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.
At a glance
This guide is best for turning a weather term into a clearer decision about timing, exposure, and confidence.
- Reading time: about 9 minutes
- Primary focus: weather science, forecast context, safety implications, and practical interpretation
- Watch for: official updates, local terrain, time of day, overlapping hazards, observation trends, and confidence language
- Decision point: Decide what the information changes: shelter, travel, outdoor plans, water safety, health, power, or continued monitoring.
- Official check: National Weather Service safety portal
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
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
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.
Learn more
Why this storm science story matters
General weather articles need enough depth to connect the headline to the atmosphere behind it. A term may sound simple, but the useful meaning often depends on scale: what is happening in the cloud, what is happening across a region, and what it means for people on the ground.
For How hail forms, the practical value is context. A reader should leave with a clearer sense of what the term means, what evidence supports it, and what choices it should influence before, during, or after hazardous weather.
The science in plain English
Most weather changes begin with uneven heating, pressure differences, moisture, and air motion. Fronts, clouds, storms, fog, drought, and wind all reflect the atmosphere trying to balance temperature and pressure while water changes phase between vapor, liquid, and ice. Local terrain and land use can sharpen or soften those patterns.
Weather is rarely controlled by one ingredient. The same headline can play out differently depending on storm timing, terrain, building quality, warning access, and how many people are exposed. That is why official meteorology sources usually describe risk as a combination of probability, severity, and confidence rather than as a single yes-or-no answer.
How to use this information
Use this article by connecting the concept to decisions. Is the issue visibility, wind, heat, water, lightning, air quality, or travel timing? Once the hazard is clear, official forecasts and local alerts become easier to interpret. The goal is not to memorize every term; it is to know which signal should change your plan.
If you are comparing this page with another guide, look for the scale of the question. Some pages explain what happens inside a storm, some explain what forecasters can detect, and others explain what a household, school, business, or community should do. Mixing those scales is how weather myths spread.
What to watch for
Watch for forecast confidence, timing, terrain effects, time of day, and whether several hazards overlap. A modest storm with bad timing can create more disruption than a stronger storm that misses populated areas. Small changes in temperature, moisture, or storm track can shift the impact zone.
Pay attention to update timing. Forecasts and warnings are snapshots of the best available information, and high-impact weather can evolve between updates. When official guidance changes, treat the change as new information rather than as a contradiction.
Common mistakes
A common mistake is treating weather terms as fixed labels instead of clues. Another is comparing events only by one number, such as temperature, wind speed, or rainfall total. Impacts usually come from the combination of intensity, duration, exposure, and vulnerability.
Another general mistake is using old experience as the only guide. People often prepare for the last event they remember, but the next event may arrive at a different time of day, affect a different road, or stress a different part of the home or community.
Reader checklist
Before moving on from How hail forms, use this quick checklist to separate useful weather information from noise:
- Can you name the main hazard: wind, water, lightning, heat, cold, visibility, or air quality?
- Do you know whether the page is explaining formation, detection, forecasting, safety, history, or recovery?
- Have you checked whether the official source is describing probability, observed damage, or immediate action?
- Can you identify the decision point: shelter, delay travel, evacuate, protect property, or keep monitoring?
- Do you have a second alert path if power, cell service, sirens, or internet access fail?
That checklist is intentionally conservative. Weather education is most valuable when it helps a reader make a calmer decision under pressure, not when it simply adds more dramatic storm vocabulary.
Tornado Hub articles are educational explainers and are not a live warning service. For immediate decisions, use official alerts from your local National Weather Service office, emergency management agency, or equivalent national weather authority.
How to read this guide
How hail forms is most useful when it is read as a decision guide, not just a definition. The goal is to connect the weather setup, the warning language, and the practical action a reader may need before conditions become dangerous.
What decision does this weather concept change?
Read this article as a concept-to-action guide. Weather terms matter most when they help a person understand timing, exposure, confidence, and what to do next.
What to compare with official guidance
Compare the article with official forecasts, observations, local terrain, time of day, and whether the hazard affects travel, buildings, health, water, power, or outdoor plans.
General weather confidence improves when a concept is tied to evidence: observations, physical ingredients, official warnings, and consistent trends across multiple tools.
Decision checklist
- Identify the main hazard first: wind, water, lightning, heat, cold, visibility, air quality, or travel disruption.
- Check whether the article is explaining a forecast ingredient, an observed hazard, a safety action, or a historical lesson.
- Compare the page with the latest official warning, local emergency instruction, or agency update before acting.
- Decide what would change your plan: sheltering sooner, delaying travel, avoiding water, preparing for outage, or checking on someone vulnerable.
- Keep a backup alert path in case power, cell service, internet, sirens, or social media updates fail.
Change the plan if the hazard overlaps with another one, timing shifts into a vulnerable period, official guidance changes, or local conditions are worse than the broad forecast suggests.
This added section is part of Tornado Hub's broader article-quality pass. It is educational context, not a live warning. During active weather, use official alerts and local instructions first.
Field notes and source map
How hail forms benefits from one more layer of context: what evidence a reader should compare, what the official sources actually cover, and what practical decision the article should support. This added section is intentionally written like a newsroom sidebar: quick to scan, but deep enough to make the page more useful than a short definition.
General weather articles become more useful when they connect a concept to a decision. The reader should know what the setup is, what signal to watch, what impact matters, and what action changes.
A weather term is only valuable if it changes understanding. Ask whether the page helps with timing, location, confidence, exposure, safety, or long-term context.
What to check next
After reading this page, compare the article with the latest official information, the local terrain or building exposure, and the time window in which the hazard matters. A weather concept becomes useful when it changes one of those things: where you go, when you travel, how you shelter, what you monitor, or whether you wait for a safer window.
For readers coming from search, the key is to avoid treating one term as the whole answer. A headline may name the storm type, but the useful details are usually smaller: the warning wording, the observation trend, the affected road or coast, the people who need extra time, and the source that will update first.
Source trail
NOAA, NWS, Climate.gov, and Weather-Ready Nation links are useful anchors because they connect public weather education to official warning and preparedness language.
- NOAA/NSSL Severe Weather 101
- National Weather Service safety portal
- NOAA Climate.gov
- NOAA Weather-Ready Nation
These links are provided so readers can move from Tornado Hub's plain-English explanation to official meteorological, warning, safety, or archive sources.