A Warmer Climate Means Bigger Hail

Regions that are often pummeled by severe storms—like the Midwestern United States under last weekend’s powerful thunderstorms and deadly tornadoes—could also face the threat of more extreme hail. 

New research published Monday in Atmospheric Science Letters for the first time linked human-caused warming with the size of hailstones in a single thunderstorm. The study examined a May 3 storm that pelted Paris and other parts of France with hail ranging in size from marbles to golfballs, destroying or damaging more than $350 million worth of property.. The researchers compared real-time data from May 3 with dozens of similar weather patterns from past decades to isolate how a warmer atmosphere changed the storm’s ingredients. 

The analysis shows that, in France and Germany, the probability of hail under similar atmospheric conditions has increased by up to 30 percent. And warming made the hailstones grow from nuisance-size to chunks of ice big enough to shred crops and damage cars and structures.

The Paris storm formed in a weather pattern similar to those that fuel severe weather across the U.S. Midwest or South, when warm, moisture-laden air surges northward and meets cooler air masses beneath strong winds high in the atmosphere. When those ingredients combine, powerful updrafts can develop inside thunderstorms, allowing hailstones to bounce repeatedly through freezing layers in the clouds and grow larger before falling to the ground.

The seasonal collision of air masses that generates intense storms is growing more volatile in a warmer and wetter world. And this winter’s extremes, with frigid snows in parts of the East and renewed drought and heat in the West, helped prime the atmosphere over North America for the March 5-7 storms that killed at least eight people and left a trail of climate scars, from an unseasonably powerful tornado in Michigan to softball-size hail in South Texas. 

For weather and climate researchers who already know that warming sets the stage for more severe weather, extreme hail outbreaks are among the outcomes that keep them up at night. 

But hailstorms are notoriously difficult to study because they form inside thunderstorms only a few miles across, far smaller than the grid cells used in most climate models. Physics can explain why more intense storms produce larger hailstones, and the new research was able to pinpoint a link between global warming and the storm in France. 

Along with other recent research, the new findings show that hail is not a marginal or localized hazard, lead author Davide Faranda, a research director at France’s National Center for Scientific Research, wrote via email. With global warming, hailstorms are becoming more damaging and affecting wider areas, including major cities, he noted.

“Understanding how climate change amplifies these risks is essential for anticipating impacts and improving preparedness,” he wrote, adding that the timing of hailstorms is still mostly driven by natural weather patterns, but global warming is amplifying their punch when they develop.

Instead of trying to simulate the May 3 storm, Faranda and co-author Tommaso Alberti focused on the large-scale weather pattern that produced it. Because that setup occurs regularly over western Europe and the region is densely monitored, they were able to compare what happened in the Paris hailstorm with dozens of earlier storms that developed under nearly identical conditions, and to answer the question of how that was affected by warming.

The research shows climate change has already made hail-producing storms over Paris more intense, said Alberti, an atmospheric physicist at Italy’s National Institute for Astrophysics who studies extreme weather and climate. When those storms happen in “a warmer and more unstable climate,” he said in a statement released with the study, “the resulting hail is larger and more damaging, generating costly damage that falls on transport networks and cascades on residents, businesses, and insurers.”

A Decade of Warnings

The insurance industry has been sounding alarms about the growing hail threat for years. In a January report, Munich Re researchers say severe hailstorms can cause billions of dollars in damage in a matter of minutes when they hit cities packed with cars, glass roofs and solar panels. In Europe, individual hail outbreaks have produced losses approaching $3 billion.

Understanding how warming affects the hail risk in individual storms could help forecasters develop more timely warnings and improve hail-disruption practices, like cloud seeding, in important agricultural areas. Attribution research also provides case studies that can help verify more theoretical climate modeling that projects increases in large hail with continued warming.

The new study affirms previous research showing that the atmospheric environments favorable for severe hailstorms are changing in a warming climate, said Iris Thurnherr, a climate scientist at Switzerland-based university ETH Zürich who studies severe storms and hail. 

“Even though theory suggests storms could become stronger in a warmer climate, there is still uncertainty about whether hailstorms will become more frequent,” Thurnherr wrote in an email. 

Showing how global warming can shape single, memorable events can help the public understand the links between climate change and extremes, and can serve as a starting point for similar research on other dangerous climate hazards, she explained. Overall, she wrote, scientists are more confident than they were just 10 years ago about links between human-caused warming and hail.

But uncertainties remain, said Abdullah Kahraman, an atmospheric scientist at the Newcastle University in England, who also has published research on severe storms and hail.

The biggest challenge is the shortage of recorded observations, he wrote via email. Hailstorms are extremely local and often go unrecorded unless they cause damage or hit populated areas. Scientists are also still working to understand how warming changes the small-scale conditions inside thunderstorms that determine whether hail forms. 

Warming enables stronger upward motions of hot, moist air inside a thunderstorm, like a lava lamp in overdrive, Faranda explained. In the churning air, hailstones stay aloft longer and grow bigger before falling to the ground. With the study, he added, “We can say with more confidence that when storms do occur, a warmer atmosphere can make them more intense and capable of producing larger hail.”