Posted on 25 March 2022 by Guest Author

This is a re-post from Yale Climate Connections by Bob Henson

When people think of the most dangerous threats spawned by thunderstorms, tornadoes typically come to mind. Yet in terms of total damage, hail really ought to be front and center. U.S. hailstorms cause far more property damage than tornadoes, and their toll is rising fast. Climate change may only accentuate the trend.

Insured U.S. hail losses now average from $8 billion to $14 billion per year, or $80-140 billion per decade, as noted by the Insurance Information Institute. This hefty bill far outpaces the total of around $14.1 billion in insured U.S. property loss from tornadoes over the decade from 2010 to 2020.

Each year since 2008 has produced at least $10 billion (USD 2021) in U.S. insured damage from severe weather, according to the reinsurance firm Aon. That’s more than four times the inflation-adjusted damage rate of the 1980s. Hail is typically the largest single culprit in such losses, according to Aon’s Steve Bowen, who called the trend toward more costly severe weather “definitely a new normal.”

One reason the financial impact of hail is getting worse: there are increasingly more things to damage in hail country.

The nation’s most destructive hailstorms occur in the Great Plains and adjacent High Plains, where metro areas such as Dallas-Fort Worth and Denver have burgeoned over the last several decades. New houses have gotten bigger, and multiple cars and trucks are a given for many suburbanites. Structural roofs and vehicles account for a large share of hail damage, so the expansion of suburbs has put an increasing amount of hail-vulnerable property in harm’s way.

How hailstones are formed

Hail is forged in the intense updrafts of strong thunderstorms. As moisture is drawn into a storm from below, it eventually reaches colder, higher altitudes. Even in midsummer, the air within the upper reaches of a powerful thunderstorm is well below freezing, allowing ice crystals to form. A hailstone begins to develop when an ice crystal collides and coalesces with water droplets that are supercooled – i.e., still unfrozen in air that is below freezing, because they lack a nucleus on which to freeze.

Updrafts can keep a young hailstone from falling even as it grows larger and larger. If a thunderstorm is large and packed with moisture, but with updrafts only modestly strong, the result could be lots of smaller hail. A more powerful updraft can keep larger stones suspended as they grow even bigger. In some cases, a frozen raindrop less than 0.04 inch wide can evolve into a baseball-sized hailstone in just 20 to 30 minutes.

Some of the most intense thunderstorms on Earth – the rotating supercells most common in the U.S. Great Plains – are known for spawning both serious tornadoes and huge hail, sometimes in the same storm. Not every tornado-producing supercell is a prolific hail producer, though, and vice versa.

Global estimated average annual probability of hail with a diameter of more than 2.5 centimeters (roughly one inch), normalized to areas of 100 by 100 kilometers, for 1979–2015. For most locations in the world, hail is a rare event. (From Raupach, et al., 2021, adapted from Prein and Holland 2018, CC BY 4.0)

Especially notorious for huge hailstones is Hail Alley, a zone spanning much of the Central High Plains, including Denver, where the higher elevation leads to deeper cold layers within thunderstorms. Other parts of the world famed for huge hail include eastern India and Bangladesh, central Europe, eastern Australia, the prairies (pampas) of central Argentina, and parts of the Sahel of central Africa.

Hail at least one inch in diameter is enough to justify a severe thunderstorm warning from the National Weather Service. “Significant” hail is at least two inches in diameter (hen-egg-sized). Only a few hailstones span four inches (softball-sized) or more in diameter.

A research team recently proposed a new class – gargantuan hail – for diameters of at least six inches. The group documented a potential world-record hailstone in Villa Carlos Paz, Argentina, on February 8, 2018, that was at least seven and one-tenth inches across, and perhaps as much as nine and three-tenths inches, based on video evidence.

A gargantuan hailstone that fell near Hondo, Texas, on April 28, 2021, spanned more than six and four-tenths inches, making it the state’s largest on record. It resulted from a string of hail-bearing thunderstorms in Texas and Oklahoma that inflicted some $3.3 billion in damage, according to NOAA.

The widest and heaviest hailstone on record for the United States (8″ in diameter and 31 ounces) fell in Vivian, South Dakota, on July 23, 2010. (Photo credit: NWS/Aberdeen, SD, via Wikimedia Commons)

Here’s what recent studies tell us about how hailstorms are evolving and how they may change in a changing climate.

Little consistent change so far

In contrast to some other widely documented trends in line with a warming climate – e.g., the intensification of extreme rainfall, or the increase in record-high versus record-low temperatures – researchers haven’t found a consistent trend in hail evolution around the world. Any such trends would be difficult to ferret out because of hail’s overall rarity and because of regional and temporal variations in how hail is measured.

