Why Does Hail Pummel Colorado and How Can We Prepare for It?

Weather’s fickle nature is something farmers must endure, and Anne Cure has survived plenty of storms since she planted her first crops in 2005 on a piece of land just northeast of Boulder. The area around her acreage at Cure Organic Farm is largely open space, farms, and ranches with plenty of windrows, fields, and grassland. The views of the Rockies come standard, too, and on most summer afternoons, she can watch clouds gather above the mountains. Not just the sweet, docile cumulus type—the fluffy ones in which you found boat or bunny shapes as a child—but the cumulonimbus variety, otherwise known as thunderstorm clouds. These behemoths can stack several miles high, and instead of white, they turn shades of gunmetal, slate, and ash. Inside, they carry the potential for destruction: lightning, rain, tornados, and hail.

There were a lot of those clouds in 2018—some would say too many. The first storm with hail hit Cure’s farm in the middle of May and lasted for, she says, an excruciating 45 minutes. Gumball-size pieces of ice shredded her newly planted kale, lettuce, chard, arugula, and pea shoots. “It looked like you had taken a weed wacker through,” Cure says. “We lost about six weeks of growing time.”

That also meant days without crops and cash flow. Cure knew her plants would grow back, but instead of focusing their strength on producing leaves and fruit—on reproducing—those plants had to shuttle energy back into rebuilding. Mother Nature wasn’t done: Cure Organic Farm would get hit two more times that summer.

Generally speaking, it’s not good form to complain about precipitation in Colorado, because water is so precious. If there is an exception to that rule, though, it is for the hailstorms that flatten crops, destroy roofs, pummel cars, and clog stormwater drains. And when it comes to hail, the last few years have been particularly difficult for the Centennial State.

On August 6, 2018, a storm near Colorado Springs killed five animals at the Cheyenne Mountain Zoo and injured 14 people. A May 2017 tempest caused more than $2.3 billion in insurance losses in the metro area. Damage from the hail and flooding shuttered the Colorado Mills mall in Lakewood for six months.

Colorado is especially prone to this destruction because it sits in what the National Oceanic and Atmospheric Administration and others call Hail Alley, a swath of the country that also includes parts of Texas, Oklahoma, Kansas, and Wyoming. Peak season is May through July, but this year hail fell in eastern Colorado in April. Nationally, annual hail damage has topped $10 billion for home and car owners and lessees for more than a decade. The damages to properties, plus the resulting insurance payouts, just keep growing.

On summer days, a humid wind travels up from the Gulf of Mexico, slides across the flat prairie, and runs up against the Rocky Mountains, where dry air waits. The two airflows smash against each other to create a physics marvel: a thunder cloud.

If the conditions are just right, there’s a sweet spot in that cloud where hail can form. There, particles bounce around and collect super-cold water, which freezes into ice. The particles keep bouncing—and collecting layers of ice—to form hailstones. If you cut a stone in half, its life history will be revealed, with distinct strata of ice wrapped around a core, just like the rings of a tree. (The largest recorded hailstone in the United States was found in Vivian, South Dakota, in 2010 and measured eight inches in diameter.)

Eventually, the updraft of humid air eases or stones grow too heavy, and hail falls to the ground—if it doesn’t melt on the way, which many stones do. That, combined with the variable movement of thunderclouds, is why meteorologists often give vague indications of hail probability. They might say that a storm “could produce” hail, meaning that all the atmospheric elements will be in place but hail may or may not reach the ground. “There are so many different physics happening at the same time,” says Andreas Prein, a scientist at the National Center for Atmospheric Research (NCAR) in Boulder. “It’s fascinating that it happens at all.”

The fact that it occurs along the Front Range, though, is not all that surprising given the elevation of the land here. There is less space between the clouds and the ground than in, say, sea-level Florida, meaning there is less space and time for hailstones to melt. “We are a mile closer to the cold air, so the thunderstorms that produce hail in other parts of the country have a lot more warm air mass to fall through,” says Mike Nelson, Denver7’s chief meteorologist. “Hail will melt back into raindrops before it hits the ground.”

