A summer thunderstorm rolls in, and suddenly chunks of ice the size of marbles, golf balls, or even softballs pelt the ground. Hail can shatter windshields, dent cars, destroy crops in minutes, and occasionally kill animals or people. Yet hail forms only in very specific atmospheric conditions — strong thunderstorms with violent updrafts. The result is one of nature's most destructive forms of precipitation.
The Short Answer
Hail forms in thunderstorms when strong upward winds carry water droplets high enough that they freeze, then accumulate additional ice as they cycle up and down through the storm. When a hailstone becomes too heavy for the updraft to support, it falls. Larger hail requires stronger updrafts to keep it aloft long enough to grow.
The Thunderstorm Setting
Hail only forms in cumulonimbus clouds — the towering thunderstorm clouds. These clouds:
- Extend from the surface to 12+ km altitude
- Have very strong updrafts (sometimes over 160 km/h)
- Reach high enough for temperatures below -40°C in their upper portions
- Contain abundant supercooled water (liquid water below freezing)
Without strong vertical motion and freezing levels, hail can't form. Light thunderstorms produce rain; only severe ones produce significant hail.
How a Hailstone Forms
The classic explanation of hail formation:
- A water droplet gets caught in a thunderstorm's updraft.
- The updraft lifts it to high altitudes where temperatures are below freezing.
- The droplet freezes around a small particle (dust, ice crystal, or another droplet).
- The frozen droplet continues to rise, picking up additional supercooled water that freezes onto it.
- Eventually the hailstone becomes too heavy or moves into less powerful air, and falls.
- As it falls through warmer parts of the cloud, it might pick up more water.
- If another updraft catches it again, it cycles back up — gaining more layers.
- Eventually the stone's weight exceeds the updraft's lift, and it falls all the way to the ground.
Hailstone Layers
Cutting a hailstone in half often reveals layers, like an onion or a tree ring. Each layer represents a "trip" through the storm:
- Clear ice layers: Form when the stone is in a region with abundant liquid water that freezes slowly, allowing air bubbles to escape.
- Opaque/white layers: Form in colder regions where droplets freeze instantly, trapping air bubbles.
The number of layers indicates how many times the hailstone cycled through the updraft. Some large hailstones have 10+ visible layers, suggesting multiple cycles.
What Determines Hail Size
Hailstone size depends on several factors:
- Updraft strength: Stronger updrafts support larger stones. To produce a softball-size hailstone, updrafts must exceed 175 km/h.
- Moisture supply: More liquid water means more material for ice growth.
- Time in cloud: Longer times allow more growth.
- Temperature profile: Specific temperatures favor different growth modes.
Typical hail sizes:
- 5-10 mm: Pea-sized hail. Common.
- 20 mm: Marble-sized. Damaging to cars and crops.
- 25-30 mm: Walnut to golf-ball sized. Significant damage to roofs and vehicles.
- 50 mm: Tennis ball sized. Major damage.
- 75+ mm: Baseball sized or larger. Can kill animals and severely injure people.
Hail Records
Some remarkable hail events:
- Largest recorded hailstone (USA): 20 cm diameter, weighing 879 g — Vivian, South Dakota, July 2010.
- Largest recorded hailstone (heaviest): 1.02 kg in Gopalganj, Bangladesh, 1986.
- Deadliest hailstorm: Roopkund, India, around 850 CE — 200+ people killed.
- Most costly hailstorm: September 13, 2010 in Phoenix, Arizona — $2.8 billion in damage.
Where Hail Is Common
Hail is concentrated in certain regions:
- US Great Plains: "Hail Alley" — eastern Colorado, Nebraska, Wyoming. Frequent severe storms.
- Northern India/Bangladesh: Some of the world's most damaging hailstorms.
- Argentina (La Plata region): Frequent giant hail.
- South African Highveld: Particularly Pretoria area.
- Northern Italy and southern France: Notable hail belt.
- Australia: Sydney basin and surrounding regions.
Hail rarely forms in tropical regions despite frequent thunderstorms there — the freezing levels are too high in the warm atmosphere.
Why Hail Falls in Warm Weather
Counterintuitively, hail mostly falls during warm seasons. The reason: strong thunderstorms require warm humid air at the surface combined with cold air aloft. This temperature contrast drives the powerful updrafts needed for hail. Cold-weather thunderstorms (rare) usually produce less hail because the convection is weaker.
Hail vs Other Ice Precipitation
Different forms of ice falling from the sky:
- Hail: True ice stones from thunderstorms. Multiple layers. Falls in warm seasons.
- Sleet: Frozen raindrops. Small, often clear ice pellets. Forms in cold winter weather.
- Graupel: Soft "snow pellets" — snowflakes coated with rime ice. Falls in winter.
