his family's cars. Tornado damage can be much more extreme, he notes, but hail is a more frequent threat: "There's never a non-year for hail."

Tornadoes often get the most attention, perhaps because of the "visual, visceral reaction" people have when they see them, says Vittorio Gensini, professor of meteorology and climatology at Northern Illinois University. But there are far fewer tornadoes than hail storms and their path is also relatively narrow, he points out. Hail storms, in contrast, "can span several kilometres and last hours", says Gensini. "It's like death by a thousand paper cuts."

Their reach and frequency means hail storms are a particular challenge for the insurance industry. According to Ian Giammanco, a meteorologist and managing director of standards and analytics at the Insurance Institute for Business and Home Safety (IBHS), only about 10% of all hail-producing thunderstorms cause damage – but those that do wreak havoc, with hail accounting for "more than $10bn in insured losses every year since 2008".

The diameter of the largest hailstone recorded in the US reached 8in (20cm), and was recorded in South Dakota in June 2010. But even smaller stones can do significant damage. At just 1in (2.5cm) hail can start to damage vegetation, Giammanco says. By 1.5in (3.8cm), it can dent cars and older, shingle roofs. By 2in (5cm) it cracks car windshields and damages most building materials typically used in US construction. At 2in and above, hail has the potential to crack plywood decking and at 4in (10.1cm) it can come through your roof.

However, if you or your property ends up in the narrow path of a tornado, then the threat is extreme: "Hail is going to cause the larger insured losses," Gensini reflects, "but a tornado is going to cost your life." 

Brooks can reel off as years that witnessed "generational outbreaks" of tornadoes: 1908, 1920, 1932, 1965, 1974 and 2011. But there's no hard and fast definition to what makes a particularly severe outbreak, he explains, not least since if a large tornado hits an unpopulated area it may not make it into the reporting database. It's a case of "I know what it is when I see it", says Brooks.

Moreover, the two threats – hail and tornadoes – commonly are found close together. The strongest, rotating and tornado-forming thunderstorms – known as supercells – are produced by the same atmospheric conditions that are also likely to create hail. 

And as human-made climate change continues to deepen its grip on global weather systems, so too is hail likely getting bigger.

Climate change is strengthening storm updrafts, according to an upcoming paper by Gensini and colleagues. Updrafts work like a hairdryer pointing upwards, Gensini explains; the stronger the thrust of air, the larger the ice-ball you could balance at its top. Therefore as the updrafts become stronger, bigger hailstones can be created. However, the warming climate is also causing more, smaller hailstones to melt before they hit the ground, he adds. 

Brooks agrees that conditions for large hail have likely become more favourable as the overall climate has warmed, and adds that the variability for tornadoes is also expected to increase. This is due to an increase in temperature and humidity in the lower atmosphere and a weakening of vertical wind shear. "It's hard to tell what the balance between sheer and energy will be," says Brooks.

Despite the considerable threat posed by hailstorms, however, there is still much that remains unknown about the phenomenon. In light of a lack of recent field studies, Gensini and his colleagues are currently processing a new field campaign specifically to study hailstorms, called ICECHIP. 

The research project will use a combination of measuring tools – including drone photography, doppler radar (which uses pulses of radiation to measure the size of hail or rain), impact disdrometers (which record kinetic energy), and weather balloons – to study what types of conditions produce certain types of hail. This, in turn, the scientists hope this will help forecasting models to better predict when and how hail will hit.

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New materials that can better withstand hail's onslaught are also being developed and tested, says Giammanco, with rubber-plastic shingles for roofing showing particular promise. But there are currently no building codes which require hail resistance, he says, adding that this is something he hopes might soon get more attention. 

When it comes to tornadoes, the US does now have tornado-resistant design requirements for critical facilities such as hospitals and schools. Yet the question remains as to whether these should be expanded to include other types of construction. 

For Giammanco, the question is a personal one. Just this month, a supercell storm produced a tornado that passed within half a mile of his house.

In terms of which Giammanco fears most – hail or tornadoes – it comes down to the cost damage versus threat to life, he says. "People used to dismiss hail, but when we hit $20-30bn (£15.7-23.6bn) loss years, everyone started to pay attention!" But tornadoes are still the more extreme event and threat to life, he adds: "They're the most violent wind storm we have on earth".

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