As a meteorologist and businessman, I respond to data. If I ignore data, evidence or trends that make me uncomfortable, I quickly go out of business. Separating out the signal of climate change from the noise of random everyday weather is challenging. When it comes to hail trends, perception and a changing observational baseline over time may be trumping reality.
A graphic from the Insurance Federation of Minnesota (below) has intrigued me over the years, showing an apparent increase in the number of reports of baseball-size hail (2.75 inches in diameter) in Minnesota since 1955. “The change in the weather patterns that we’re seeing in the last decade, the extreme weather, is bringing bigger claims. Both the number of them and the severity is up,” says Bob Johnson, its president. “That’s nationwide.”
Scientists who study attribution, linking specific weather events to a warmer, wetter climate, have doubts.
“I’m not aware of an observational study that has concluded hail frequency and large hail frequency is increasing,” says Michael Tippett, a climate researcher at Columbia University. “Difficulties with the data makes this a vexing challenge.”
Hail and wind-related losses from severe thunderstorms are, in fact, trending upward, according to data from reinsurance companies such as Munich Re. But multiple factors may be responsible for these trends, including expanding cities, dubious observations, untrained storm chasers, rising building costs and even unscrupulous roofing contractors filing false claims. Pinning any apparent increase in hail frequency and cost entirely on a warming world may be, at best, simplistic.
"The consensus opinion right now is that the shift in hail reports might be an artifact of changes in operating practices and rollout of new technology,” says hail researcher Julian Brimelow at Environment Canada. “The data aren’t there. We’ve had similar problems with tornadoes. You’re dealing with a relatively rare phenomenon; people are very subjective, and things change.”
A reliable, bias-free baseline for hail observations simply doesn’t exist, not yet. Noah Diffenbaugh, a climate scientist at Stanford University, says “we need well-developed, long-term observational records” to detect trends in hail.
Hail records are a hodgepodge. In the 1990s, cable TV and the movie “Twister,” among other factors, encouraged storm chasers to converge on severe supercell thunderstorms, and they tended to field far more hail reports than tornado videos. Now, social media is an even stronger influence.
“What you might call the sampling effort isn’t a constant” said Diffenbaugh. “There are different incentives to find severe events and chase them in some cases.”
Moreover, when hail does fall, there are more people observing it. What was farmland in the 1970s is now subdivisions and strip malls.
Complicating matters, NOAA’s definition of “severe hail” went from 0.75 inches in diameter to one-inch in 2009, and hail often melts before it can be measured properly, so there are reporting inconsistencies.
"Changes in individual National Weather Service offices and changing procedures in how hail reports are gathered increases confusion,” said Diffenbaugh.
Although reliable hail statistics don’t exist, homeowners and insurers are paying more for hail damage. A single hailstorm in the northern suburbs of the Twin Cities on June 11, 2017, left behind $3.2 billion in damage, one of the world’s most expensive storms that year.
But discerning hail size from damage is not very easy or accurate. It is confounded by rising costs of building supplies (shingles aren’t getting cheaper) and lighter, more fuel-efficient vehicles that aren’t as hail resistant as cars and trucks built a generation ago.
Still, scientists are developing conceptual models for how hail may change in a warming world.
There is emerging consensus that a warmer, wetter world will increase hail frequency in some regions. Increased warming would potentially strengthen thunderstorm updrafts critical for hail formation.
However, hail also requires wind shear, a turning of wind with altitude that helps sustain thunderstorms. Overall, wind shear may decrease in a warming world, but changes will depend on location and the time of year. (In some places, at some times, shear would be expected to increase.)
In a warming world, “we might get fewer hail days, but when it does hail, because of increased buoyancy and increased moisture and lapse rates, we could be seeing more frequent instances of large, damaging hail,” predicts Brimelow, who sees a possible increase in the number of large summer hail events over the foothills of the northern Rockies and Canadian Prairies.
Warming may reduce hail frequency over southeastern states, where smaller hail will be more prone to melting before reaching the ground.
“More moisture fuels stronger updrafts, but warmer temperatures mean that some of the small hail won’t reach the ground,” Tippett explains. He predicts hail season may last longer, extending farther north and west.
Tippett says less small hail has already been observed in recent research in France and China. “Changes in the melting level will be accompanied by less small hail, but the same or more large hail” he added.
Larger hail can travel around 100 mph; the time large hail spends below the melting level is much shorter than small hail. Accordingly, both Tippett and John Allen, a professor of meteorology at Central Michigan University conclude a warmer world may support more large hail.
Diffenbaugh has a similar long-range hail outlook, calling for “an increase in severe thunderstorm days overall east of the Rockies,” which could potentially be hail-producers.
Up to now, although any perceived increase in hail frequency or size cannot be supported with data, a warmer, wetter world should increase hail frequency and size across parts of the planet in the years to come. Consider any near-term close encounter with hail an icy shot across the bow.