How does fog form?
Fog is a cloud that has come down to meet you. Thicken it over a village road until the 1 km board disappears, cool a case of air to its dew point, seed it with smoke, build five kinds of fog on one landscape, watch the morning Sun lift the lid, and see why a high beam turns fog into a white wall.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- A cloud on the ground. Fog and cloud are the same thing: air filled with tiny droplets of liquid water that scatter light. It is fog when you can see less than 1 km, and mist when you can see further. Thick fog holds only 0.1 to 0.3 grams of water in each cubic metre: a classroom of it contains a few teaspoons, split into billions of droplets.
- The dew point. There is a limit to how much water vapour can stay a gas, and the limit falls as the air cools: about 30 grams per cubic metre at 30 °C, 9 at 10 °C, under 5 at 0 °C. Cool damp air and its relative humidity climbs. At 100 % it has reached its dew point. Any colder and the surplus vapour condenses, as dew on the grass and as fog in the air.
- Droplets need a seed. Vapour will not condense on nothing. Every droplet, 10 to 20 micrometres across, forms on a speck of dust, salt, smoke or sulphate: a condensation nucleus. Share the same water between more seeds and you get more, smaller droplets, which scatter more light and fall more slowly. That is the honest link between winter fog and polluted air.
- Five kinds of fog. Fogs are named after what brought the air to its dew point. Radiation fog: the ground loses heat to a clear, calm night sky. Advection fog: warm, moist air drifts over a cold surface, as with sea fog. Upslope fog: wind pushes air up a hill and it cools as it expands. Valley fog: cold air drains downhill and pools. Steam fog: cold air passes over warmer water.
- The lid and the lift. On a foggy morning cold air lies under warmer air: a temperature inversion. It works like a lid, so fog and smoke cannot mix away. Fog does not burn off. Sunlight warms the ground, the air stirs, and the droplets evaporate from the bottom up, so the fog lifts into low cloud and then breaks. Deep fog reflects most of the weak winter Sun and can last all day.
- Fog in India and staying safe. Each December and January fog covers the Indo-Gangetic Plain from Punjab to Bihar. IMD grades it by visibility: shallow under 1,000 m, moderate under 500 m, dense under 200 m, very dense under 50 m. A high beam scatters back at the driver as a white wall. Use low beam and fog lamps, slow down until you can stop within what you can see, leave more distance, follow the lane markings, and never stop on the carriageway.
| Term | Meaning |
|---|---|
| Fog | A cloud touching the ground: tiny water droplets that cut visibility to less than 1 km. |
| Mist | The same droplets as fog, but thin enough that you can still see 1 km or more. |
| Dew point | The temperature to which air must cool for its water vapour to start condensing. |
| Relative humidity | The water vapour in the air as a percentage of the most it could keep at that temperature. |
| Condensation nucleus | A tiny particle of dust, salt, smoke or sulphate on which a droplet forms. |
| Radiation fog | Fog formed on clear, calm nights when the ground loses heat to the sky and chills the air above it. |
| Advection fog | Fog formed when warm, moist air is carried over a colder surface, such as a cold sea. |
| Temperature inversion | A layer where warmer air lies above colder air. It acts as a lid that traps fog and pollution. |
| Backscatter | Light bounced back toward its source by droplets: the white glare of a high beam in fog. |
2,400 years from Aristotle's moist vapours to fog forecasts for Delhi's runways.
- 1802 · The clouds get names (Luke Howard, London, England)
- 1880 · No dust, no fog (John Aitken; Paul-Jean Coulier (1875), Falkirk and Edinburgh, Scotland)
- 1943 · Burning the fog off a runway (Petroleum Warfare Department; Arthur Hartley, RAF Graveley, Cambridgeshire, England)
- 1952 · The Great Smog of London
- 1965 · The aircraft lands itself (British European Airways; Blind Landing Experimental Unit, London Heathrow, England)
- 1992 · A village drinks the fog (Pilar Cereceda, Robert Schemenauer and colleagues; after Carlos Espinosa, El Tofo and Chungungo, Chile)
- 2001 · Delhi lands in 200 metres (Airports Authority of India, Indira Gandhi International Airport, Delhi, India)
- 2007 · An alarm about Delhi's fog (Rajendra Kumar Jenamani, New Delhi, India)
The full story, with 28 moments, charts, people and 60 sources: glassbox.how/e/fogclear/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 |
|---|---|
fogclear.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/fogclear”. - Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
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Found a mistake? Open an issue. Corrections happen in public.
It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.
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/fog.js, js/fogphys.js |
FogClear's own helpers: the village, fog banks and chart boards, and the physics (Magnus formula, droplets and visibility, the five kinds of fog, the morning energy budget, headlight scattering) |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
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/ |
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