Construction Weather Data Methodology
See how forecasts, historical archive data, trade advisories and ten-year weather benchmarks are sourced and calculated — including the limitations project and contracts teams should understand.
- Where the data comes from
- Observation, model and reanalysis data — what is the difference?
- How your site location is matched to a grid point
- Archive vs. forecast data
- What data fields are used
- Trade thresholds
- Tender planning averages and benchmark values
- Reproducibility and independent verification
- Limitations
- When a physical on-site station may be preferable
- What each output is suitable for
Use the right weather output for the decision
Construction Weather provides location-based weather information for review. It is not a site measurement, a safety authorisation or an automatic contractual decision. The table below describes a sensible boundary for common construction uses.
| Use | Weather basis | Useful for | Must be added or checked |
|---|---|---|---|
| Tender | Historical archive / reanalysis and the configured trade thresholds | Comparing seasonal weather risk and informing an allowance or risk register | The proposed programme, method, contract risk allocation and estimator's judgement; it is not a promise of future conditions |
| Planning | Forecast information, with radar shown separately where available | Sequencing work, checking likely weather constraints and considering mitigation | The method statement, manufacturer limits, site observations and a competent person's decision; an advisory is not a go/no-go safety authorisation |
| Early warning | Near-term forecast information, with the retrieval time recorded | Drafting a prospective notice for project-team review before a potential weather risk | The governing clause, notice route and deadline, affected activities, mitigation and project-specific facts before issue |
| EOT review | Historical archive / reanalysis for the affected period, with provenance retained | Identifying potential weather events and organising a reviewable evidence pack | Site records, notices, affected work, programme and critical-path analysis, causation and clause tests; weather data does not create automatic entitlement |
Historical reanalysis reconstructs past conditions; a forecast estimates upcoming conditions; radar is a separate recent precipitation observation view. None is a physical measurement at the site, and none should be presented as a safety clearance or a contractual entitlement decision.
Where the data comes from
Construction Weather uses Open-Meteo as its primary weather data provider. Open-Meteo provides a documented weather API with source-specific archive, forecast and related data products. The relevant source and date range should be recorded with any project output.
For historical data the platform uses the Open-Meteo Archive API, which draws on ERA5 reanalysis produced by ECMWF. Reanalysis reconstructs past weather by combining observations with a weather model; it is a historical dataset, not a reading from an instrument at your site.
Open-Meteo documents its API and methodology. This means the source basis of a historical query can be recorded and independently examined, subject to the provider's availability and any missing values.
Who produces ERA5?
ERA5 is produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) and published through the Copernicus Climate Change Service (C3S).
ERA5 is a historical reanalysis record. Open-Meteo's documentation identifies it as the source for its Archive API. The dataset is useful for comparisons across dates, but local terrain, exposure and microclimate can differ from a gridded value.
An independent party can query the same public archive service for the same coordinates, date range and parameters. Retain the source and query details with any report so the basis of the comparison is clear.
Observation, model and reanalysis data — what is the difference?
These three terms are used throughout this page and in the wider construction contract context. Here is what they mean in plain language.
Station observations
A physical instrument — an anemometer, rain gauge or thermometer — at a specific location measures conditions and records them at regular intervals. Station data is a direct measurement at that point, but coverage, siting and historic completeness vary.
Construction Weather does not present a raw station reading as the platform's weather output. It provides the provider's model, forecast and reanalysis products for the requested location and dates.
Numerical weather models
A weather model represents the atmosphere using mathematical equations and input data. Forecast models run forward to estimate upcoming conditions. Forecasts should be read with their issue time and lead time in mind; they are not observations of what has already happened.
Reanalysis data
Reanalysis combines available historical observations with a weather model to reconstruct past conditions across a grid. It provides a consistent dataset for comparison, but it is not a site instrument and may not reflect local effects.
For a contractual review, retain the source, query details and retrieval date, then check the reanalysis against the contract and any contemporaneous project records.
