How Windfield works

This page walks through what actually happens between you drawing a box on a map and seeing wind flow over the ground. Plain language first; open any Technical note for the mechanics. See also About & Methodology for the short version, and Help for how to use the app and the full public API.

1. The big picture

You draw a rectangle on the map — that's the ground you want modelled. Everything after that happens on our server, not in your browser: it downloads real elevation data for that exact area, builds a 3D terrain model at the resolution you asked for, burns in any structures you drew, runs the WindNinja wind model on that terrain, and sends back pictures (a speed map, arrows) and numbers (a grid of values you can probe). Your browser's whole job is to draw what comes back and let you interact with it — it never runs the wind model itself.

2. The engine: WindNinja 4.0

Windfield runs WindNinja 4.0, built by the USDA Forest Service's Rocky Mountain Research Station at the Missoula Fire Sciences Lab, originally for wildfire behaviour prediction. It's a diagnostic wind model: it solves one physically consistent snapshot of the wind for the terrain and inputs you give it, rather than forecasting how the atmosphere will evolve over time. Give it the same terrain and the same starting wind twice and you get the same answer — it's a calculation, not a weather forecast.

The fast solver: mass-conserving

By default Windfield uses WindNinja's fast mass-conserving solver. It starts from an initial wind (one you set, a station, or a model) and adjusts it, cell by cell over a 3D terrain-following mesh, so that air doesn't pile up or vanish anywhere — the amount of air flowing into a cell has to equal the amount flowing out. Enforcing that single rule over real terrain already produces the effects you'd expect to see outdoors: wind speeds up over ridgelines and passes, slows and backs up on the windward side of a hill, gets deflected sideways around high ground, channels down valleys, and runs faster downslope at night than during the day. It does all of this in seconds, which is why it's the default.

What it can't do: it has no concept of flow separating off a sharp edge, no wakes, no eddies, no recirculation behind a ridge, cliff or building. It smooths the wind over the terrain's shape; it doesn't simulate the turbulence that shape would actually cause.

The high-fidelity solver: momentum (OpenFOAM)

For cases where that separated flow matters — wakes behind buildings or cliffs, eddies, recirculation zones, gully flows merging — Windfield can opt in to a momentum solver built on OpenFOAM. Instead of only conserving mass, it solves the full Navier–Stokes momentum equations with a turbulence model, which is what actually produces wakes and recirculation in the first place. The cost is time: minutes rather than seconds, and a much smaller cell budget, because resolving that level of detail is far more expensive per cell.

The log wind profile, roughness, and diurnal slope winds

Near the ground, wind speed doesn't change linearly with height — it follows a logarithmic profile: close to the surface friction slows it down a lot, and speed increases quickly at first, then more gradually higher up. How quickly depends on vegetation roughness: short grass lets wind run fast close to the ground, while a forest canopy drags on the air much more, flattening the near-surface profile and effectively pushing the "free-flowing" wind higher up. That's why vegetation is one of the inputs you set.

When you turn on diurnal winds, Windfield also models the daily heating/cooling cycle: during the day, sun-warmed slopes heat the air next to them, which rises and pulls air upslope; at night, slopes cool faster than the free air above them, and that denser cold air sinks and drains downslope and down-valley. Pair it with non-neutral stability and the model also accounts for how stable or unstable the atmosphere is, which changes how strongly it resists vertical motion — stable air suppresses turbulence and mixing near the ground, unstable air encourages it.

3. Ways to start the wind

WindNinja always needs a starting wind to adjust — Windfield gives you five ways to provide one:

• One wind for the whole area — you set a single speed, direction and height yourself (Simulate). The simplest option, and a good way to see how terrain alone bends a given wind.
• Observation points — you place one or more points on the map and give each its own speed/direction/height, letting the model start from several different readings across the area rather than one uniform value.
• Live stations — real current observations: NOAA's National Weather Service stations, airport METAR reports, NDBC ocean buoys, and RAWS/mesonet stations (via Synoptic Data) feed the model with what's actually blowing right now, nearest to your area.
• A vertical profile — wind speed and direction at several heights at once, rather than one height. This comes from model analysis pressure levels (Open-Meteo, which assimilates balloon, aircraft and satellite data into its weather models) and from radiosonde (weather balloon) soundings at nearby launch sites. Windfield interpolates between the levels you give it to drive each output height.
• A forecast model — GFS, HRRR or NAM (via NOAA NOMADS) drives the wind over several hours at a time, one WindNinja run per forecast hour, with a time player to step through them.

