Data Methodology
Where the weather data comes from, how it is processed, and what the numbers on the platform actually mean.
Weather data sources
Construction Weather uses Open-Meteo as its primary weather data provider. Open-Meteo is an open-source weather API that aggregates data from national meteorological agencies — including NOAA, DWD (Germany), Météo-France, ECMWF and the UK Met Office — and makes it available as a unified, gridded dataset.
Open-Meteo combines station observations with numerical weather model reanalysis (ERA5 and ERA5-Land from ECMWF) to produce a consistent historical archive with global coverage. This means the platform can retrieve data for any location worldwide, not just locations near physical weather stations.
Archive vs. forecast data
The platform distinguishes between two data regimes depending on the date range you request:
- Historical archive — dates more than 90 days in the past use the Open-Meteo Archive API, which draws on ERA5 reanalysis. This data is verified and stable.
- Recent and forecast — dates within the last 90 days and up to 14 days ahead use the Open-Meteo Forecast API, which blends recent station observations with model output. Accuracy decreases further into the future.
Where a requested date range spans both regimes the platform merges the two datasets and aligns them by date, so your output is seamless regardless of the period.
What data fields are used
The platform retrieves and processes the following parameters from Open-Meteo for each day in a requested range.
Precipitation
Total precipitation (mm), rainfall (mm), snowfall (cm) and the number of hours with measurable precipitation. Used for rain, snow and general wet-weather thresholds across all trades.
Temperature
Maximum, minimum and mean air temperature at 2 m, plus apparent (feels-like) temperature. Used for frost, freeze, heat-stress and concrete-pour thresholds.
Wind
Maximum sustained wind speed and maximum wind gust speed at 10 m, with dominant wind direction. Used for elevated-work, crane, roofing and open-area trade thresholds.
Humidity & dew point
Mean relative humidity and mean dew point temperature derived from hourly data. Used for painting, coating, waterproofing and finishing-trade thresholds.
Sunshine & UV
Total daily sunshine duration (seconds) and maximum UV index. Used as supporting indicators for worker exposure and scheduling context.
Weather code
WMO weather interpretation code summarising the dominant condition for the day (clear, cloudy, rain, snow, thunderstorm, etc). Used for headline day classifications.
Trade thresholds
Each construction operation on the platform has a set of weather thresholds that define an "unsuitable" day for that trade. These thresholds are derived from a combination of:
- Published British Standards (BS) and industry guidance notes where applicable
- NEC and JCT contract clause conventions for adverse weather events
- CIOB and RICS guidance on weather risk and delay analysis
- Practical site experience for operations not covered by formal standards
Thresholds are applied independently per parameter. A day is flagged as unsuitable if any threshold is breached — for example, a day with acceptable rain but excessive wind would still be classified as unsuitable for crane operations.
Tender planning averages
The tender planning tool calculates average unsuitable days per month by applying trade thresholds to ten years of historical archive data (ERA5 reanalysis) for the requested location.
The result is a statistical average — the expected number of unsuitable days in a given month, based on the decade of historical data. Individual years will vary. The averages are intended to inform programme allowances and contract risk registers, not to predict the specific conditions in a future project month.
Ten years of ERA5 data represents a robust climatological baseline for most locations, balancing data stability against the need to capture recent climate trends.
Accuracy and limitations
Gridded vs. station data
Open-Meteo uses gridded reanalysis rather than point-in-time station observations. Grid cells are approximately 9 km × 9 km (ERA5-Land). Conditions at a specific site may differ from the nearest grid point, particularly in hilly, coastal or urban terrain.
Elevation effects
The platform retrieves the elevation of the nearest grid point from Open-Meteo. Temperature and precipitation estimates are adjusted for elevation by the model, but site-specific microclimates (valley frost pockets, ridge exposure) are not captured.
Forecast uncertainty
Forecast accuracy declines with lead time. Days 1–3 ahead are generally reliable; days 8–14 are indicative only and should not be relied upon for precise operational decisions. Historical archive data is stable and does not change after the fact.
Missing data
In rare cases archive data may contain gaps or null values for specific parameters and dates. The platform displays null values as absent rather than substituting estimates, to avoid misleading outputs in contractual contexts.
Threshold applicability
Unsuitable-day thresholds are general guides. The applicable threshold for any specific contract event will depend on the exact wording of the contract, the nature of the work, and the judgment of the contract administrator. Always verify threshold applicability against your contract documents.
Not a substitute for expert advice
Platform outputs — including EOT evidence reports and Early Warning Notice drafts — are provided to assist review, not to replace it. Contractual and legal decisions should be made with qualified professional advice.