How your site location is matched to a grid point
When you enter a site address or postcode, the platform converts it to a latitude and longitude coordinate and requests weather data for that location. The service returns a gridded value for the requested area; it is not a reading from a sensor installed at the project.
The location, source and requested date range should be retained alongside an export. This makes the basis of the weather comparison clear and lets the project team check whether the contract calls for a named station or another source.
Local terrain, exposure, coastal effects and urban conditions can cause the weather at the works to differ from a gridded value. Treat the output as location-based decision support and compare it with site or station records where the distinction matters.
Elevation handling
The weather service may return elevation metadata with a location. That metadata describes the source data, not a survey of the construction site.
If the works are at a materially different elevation or exposure from the requested location, note that as an assumption and check available local records. A gridded value can smooth or miss conditions created by terrain.
Precipitation and wind can also vary across a project area. Use the contract, method statement and competent-person assessment to decide whether the available weather basis is appropriate for the decision.
Archive vs. forecast data
The platform distinguishes between historical and forward-looking weather information depending on the date range you request:
- Historical archive / reanalysis — past dates use the Open-Meteo Archive API and ERA5 reanalysis. This is a reconstructed historical record for comparison, not a measurement at the site.
- Recent and near-term forecast — recent and upcoming dates use the Open-Meteo Forecast API and forecast output. The issue time and lead time should be recorded when the forecast informs a notice or plan.
- Radar view — where shown, the radar is a separate recent precipitation observation view. It should not be described as historical reanalysis or as a forecast.
Where a requested range spans more than one source, record the source boundary in the evidence. Do not imply that forecast or radar information has the same provenance as historical reanalysis.
Why can the source vary by date?
Archive and forecast availability depends on the provider and the requested dates. The important distinction is provenance: an archive query is a historical reanalysis record, while a forecast is an estimate issued for a future or recent period. A radar frame is a separate recent precipitation observation.
For contractual purposes, preserve the source label, retrieval or issue time and any missing values. Do not present a forecast or radar view as if it were a settled historical record.
What data fields are used
The platform retrieves and processes the following parameters from its weather data sources for each requested range. Availability and aggregation depend on the source and date 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 single threshold is breached — for example, a day with acceptable rain but excessive wind would still be classified as unsuitable for crane operations.
How thresholds are applied
Each day's weather data is compared against the threshold set for the selected trade. The comparison uses daily aggregates — maximum wind, total precipitation, minimum temperature — rather than hourly data, which reflects the way adverse weather clauses are typically assessed in standard construction contracts.
The resulting unsuitable-day count is a binary classification: each day either breaches one or more thresholds (unsuitable) or does not (suitable). The platform does not apply partial or proportional counting.
Threshold values are visible within the platform for each trade. If you believe a threshold is not appropriate for a particular contract or operation, you should verify the applicable threshold with the contract administrator before relying on the output.
→ See how trade thresholds power Construction Weather Advisories
→ See how unsuitable-day counts support EOT Evidence reports
Tender planning averages
The tender planning tool calculates average unsuitable days per month by applying trade thresholds to ten years of historical archive data retrieved from the Open-Meteo Archive API (ERA5 reanalysis) for the requested location.
The result is a statistical average: the expected number of unsuitable days in a given calendar 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 archive data gives the planning team a stated comparison period. It remains a historical average: individual years and project conditions can differ, and the result is not a prediction or a site measurement.
How benchmark values are calculated
For each month in the 10-year window, the platform counts the number of days on which the trade's threshold conditions were breached. These annual monthly counts are then averaged to produce a single figure — for example, "2.3 unsuitable days in March" for a particular trade at a particular location.
The calculation applies the selected thresholds to the retrieved historical data. An independent party can inspect the same Open-Meteo Archive API source and repeat a query for the same location, dates and parameters, subject to the provider's response and any missing values.
The tender planning tool is intended for historical comparison and allowance planning. It does not predict the specific weather in a future project month or decide whether a tender allowance is adequate.
Reproducibility and independent verification
For contractual and dispute-resolution purposes, it matters whether data can be verified independently. Here is how Construction Weather data can be checked.