4. Heights: above the ground, not above sea level

Every wind value Windfield shows you is a height above the ground directly beneath it (AGL — above ground level), following the terrain up and down, not a fixed altitude above sea level (MSL). "10 m" always means ten metres above whatever the ground is doing right there — on a ridge, in a valley, on a slope.

• 2 m — roughly head height; dominated by local surface friction and the most sheltered by vegetation and terrain roughness.
• 10 m — the standard meteorological reference height, above most ground-level friction effects; what most station wind reports use.
• 100 m — well above the surface boundary layer on flat ground, closer to the wind that's steering the larger flow rather than being dragged by the ground.

The 3D view works the same way: it's a volume of AGL wind following the terrain surface, but when you slice through it at a fixed altitude (MSL), that slice can clip through the ground on high terrain and float above it over low terrain — a 500 m MSL slice is inside the mountain on one side of your area and still well above the valley floor on the other. Where the model shows air rising, it's being pushed up a windward slope; where it shows air sinking, it's descending a lee slope or draining downhill at night.

5. Structures

You can draw a building, wall, barn, or any polygon/rectangle/circle shape on the map, give it a height (or mark it infinitely tall), and Windfield raises the terrain under it before solving.

• With the fast solver, a structure is modelled as raised terrain — the solver doesn't know it's a building, only that the ground there is suddenly taller. That means your mesh has to be clearly finer than the object: a structure smaller than one grid cell barely registers at all, and a coarse 100 m mesh will simply erase a 10 m shed. Pick a mesh resolution well under the structure's own size to see it at all.
• With the momentum solver, structures behave as real obstacles the flow has to go around — this is what actually produces a wake, separation and recirculation behind them, which the fast solver can't represent regardless of mesh size.
• Infinite height marks a structure as effectively unbounded (burned far above any real terrain in the area) — useful for seeing a wall-like barrier effect without picking a real height.

6. The pictures

• Speed drape — a colour-mapped raster of wind speed over the terrain, opacity adjustable so the map underneath stays visible. Colours follow the legend shown with the result.
• Streamlines — traced by stepping along the local wind direction from a starting point in small increments, again and again, so the resulting curve follows the path air would actually take through the field; good for seeing flow wrap around a ridge or a structure at a glance.
• Particles — small animated points carried by local speed and direction in real time, the most intuitive read of where flow is fast, slow, or turning, at the cost of giving exact values at a glance.
• Arrows — a decimated grid of direction/speed arrows, useful when you want a value at a specific spot rather than an animation.
• Contours, relief and slope — drawn from the same model terrain WindNinja actually solved on (after any structures were burned in), not a generic basemap layer, so what you see lines up exactly with what was modelled.
• Cross-sections — draw a line and see a side-on profile of the terrain, and where available, wind speed, along it.
• Probe values — click any point on a finished result for the exact speed, direction and elevation there.
• Eddies, recirculation and convergence overlays — highlight where the flow curls back on itself or where air from different directions meets; most meaningful with the momentum solver, which is the one that actually resolves that behaviour.

7. Accuracy and limits

Windfield's results are a model, not a forecast. It computes a physically plausible wind field for the terrain and inputs it's given — it doesn't know what the wind is actually doing right now unless you chose Current conditions or Forecast, and even then it's only as good as the weather source behind it. Treat it as a planning and situational-awareness tool, not a substitute for an official forecast, especially for life-safety decisions.

There's a direct trade-off between resolution and run time: a finer mesh resolves smaller terrain features and structures but costs more cells and more solver time, so the app shows a live estimate (cells · seconds · terrain resolution) before you run, and offers the coarsest resolution that would fit if a custom setting is too fine for the area.

Results aren't kept forever: a finished job's files expire roughly 5 minutes after you stop watching it (close the page or move on). A share link doesn't point at a file kept on disk indefinitely — it re-POSTs the original request and rebuilds the run from scratch, so it still works after the original job has expired, just with a fresh wait. Everything above is also reachable as a plain public JSON API — see Help › API for every endpoint and example request.

8. Data sources and credits

Windfield runs on, and is grateful to: the USDA Forest Service Rocky Mountain Research Station / Missoula Fire Sciences Lab's WindNinja; AWS Terrain Tiles (SRTM, USGS 3DEP, GMTED) for elevation; Open-Meteo and the NOAA National Weather Service for current conditions; NOAA NOMADS (GFS, HRRR, NAM) for forecasts; NDBC and Synoptic Data for additional live stations; OpenStreetMap Nominatim for place search; and OpenStreetMap, OpenTopoMap and Esri for basemaps. Full attribution, licences and links: Credits.

Windfield has no login and no accounts. Its privacy policy and terms are SummitFlux's own: summitflux.com/privacy-policy.