How do hailstorms happen?
On a hot afternoon, a cloud taller than Everest can drop ice the size of cricket balls. Watch a hail streak cross a village, grow a stone ring by ring and cut it open, find the updraft that holds it up, and see what actually protects a crop.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- A hailstorm arrives. Hail comes from a cumulonimbus, a thunderstorm cloud whose top can pass 12 km. Above about 4 to 5 km the cloud is colder than 0 °C even on a hot day. Hail falls from one small part of the storm, the hail core, so it draws a narrow strip on the ground: the hail swath. How long it hails at one spot is the size of the core divided by the speed of the storm, usually a few minutes.
- The seed. Cloud droplets are tiny and clean, so they stay liquid far below 0 °C, down to about −38 °C. This supercooled water freezes the instant it touches ice. A hail embryo is a small piece of ice, either soft white graupel or a frozen raindrop, that falls through the droplets and collects them. At 5 mm across it counts as hail.
- Growing in layers. Freezing releases heat, 334 kJ for every kilogram. Where the cloud is cold or thin, droplets freeze at once and trap air: white ice (dry growth). Where it is warm and soaking, water arrives faster than it can freeze, the surface stays wet at 0 °C and the ice sets clear (wet growth). The rings in a hailstone record those changes along its path, not the number of loops it made.
- How big can it get. A stone stays in the storm only while the updraft blows up as fast as the stone falls. Fall speed comes from weight against drag: about 25 km/h for a pea, about 90 km/h for a cricket-ball stone. So big hail means a violent updraft. Energy on landing rises about sixteen times for each doubling of size. The largest stone measured, at Vivian, South Dakota, in 2010, was 20 cm across.
- Damage and defence. Pea-sized hail tears crops, glass breaks at about 2 to 3 cm, car bodies dent from about 2.5 cm and tiled roofs crack at 3 to 5 cm. An aircraft in flight meets hail at its own speed, so even small stones hit hard. Nets over orchards work because they are a barrier. Careful trials of cloud seeding found no clear effect, and trials of hail cannons found none.
- Hail in India, and staying safe. Hail melts on the way down, so it reaches the ground when the freezing level is low and the ground is high: February to May, with western disturbances, and most often in the hills. The Moradabad hailstorm of 30 April 1888 killed 246 people, the highest toll in World Meteorological Organization records. When hail starts, get indoors, keep away from windows and skylights, pull over under cover if driving, and protect your head in the open.
| Term | Meaning |
|---|---|
| Hail | Balls or lumps of ice 5 mm across or more that fall from a thunderstorm cloud. |
| Supercooled water | Liquid water colder than 0 °C. It freezes the instant it touches ice. |
| Hail embryo | The small piece of ice, graupel or a frozen raindrop, that a hailstone grows around. |
| Wet and dry growth | Wet growth lays down clear ice from a film of water. Dry growth freezes droplets at once and traps air, making white ice. |
| Updraft | The current of air rising through a storm. It must match a stone's fall speed to hold it up. |
| Terminal speed | The steady speed of a falling stone once air drag equals its weight. |
| Hail swath | The narrow strip of ground that a storm's hail core passes over. |
| Freezing level | The height where the air is 0 °C. Hail starts to melt once it falls below it. |
| Hail net | A mesh stretched over a crop. The one defence against hail with solid evidence. |
2,400 years from a puzzle about ice in warm weather to radar that can see hailstones inside a storm.
- 1788 · Two bands of ruin across France (Henri-Alexandre Tessier, Jean-Baptiste Le Roy and Jean-Nicolas Buache, From the Loire to the Low Countries)
- 1806 · Volta's dancing hailstones (Alessandro Volta, Pavia and Bologna, Italy)
- 1888 · The Moradabad hailstorm (People of Moradabad and nearby districts, Moradabad, North-Western Provinces (now Uttar Pradesh))
- 1896 · A winegrower fires at the clouds (Albert Stiger, Windisch-Feistritz, Styria (now Slovenska Bistrica))
- 1904 · The cannons are put on trial (Pietro Blaserna, Josef Pernter and colleagues, Castelfranco Veneto, Italy, and Windisch-Feistritz)
- 1958 · Wet growth and dry growth (Frank Ludlam, Imperial College London)
- 1964 · Soviet rockets against hail (G. K. Sulakvelidze and colleagues, Georgia and the North Caucasus)
- 1972 · A fair test in Colorado (US National Hail Research Experiment, NCAR, North-east Colorado)
The full story, with 30 moments, charts, people and 41 sources: glassbox.how/e/hailclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
Instagram carousel | 1080×1350 |
glassbox/thumb.jpg |
YouTube thumbnail | 1280×720 |
glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
History carousel | 1080×1350 |
glassbox/post.json |
Post copy and schedule used by the publish kit |
This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.
It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
hailclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
Exactly what each one sees is at glassbox.how/privacy.
- Code: MIT. Use it, change it, ship it.
- Explanations, text, images and videos (
glassbox.json,glassbox/): CC BY 4.0. Credit “Glassbox, glassbox.how/e/hailclear”. - Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
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python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
js/hailphys.js |
The physics, with no drawing code: fall speed, wet and dry growth, melting, damage thresholds, with sources |
js/hail.js |
Shared drawing helpers: the storm cloud, town, chart boards, the cut-open hailstone and the rattle of hail |
glassbox.json |
Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how |
reel in each chapter |
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