Large Hail — How Giant Hailstones Form and Why They Cost Billions
How does large hail form?
Hail forms when a thunderstorm updraft carries raindrops above the freezing level, where they become ice embryos that grow by colliding with supercooled water droplets — liquid water colder than 32°F that freezes on contact. The stronger the updraft, the larger the stone it can keep suspended in the growth zone: quarter-size hail rides updrafts around 50 mph, while softball-size stones require the 100+ mph updrafts found almost exclusively in supercells. When the stone outgrows the updraft — or gets ejected — it falls, with the largest stones hitting the ground at highway speeds. Hail is rarely deadly in the US but is the country's costliest severe-thunderstorm hazard, driving tens of billions of dollars in annual insured losses.
What happens inside the hail growth zone?
The action is in the layer of the storm between roughly -10°C and -30°C, loaded with supercooled water — droplets that remain liquid well below freezing until they touch something. An ice embryo circulating there grows layer by layer: in wetter regions, water spreads before freezing into clear ice; in colder, drier regions it freezes instantly into opaque rime. Slice a big hailstone and the alternating clear/opaque rings record its journey through the storm.
Giant hail isn't from stones "recycling" up and down repeatedly, as the old textbook picture had it — modern studies show large stones mostly ride a single sustained pass through the growth zone, steered along the updraft's edges where they can linger longest. That's why updraft strength and breadth, not just storm intensity, decide the maximum stone size.
How fast does hail fall, and what can each size do?
Fall speed scales with size, which is why damage escalates so steeply:
- Quarter (1.00") — around 25-40 mph. Dents soft metal, cracks old shingles; the severe threshold.
- Golf ball (1.75") — 50-60 mph. Windshields crater, asphalt shingles fail, gardens are shredded; "considerable" warning tag territory.
- Baseball (2.75") — 70-85 mph. Punches through car glass and some roofs; triggers the "destructive" tag and WEA phone alerts.
- Softball and larger (4.00"+) — 100+ mph. Penetrates roofs into living spaces, totals vehicles, breaks bones. Grapefruit-class stones are falling roughly as fast as a major-league pitch, with several times the mass.
The US record stone fell at Vivian, South Dakota on July 23, 2010: 8.0 inches in diameter, 1.94 pounds.
Where and when is large hail most common?
The US hail capital is the High Plains — a belt from West Texas through eastern Colorado, Kansas, Nebraska, and Wyoming — where high cloud bases and cold air aloft let stones reach the ground before melting. "Hail Alley" around the Colorado-Nebraska-Wyoming junction sees the most hail days per year. Season peaks March-June in the southern Plains and May-August further north, in the late afternoon and evening when updrafts peak.
Melting is the underappreciated variable: humid, warm-based Southeast storms shed much of their hail as rain before it lands, while a Denver storm's hail survives a short, cold descent. The same supercell can be a windshield-smasher in Colorado and merely a downpour in Georgia.
Why is hail the costliest severe-storm hazard?
Because it attacks area, not points. A tornado's violence is confined to a path a few hundred yards wide; a supercell can drop damaging hail along a swath miles wide and dozens of miles long, hitting every roof, window, car, solar panel, and crop in it. Insurance-industry tallies routinely attribute tens of billions of dollars per year in US losses to hail — the majority of all severe-convective-storm claims, and single metro-area hailstorms (San Antonio 2016, the Front Range repeatedly) have each exceeded a billion dollars.
Deaths, by contrast, are rare — typically zero to a few per year in the US — because shelter is so effective: essentially any roof defeats hail. The casualties that do occur are almost always people caught outdoors or in vehicles.
What should I do when large hail threatens?
Get indoors and stay away from windows and skylights — flying glass is the main injury path once you're inside. If you're driving, don't stop under an overpass (it blocks traffic and invites collisions); NWS guidance is to pull safely off the road, angle the vehicle so the windshield — the strongest glass — faces the hail, and stay away from the other windows. Full storm safety guidance is at weather.gov/safety/thunderstorm.
Warnings give you the heads-up: hail size estimates appear in every severe thunderstorm warning, and baseball-plus stones buzz phones via WEA. On the VORTEX map, warned hail cores track with their storms so you can time a car's dash into the garage — the single highest-value minute in hail preparedness.