Open data source
ERA5 reanalysis data is publicly available via the Open-Meteo Archive API. An independent reviewer can query the same endpoint for the same coordinates, date range and parameters, subject to the provider's response and any missing values.
Location and source in exports
Downloaded exports — including Excel weather reports and single-day certificates where available — record the location and query context supplied to the platform. These are location-based data outputs, not site measurements. An independent party can query the Open-Meteo Archive API with the same coordinates, date range and parameters to inspect the source basis.
Deterministic thresholds
The unsuitable-day thresholds applied to each trade are configured rules, not a safety authorisation or a probability of delay. Review the chosen rule, source data and missing values before using a classification in a project record.
Recorded historical source
Historical reanalysis is a source for comparing past dates. Record the provider, dataset, query parameters and retrieval date with the export so a later reviewer can understand the basis and any source-version differences.
Third-party verification
An independent reviewer can inspect the public source, apply the contract's test and compare the output with station and project records. Platform outputs are a starting point for expert and contract review, not a substitute for it.
Time-extension claim documents
The time-extension evidence Word document records the weather parameters and threshold values applied to each qualifying day, supporting disclosure to the other party. For very high-value claims, an independent meteorological expert should be instructed to review and, where appropriate, supplement the platform-generated data with their own analysis.
Limitations
We believe transparency about what the platform cannot do is as important as explaining what it can. The following limitations are inherent to the approach and should be understood before relying on outputs for contractual purposes.
Gridded vs. point measurements
Historical reanalysis is gridded data rather than a point measurement. Conditions at a specific construction site may differ from the represented area, particularly in hilly, coastal or dense urban settings where local terrain creates its own microclimate.
Elevation and microclimate effects
Valley frost pockets, ridge exposure, sea-breeze effects and urban heat islands may not be represented in the same way as the wider gridded dataset. A site can experience conditions that differ from the value returned for its requested location.
Forecast uncertainty
Forecasts are estimates issued at a particular time. Uncertainty can change with lead time and weather situation, so retain the issue time and use the forecast alongside site controls and current observations. A forecast is not a historical measurement.
Missing or sparse data
Archive data may contain gaps or null values for particular parameters and dates. Treat missing values as missing, record them in the evidence and do not assume that an absent value represents a zero or a safe condition.
Threshold applicability
Unsuitable-day thresholds are general guides aligned to published standards and common contract practice. 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 time-extension 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. In high-value disputes a meteorological expert witness should be instructed to review and, where appropriate, supplement platform-generated data.
When a physical on-site station may be preferable
Construction Weather uses gridded reanalysis and forecast data rather than a physical instrument on your site. A station or other local source may be needed where the contract, specification or decision requires a measurement at a particular point. There are also situations where an on-site station delivers material advantages.
Consider a physical station when:
- Your site is in a location with strong local microclimate effects — a narrow coastal valley, a high-altitude plateau, an area prone to fog or orographic rainfall — that may not be represented by the gridded data.
- The contract or project specification explicitly requires weather data from a named local meteorological station or from an instrument installed on site.
- The project is of very high value and weather-related claims are anticipated from the outset, making a contemporaneous on-site record a worthwhile investment in evidence quality.
- The relevant weather parameter is highly localised — for example, precise wind gust readings at height on a tall structure, or ground temperature at the exact location of a concrete pour.
Gridded data vs. an on-site station — a fair comparison
A physical station installed on a specific site produces a direct measurement of conditions at that point. Its siting, calibration, maintenance and record completeness still need to be checked.
However, on-site stations also have limitations that gridded reanalysis does not:
- Coverage gaps: station data only exists from when the instrument was installed. ERA5 reanalysis covers the full project period, including dates before the station was commissioned.
- Maintenance: instruments require calibration and maintenance. Siting or record-quality issues should be documented before relying on the readings.
- Retrospective analysis: for time-extension claims based on conditions that occurred in the past, an on-site station provides no advantage unless it was in place and recording during the period in question.
For a high-value review, compare historical reanalysis with on-site or nearby station records where available. Use the contract and expert review to decide how the sources should be weighed.