Storm science
How hail forms
How a single ice pellet grows into a softball — and why some storms produce baseballs while others just spit pea-sized. The science of hail.
The basic recipe
- Water vapor rises in updraft.
- Freezes at -20 to -40°C.
- Ice crystal grows as it collides with supercooled droplets.
- Falls partway through downdraft, then caught in updraft again.
- Cycle repeats — each cycle adds a layer of ice.
- When too heavy for updraft, falls to ground.
The updraft matters
- To hold a golfball-sized hailstone, need ~65 mph updraft.
- Baseball-sized: ~85 mph.
- Softball-sized: ~100 mph.
- Grapefruit-sized (world record): ~120+ mph.
- Weaker storms can't produce giant hail regardless of moisture.
- Supercell updrafts (100+ mph) are giant-hail machines.
The temperature layer
- Optimal hail growth zone: -10°C to -30°C.
- This layer must be at right altitude (7-25,000 ft).
- Below optimal: hailstones melt during fall.
- Above optimal: hailstones stay small.
- Dry hail layer (dry adiabatic profile) suppresses growth.
- Wet layer favors growth.
The hailstone rings
- Cross-section reveals concentric rings — like tree rings.
- Each ring = one updraft/downdraft cycle.
- Some hailstones cycle 15+ times before falling.
- Different air temperatures produce different textures.
- Layers alternate between clear ice (rapid growth) and opaque (slow growth).
- Scientists study rings to reconstruct storm updraft history.
Hailstone shapes
Spherical
Smallest hailstones. Simple growth.
Conical
Grows around a preferred axis.
Multi-lobed
Multiple growth centers.
Spiky
Rare. Very cold, dry growth.
Sheet-like
Extremely rare accumulation.
Aggregations
Multiple hailstones fused together during fall.
The hail size scale
- Pea (1/4"), Marble (1/2"), Dime (3/4"), Nickel (7/8"), Quarter (1"), Half dollar (1 1/4"), Ping pong ball (1 1/2"), Golf ball (1 3/4"), Hen egg (2"), Tennis ball (2 1/2"), Baseball (2 3/4"), Softball (4"), Grapefruit (4 1/2"), World record (approximately 8" in Vivian SD 2010).
NWS Severe Thunderstorm Warning triggers at 1" hail (quarter). Damaging hail begins at 1.75" (golf ball).
The Vivian record
July 23, 2010 — Vivian, South Dakota. Hailstone 8 inches diameter, 1 pound 15 ounces.
- Broke previous records set at Aurora NE 2003.
- Sample preserved at NCAR.
- Fell from a supercell with 100+ mph updraft.
- Photographed by local rancher Les Scott.
- Confirmed by Storm Damage Survey team.
Hail damage
1"-1.5"
Vehicle windshields cracked. Roof shingles damaged.
1.5"-2.5"
Auto body damage. Roof shingles removed.
2.5"-4"
Windshields broken through. Siding destroyed.
4"+
Metal roofing punctured. Concrete damage possible.
Hail cores (dense concentrations)
Vehicles destroyed. Homes uninhabitable.
The Denver hail alley
Denver metro is called Hail Alley — most hail losses in the US.
- Elevation puts optimal hail growth zone near surface.
- Frequent afternoon supercells in summer.
- Population density = damage costs.
- 2017 May 8 hailstorm caused $2.3B damage — most expensive single hail event in US history.
- Multiple $500M+ hail events per year.
Hail forecasting
- HRRR hail size algorithm forecasts hail diameter.
- GFS/RAP indicate storm intensity but not size.
- Warning issued when radar detects reflectivity > 60 dBZ above freezing.
- Dual-pol radar detects hail signature.
- MRMS produces national hail size maps.
- Local NWS offices customize thresholds.
If hail is falling
- Do NOT drive.
- Get vehicle under cover (garage, overpass).
- Do not stand under overpass in traffic.
- Get indoors.
- Keep away from windows.
- Cover head with hands, backpack, book, whatever available.
- Cover animals if outside.
- Do NOT try to catch hail with hands.
- Photograph after for insurance.
The specific storms famous for hail
- 2010 Vivian SD — world record.
- 2003 Aurora NE — 7" hail.
- 2017 Colorado Front Range — $2.3B damage.
- 2018 Colorado — massive.
- 2019 Guajira Colombia — softball hail near equator.
- 2020 Ontario Canada — 2.5" hail through farm belt.
- 2023 Croatia — 4" hail on grape harvest.
- 2024 Corpus Christi TX — 5" hail.
Hail insurance
- Comprehensive auto covers hail damage.
- Homeowners covers roof, siding, gutters.
- Some Colorado policies exclude hail cosmetic damage on roofs.
- Deductibles: $1,000-$5,000 typical.
- Payment issued for actual cash value or replacement cost — read policy.
- Hail claim frequency affects future rates.
- IIHS crash tests and hail resistance ratings for vehicle protection.