- Freezing rain: Falls as liquid, freezes on contact with cold surfaces.
- Ice crystals/diamond dust: Tiny ice particles in very cold air, not from clouds.
Hail Damage
Hail causes massive economic damage:
- Agricultural losses: Hail destroys crops in minutes. Annual US crop losses average $1+ billion.
- Vehicle damage: Dented hoods, broken windshields. Insurance claims billions/year.
- Roof damage: Especially asphalt shingles. Major impacts to home insurance.
- Aircraft damage: Even small hail can damage aircraft surfaces and engines.
- Solar panels: Vulnerable to hail damage.
Hail Suppression Attempts
Various methods have been tried to reduce hail damage:
- Cloud seeding: Adding silver iodide or salt to clouds, intended to produce more numerous smaller hailstones instead of fewer larger ones.
- Anti-hail cannons: Shock-wave-producing devices popular in some agricultural regions.
- Hail nets: Protective netting over orchards and vineyards.
Scientific evidence for cloud seeding's effectiveness is mixed. Anti-hail cannons have no convincing scientific support. Hail nets do work but are expensive.
The Mystery of Giant Hail
How some hailstones reach extreme sizes (10+ cm) puzzles meteorologists. Modern thinking suggests:
- Particularly intense storms with very strong, persistent updrafts
- Multiple growth cycles in a sustained storm
- Possibly multiple stones merging
- Specific moisture and temperature profiles
Storm-chasing scientists collect large hailstones to study their internal structure.
Hail Safety
If caught in a hailstorm:
- Get indoors immediately; stay away from windows.
- In a car: continue to a covered area if safe; otherwise stop in a sheltered location, lean away from windows.
- Outside with no shelter: protect head with arms or anything available.
- Do not attempt to drive through severe hail.
- Hail can indicate severe weather; tornadoes sometimes accompany large hail.
Key Facts
- Hail forms in thunderstorms with strong updrafts.
- Hailstones grow by cycling through updrafts, adding ice layers.
- Layers in hailstones indicate multiple growth cycles.
- Giant hail (>10 cm) requires updrafts over 160 km/h.
- Hail mostly falls in warm seasons, not winter.
Fun Facts
- The largest US hailstone was 20 cm across — about 0.9 kg.
- The world's deadliest hailstorm killed 200+ people in India around 850 CE.
- Hail is rare in tropical regions despite frequent thunderstorms there.
- Hailstones can cycle through a thunderstorm 10+ times before falling.
- Hail damages billions of dollars in crops and property annually.
Recent Major Events
Recent decades have seen several extraordinary hailstorms. The 2017 Texas hailstorms produced softball-sized stones causing $5 billion in damage across the Dallas-Fort Worth area. The 2014 Abilene event spawned hail up to 13 cm wide. A 2020 Calgary, Alberta hailstorm caused $1.4 billion in damage in a single afternoon. Argentina experienced massive hail in 2018 affecting wine country. Cleaning up from such storms can take months, with backlogs at auto body shops and roofing contractors. Climate scientists are studying whether hail events are intensifying, though the relationship between climate change and hail remains complex and somewhat uncertain.
Hailstorm Insurance and Economics
Hail damage costs insurance companies billions of dollars annually, particularly in "Hail Alley" running through the central United States. After major hailstorms, insurance claims can overwhelm local adjusters, with some events generating 100,000+ claims in a single state. Auto insurers, homeowners insurers, and crop insurers all track hail risk carefully. Premiums in hail-prone regions are noticeably higher. Some insurers offer specialized hail protection products. The rise of "chasing" hail damage has also created markets for repair specialists who travel to hail-hit regions. Roofing companies particularly benefit, often working for years repairing storm damage.
How Scientists Study Hail
Storm-chasing scientists pursue hailstorms to study them firsthand. Specialized vehicles with reinforced shells follow severe thunderstorms, collecting hailstones for analysis. Project IceBridge and other research efforts use Doppler radar to map hail in storms before it reaches the ground. Computer models simulate hailstone trajectories within thunderclouds, helping understand growth processes. Recent research uses 3D scanning to study hailstone internal structures, revealing growth patterns. Some scientists collect hailstones during chases and study them in laboratories to understand temperature, density, and crystal formation. This research informs storm forecasting and weather modification efforts.
The Bottom Line
Hail falls because thunderstorm updrafts lift water droplets to freezing altitudes, where they accumulate layers of ice as they cycle up and down within the storm. The result is solid ice projectiles ranging from peas to softballs that can be highly destructive. Hail is concentrated in regions where powerful continental thunderstorms are common, particularly the US Great Plains, Indian subcontinent, and parts of South America and Australia.