Unfortunately, we’re not so lucky. Just ask Hal Alguire, director of public works at Fort Carson, where two separate storms totaled 52 roofs last year. Or Brien Darby, Denver Botanic Gardens’ manager of urban food programs, who remembers a hailstorm that broke every window in 12 greenhouses a few years ago. Or arborist Kevin Marks, who says a perfectly timed storm last spring helped fire blight, an airborne bacterium that infects and slowly kills, attack injured apple trees. Or Phil Long Dealerships’ chief administrative officer, Scott Arnold, who accepts that it’s a regular occurrence for hundreds of cars to be damaged in a matter of minutes. “If conditions are right, you just basically get your employees out of the way,” Arnold says.

All of which has made hail insurance for cars and roofs—often covered under auto or homeowner’s insurance—a virtual necessity along the Front Range. Colorado is second in number of hail insurance claims (behind Texas), which is driving up premiums, says Carole Walker, executive director at Rocky Mountain Insurance Information Association, a trade organization. A 2018 study by the National Association of Insurance Commissioners showed that Coloradans have the third fastest-rising homeowners’ rates.

So, are we doomed to pay high insurance premiums and suffer hail damage regularly? Maybe not. “We’re trying to figure out how we could build safely, build better, build more fortified,” Walker says. On top of that, there’s a small army of scientists, based in Colorado, working to better understand these unique and destructive little balls of ice.

In his Boulder office at NCAR, senior scientist Andrew Heymsfield holds a 3D model of a hailstone to demonstrate that these pieces of frozen ice are rarely smooth; they have peaks, valleys, and bulges. That’s critical, because Heymsfield wants to update data on terminal velocity—in this case, how fast hail falls—that was generated more than 70 years ago using smooth spheres. “Natural hailstones fall much more slowly than tennis balls of the same size and weight,” he says. With better data, insurance companies can adjust forecast models to accurately predict damage and manufacturers can create effective building materials to withstand storms.

His colleague, Prein, is looking at how climate change may impact thunderstorm clouds and, ultimately, hailstone formation. It’s possible that warming temperatures will produce stronger updrafts and create storms that carry and build larger stones. But it’s also feasible that warmer temperatures will melt falling hail before it hits the ground. “We know for certain that climate change is increasing temperatures,” Prein says. “How it affects hail and supercell storms is still [unknown].”

Scientists are asking volunteers—called “citizen scientists”—to help them track and record hail data, including storm frequency and stone size. Fort Collins’ Community Collaborative Rain, Hail & Snow Network, known as CoCoRaHS, receives more than 12,000 daily weather reports. The University of Colorado Boulder’s Department of Atmospheric and Oceanic Sciences is running a project called Colorado Hail Accumulation from Thunderstorms (CHAT), which focuses on collecting information about accumulation rather than size. Softball-size hailstones might grab headlines, but Dr. Katja Friedrich and researcher Bobby Wallace are interested in what happens when six inches of pea-size hail accrues on the earth. That’s a scenario that might require cities to dust off snowplows in July to clear roads (and one reason it might not be a good idea to put away your snow shovel for the summer).

Wallace has developed an algorithm that estimates, in real time, how much hail might accumulate from a storm. Information like this could eventually help meteorologists and homeowners forecast what damage a storm might cause. To test Wallace’s algorithm, CU’s CHAT initiative collects pictures of hail sent in from people who use rulers, dollar bills, or whatever’s available to show accurate measurements. The program is seven years old, but reports picked up last summer. All of that detailed data could make a difference in predicting and preparing for hail.

In the meantime, Anne Cure has an alternate plan. As the proprietor of a small, organic farm, she doesn’t have hail insurance. (Those programs can be cost-prohibitive for tiny, multicrop producers.) Instead, she overplanted this season to create a seedling bank in her greenhouse. If an early storm hits, she’ll have a stock of plants with which to replace the damaged ones. It’s a backup she may not need, but Cure knows she can sell the leftover starters at a farmers’ market or donate them. It won’t stop the hail, but it will give her a little bit of security. Perhaps that’s enough—along with some luck. “We’re all crossing our fingers,” she says, “that it won’t be like 2018.”