For example, a doubling of hail reports across the United States since the 1980s is likely a product of more people, greater interest in storms, and more ways to file reports, according to a 2021 overview paper on hailstorms and climate change published in Nature Reviews.

Hail pads – simple devices that allow falling hail to leave an imprint on a Styrofoam pad topped with aluminum foil – are one useful way to measure hail. Regional hail-pad networks across Europe since the 1970s have found varying trends in overall hail frequency, as noted in the Nature Reviews paper.

Though there’s no sign of a broad global shift toward more hail, there are hints that hail is becoming more severe in at least some areas. A 45-year analysis in northeast Italy found a 2% rise per year in the kinetic energy (a proxy for destructive power) delivered by 90th-percentile hailstorms, or the most intense 10% of all hailstorms, even though the total number of hailstorms did not change dramatically.

In the United States, observations from the nationwide network of Doppler radars installed in the 1990s now serve as a unique multi-decade dataset on storm behavior, one that’s unaffected by variations in human hail reports. One analysis, based on maximum estimated hail size as indicated by Doppler radar from 1995 to 2016, showed an overall increase in the geographic extent of radar-estimated hail at least two and one-half inches in diameter, especially over the Rocky Mountains and central United States.

Hail may become less frequent, but trending larger when it does happen

Averaged across the world, the future of hailstorms in a warmer climate will hinge on several competing factors.

More warm, moist air to fuel thunderstorms. More moisture is evaporating from oceans as temperatures rise, so the warm, moist air masses that fuel severe weather may become more unstable on average – a factor that would favor thunderstorm growth and large hail, all else being equal.

A higher melting height. In a warming climate, the average melting level will tend to rise within thunderstorms. Not only would this reduce the depth of a storm’s hail-producing upper layer, but it would give small hailstones more of a chance to melt as they fall to the ground through a deeper layer of air that’s above freezing. (Larger hailstones would be less affected.)

Changes in wind shear. Early studies examining thunderstorms and climate change hypothesized that supercell storms could be less potent on average in a warmer world. The reasons: Although instability should increase overall, a weakening jet stream is expected to lead to a general decrease in vertical wind shear (the change of winds with height that helps supercells to stay organized). The result would be plenty of thunderstorms, but fewer of the intense supercells that spit out tornadoes and huge hail.

Subsequent work has focused on how the changes in instability and wind shear will align in time and space. As it turns out, wind shear may increase in precisely those situations that lend themselves to supercell formation, so the overall global drop in wind shear may not hinder supercells after all.

The current thinking, as summarized in Nature Reviews, is that “vertical wind shear changes are unlikely to strongly affect hailstorms.”

A modeling innovation called dynamic downscaling now allows researchers to blend large-scale global climate projections with fine-scale models that can simulate individual thunderstorms. As a result, scientists can zero-in not just on the kind of environments that would produce hail in a warmer climate, but on thunderstorms themselves. A future next step would be to bring the spectrum of hail sizes into such models, to see if and how that spectrum itself might change.

As of early 2022, the dynamic-downscaling studies thus far suggest that the United States could experience a more prolonged hail season. Summertime is projected to shift toward fewer hailstorms overall, especially in the eastern United States, but with an increase in potentially damaging summer hail in the central U.S., where hailstorms are already most frequent and costly.

Plenty of questions are yet to be resolved about regional hail trends, and uncertainty remains high. However, as cities in the most hail-vulnerable areas, such as Dallas-Fort Worth, continue to expand their urban footprints, it’s quite possible that climate change will compound the effects of that growth on total U.S. hail risk – thus keeping the nation’s bill for hail damage increasing ever more steeply over time.

References

CoCoRahs, “Measuring Hail.”

Eccel, E., et al., Quantitative hail monitoring in an alpine area: 35-year climatology and links with atmospheric variables. International Journal of Climatology 32, 503–517 (2012).

Matthew R. Kumjian, et al., Gargantuan Hail in Argentina, Bulletin of the American Meteorological Society (BAMS) 101, E1241-E1258 (2020).

Timothy H. Raupach, et al., The effects of climate change on hailstorms. Nature Reviews Earth & Environment 2, 213-226 (2021).

Brian H. Tang, et al., Trends in United States large hail environments and observations. NPJ Climate and Atmospheric Science 2, 45 (2019)

Robert J. Trapp, et al., Future Changes in Hail Occurrence in the United States Determined through Convection-Permitting Dynamical Downscaling. Journal of Climate 32, 5493–5509 (2019).