DewClear

How does dew form?
Nobody watered the lawn, yet it is soaking at dawn. Watch grass and a car roof radiate their heat to a freezing-cold clear sky, run a night hour by hour until the surface crosses the dew point, grow feathery frost on a Punjab wheat field, size a dew condenser for Kutch, and redo the 1814 experiment that proved dew never falls.

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

In 60 seconds

1. Water that was in the air. Dew is water vapour from the air that condenses on a surface colder than the air's dew point. It does not fall and it does not come out of the grass. A good night leaves only 0.1 to 0.2 mm, but that is 100 to 200 grams of water on every square metre. Drops that sit only on the tips and edges of a leaf are different: that is guttation, water pushed out by the roots.
2. The cold surface. Everything glows in infrared. A lawn at 15 °C radiates about 380 W/m², and a clear, dry sky sends back only about 300, so a surface at air temperature makes a net loss of 60 to 100 W/m². Still air pays back little of it. Thin, insulated things such as grass blades and car roofs cool several degrees below the air; a road or bare soil keeps drawing the day's heat from below, a wall sees only half the sky, and they stay dry.
3. A night, hour by hour. After sunset the surface falls to the dew point and condensation begins. Condensing water gives out about 2.45 MJ per kilogram of latent heat, which slows the cooling. Clouds glow back and stop dew; strong wind stirs down warmer air and stops it too; but dead calm starves the surface of fresh vapour, so a light breeze gives the most. Measured yields average about 0.13 mm a night, and physics caps them near 0.8 mm.
4. Frost. When the surface goes below 0 °C, vapour deposits straight onto it as ice. If dew formed first and then froze, you get white globules; if the dew point itself is below zero, feathery hoar frost grows instead. Grass can freeze while the air a metre up is still 2 or 3 °C: a ground frost without an air frost. IMD warns of ground frost over Punjab, Haryana and Rajasthan, with impact on wheat, potato, mustard and other rabi crops.
5. Dew at work. Insulated, sky-facing panels in Kutch, Gujarat, collect dew as drinking water in the dry months: the best night measured gave just over half a millimetre. A cold glass on a humid day sweats because it is below the room's dew point. A car's demister clears the inside of the windscreen by warming the glass and drying the air on the AC coil. In evening cricket, dew wets the ball, and since 2025 the IPL lets the side bowling second ask for a replacement.
6. Wells weighs the dew. For two thousand years people thought dew fell like fine rain. In 1814 the London doctor William Charles Wells published his weighings of wool left out at night: 16 grains of dew in the open, 2 under a little pasteboard roof, 14 on top of a raised board and 4 underneath. Grass was usually 7 to 9 °F colder than the air. Surfaces radiate their heat to the sky and cool first; the dew is the result.

Words worth knowing

Term	Meaning
Dew	Liquid water condensed from the air onto a surface that has cooled below the dew point.
Dew point	The temperature at which air becomes saturated with the water vapour it holds.
Stefan–Boltzmann law	A surface radiates εσT⁴ watts per square metre, with T in kelvin and σ = 5.67 × 10⁻⁸ W/m²K⁴.
Net radiation	What a surface radiates minus what it receives; at night under a clear sky it is a loss.
Latent heat	Heat given out when vapour condenses, about 2.45 MJ/kg, which slows a dewy surface's cooling.
Contact angle	The angle where a drop meets a surface: large on waxy leaves (beads), small on clean glass (film).
Hoar frost	Ice grown straight from vapour on a surface below 0 °C, by deposition.
Ground frost	The ground or grass below 0 °C, which often happens while the air above is still a little warmer.
Guttation	Drops pushed out at leaf tips and edges by root pressure, often mistaken for dew.

A short history

From Aristotle's falling dew to dew-harvesting panels in Kutch: 2,400 years of looking at wet grass.

• 340 BCE · Dew and hoar frost need a clear, still night (Aristotle, Athens)
• 1814 · Wells shows that the cold comes first (William Charles Wells, London)
• 1859 · Water vapour blocks the night's escape (John Tyndall, Royal Institution, London)
• 1885 · Some dew rises from the ground (John Aitken, Falkirk)
• 1957 · Dew in watts and millimetres (John Lennox Monteith, Rothamsted Experimental Station, Harpenden)
• 2001 · Water on a cottage roof in Kutch (Girja Sharan, Kutch, Gujarat)
• 2017 · Drinking water from dew at Kothara (Girja Sharan (who conceived it), Anil Kumar Roy, Laurent Royon, Anne Mongruel and Daniel Beysens, Kothara, Kutch, Gujarat)
• 2021 · IMD warns of ground frost (India Meteorological Department, New Delhi)

The full story, with 29 moments, charts, people and 36 sources: glassbox.how/e/dewclear/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
dewclear.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/dewclear”.
• 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/dewphys.js	The physics, with sources: FogClear's Magnus saturation model, Stefan–Boltzmann night radiation with Brutsaert's clear-sky emissivity, and the minute-by-minute night model of a surface's temperature, dew and frost
js/dew.js	Drawing helpers shared by the chapters: chart boards, lawn, generic car, water beads, frost needles
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)