WebGL is not available in this browser. The numbers and charts on the right still run.
Clock 19:00 Air 0 °C Leaf 0 °C Dew point 0 °C Dew 0 mm dry
Drag to orbit, wheel or pinch to zoom. The white louvred box is a weather screen (it measures air temperature). Water drops grow on the blades only while the leaf is colder than the dew point. Drop size is computed from the real deposited depth; drop count and positions are artistic.
Dew Formation Lab
A grass blade loses heat to the cold night sky, cools below the air and, if it gets colder than the dew point, water vapour condenses on it. The night is computed from an energy balance, then played back as a time-lapse.
Pause Reset Thermal view Quality: High
Meadow Grass close-up Weather screen From above
Time of night
Time since sunset 19:00
Time-lapse speed
Air at sunset
Sky and wind
Air cooling
The air temperature follows a prescribed curve (it cools by the slider value on a clear night and by less under cloud). The leaf temperature, dew and fog come from the energy balance.
Readouts
Night temperatures and dew Orange: air. Cyan: grass leaf. Green dashed: dew point of the air. White dotted: dew depth (scale at the right). Blue band: leaf below dew point. Final dew:
Experiment: which nights give dew?
Runs 15 full nights at the current temperature and humidity, sweeping cloud cover and wind, and compares the full run with a closed-form linearised prediction (leaf-air drop = net sky loss / (2h + 4εσT³)).
Run experiment (15 nights)
Independent numerical checks (run in this page)
Legend and controls Presets set a whole night. Sliders recompute the night immediately; the time slider scrubs through it. Thermal view colours the blades by leaf temperature relative to the air: blue is colder. Dew forms when the leaf temperature is below the dew point, so the chart's blue band is the dew window. The science
Dew point. Air holds vapour at partial pressure e. It is saturated at the temperature where es (Td ) = e. The page uses the Magnus formula es = 610.94 exp(17.625 T/(T+243.04)) Pa (T in °C) and its inverse. When the air itself cools below its dew point, the vapour above that is removed as fog (the page caps e at es (Tair )).
Energy balance of one leaf. C dTs /dt = −εσ(Ts 4 − Tsky 4 ) + 2h (Ta − Ts ) + Lv m″, with leaf heat capacity C = 800 J/m²/K, emissivity ε = 0.96 and Lv = 2.5 MJ/kg. Only the sky-facing side radiates to the sky; the ground side is assumed to exchange with air-temperature surroundings. The convection coefficient per side is h = 3 + 3.8 u (W/m²/K, wind u in m/s) and the surface also gains heat from condensation.
Sky temperature. The clear-sky emissivity uses a Brunt-type formula εsky = 0.52 + 0.065 √ehPa ; cloud cover N blends it toward 0.96 (cloud base nearly a black body): εeff = εclear + N (0.96 − εclear ); Tsky = εeff 1/4 Ta . This is a textbook-style fit; real nights vary.
Condensation. Mass flux m″ = gm (e − es (Ts )) / (Rv Tfilm ) with gm = 2h/(ρa cp ) (heat-mass analogy with Lewis number 1). The flux is zero exactly at Ts = Td , positive (dew) below it. One kg/m² of water is 1 mm deep. Dew evaporates again if the leaf warms, but a dry leaf does not transpire in this model.
Why wind and cloud matter. Clouds return long-wave radiation, so the net loss falls and the leaf stays near the air temperature. Wind raises h, which ties the leaf to the air. Both reduce the leaf-air gap and so the dew. The linearised drop is ΔT ≈ εσ(Ta 4 −Tsky 4 )/(2h + 4εσTa 3 ) with time constant τ = C/(2h + 4εσTa 3 ); the page checks the full run against both.
Simplifications and honesty. One flat leaf, no canopy air layer, no soil heat flux, no guttation, no surface roughness effects, no droplet size distribution. The leaf constants, the sky fit and the prescribed air-cooling curve are typical round values, not measurements of a particular night. Typical measured dew is about 0.1 to 0.5 mm per night; results of this model in the same range should be read as plausible, not forecasts. Drops in the picture are placed artistically; their radius follows the computed depth spread over about 4.5 drops per blade. The 3D models are made with Blender.
Independent educational model. Not for agricultural or frost-protection decisions.