RainClear

How does rain happen?
A raindrop holds about a million cloud droplets, and nature builds one in under half an hour. Cut a rain cloud open, push monsoon air over the Western Ghats, grow a drop by collisions or by stealing vapour with ice, and watch a 5 mm drop flatten into a bun and burst.

▶ Play with it  ·  Read the 60-second explainer  ·  Watch the 40-second video

In 60 seconds

1. A rain cloud, cut open. Rain needs three steps: lift moist air so it cools, condense its vapour into cloud droplets 10 to 20 µm across on tiny specks of salt, dust and smoke, then grow those droplets into drops a hundred times wider that can fall. Above the 0 °C level droplets stay liquid (supercooled) down to about −38 °C, sharing the cold top of the cloud with ice.
2. Lift it, cool it, wring it out. Rising clear air cools 9.8 °C per km; cloud forms where it reaches its dew point, about 125 m up per °C of difference; inside cloud it cools only 4 to 6 °C per km because condensing vapour releases latent heat. Air is lifted by hot ground, fronts, mountains and converging winds. The Western Ghats wring rain out of the monsoon for the coast and leave the Deccan in a rain shadow; Cherrapunji's IMD normal is about 11,200 mm a year.
3. Growing a raindrop. Condensation alone would need about two weeks to make a 1 mm drop. In warm clouds a slightly bigger droplet falls faster and swallows smaller ones, and growth runs away in 20 to 40 minutes. In cold clouds, ice holds its vapour more tightly than water (2.60 against 2.87 hPa at −10 °C), so ice crystals grow about 10 % oversaturated while droplets evaporate: the Bergeron–Findeisen process. The snow melts into rain on the way down.
4. Falling: buns, not teardrops. Drops reach a steady terminal speed: about 4 m/s at 1 mm, 6.5 m/s at 2 mm and only about 9 m/s at 5 mm (Gunn and Kinzer, 1949). Small drops are spheres; bigger ones are flattened underneath like a burger bun. Past about 4.5 mm the air blows them into a parachute that bursts. Under a dry cloud small drops evaporate before landing: virga.
5. The monsoon. Land heats faster than sea, the winds reverse and the rain band swings north. IMD's normal onset over Kerala is 1 June (24 May in 2025, 8 June in 2023); the monsoon normally covers India by 8 July. June to September brings 868.6 mm to India as a whole, the 1971–2020 long-period average. In October to December the northeast monsoon gives Tamil Nadu about 48 % of its year's rain.
6. Rain and us. Warmer air holds about 6 to 7 % more vapour per °C, so the heaviest downpours get heavier; widespread extreme rain events over central India tripled between 1950 and 2015. Cloud seeding only nudges suitable clouds: IITM's CAIPEEX found about 18 % more rain over 100 km² downwind. If you hear thunder, go indoors; never camp or park in a stream bed.

Words worth knowing

Term	Meaning
Dew point	The temperature at which air becomes saturated and its vapour starts to condense.
Cloud base	The flat bottom of a cloud, where rising air reaches its dew point: about 125 m up per °C between temperature and dew point.
Lapse rate	How fast rising air cools: 9.8 °C per km when clear, about 4 to 6 °C per km inside cloud.
Orographic lift	Air forced upward by a mountain, the reason the Western Ghats and the Khasi Hills are so wet.
Rain shadow	The dry side of a mountain range, where air comes down warm after raining on the other side.
Collision and coalescence	Drops growing by bumping into and merging with smaller droplets: how warm rain forms.
Bergeron–Findeisen process	Ice crystals growing at the expense of supercooled droplets, because ice holds its vapour more tightly.
Terminal speed	The steady speed of a falling drop when air resistance balances its weight: no more than about 9 m/s for raindrops.
Monsoon	A seasonal reversal of the winds and the rainy season it brings; in India, June to September.

A short history

From Aristotle's circle of water and Kautilya's storehouse gauges to radars in orbit: 2,400 years of working out how a cloud makes rain.

• 340 BCE · Water rises as vapour and falls as rain (Aristotle, Athens)
• 150 BCE · Rain measured at the king's storehouses (Attributed to Kautilya (Chanakya), Magadha)
• 1686 · Halley explains the monsoon (Edmond Halley, London)
• 1861 · Cherrapunji's record year (Rain gauge at Cherrapunji (Sohra), Cherrapunji, Khasi Hills)
• 1875 · India gets a weather service (Government of India; Henry F. Blanford, Calcutta (Kolkata))
• 1884 · Forecasting the monsoon from Himalayan snow (Henry F. Blanford, Calcutta (Kolkata))
• 1933 · Most rain starts as snow (Tor Bergeron, Lisbon)
• 1946 · The first cloud seeded from the air (Vincent Schaefer, Berkshire Mountains, Massachusetts)

The full story, with 29 moments, charts, people and 40 sources: glassbox.how/e/rainclear/history. The data lives in history.json.

Video and slides

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

Privacy

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
rainclear.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.

Licences

• 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/rainclear”.
• Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
• The Glassbox name and logo aren't covered by either licence. See the terms.

Found a mistake? Open an issue. Corrections happen in public.

Run it

It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.

python3 -m http.server 8000

Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.

Then open http://localhost:8000.

How it's built

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/rainphys.js	The physics with no drawing: saturation vapour pressure over water and ice, lapse rates and the parcel, droplet growth, fall speeds, drop shapes and evaporation, with sources
js/rain.js	Drawing helpers shared by the chapters (boards, clouds, terrain, rain, gauges, maps), mostly borrowed from StormClear and CloudburstClear
glassbox.json	Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how
reel in each chapter	The storyboard the Glassbox studio records into short videos
glassbox/	The published video, slides, thumbnail and post copy
fonts/, vendor/three/	Self-hosted Geist and Instrument Serif (SIL OFL 1.1) and three.js (MIT)