Why weather claims fail
The contractor's burden of proof for an adverse weather extension of time is higher than most practitioners appreciate. It is not enough to show that it rained, or that it was cold. The contractor must show all of the following:
- The weather was exceptional by reference to the historical average for that location and time of year
- The exceptional weather caused delay to a specific critical path activity
- The activity could not reasonably have been re-sequenced or mitigated
- Notice was given in accordance with the contract
Adjudicators and arbitrators see weather claims supported by nothing more than a site manager's recollection of a bad week in January. These claims fail. The solution is data — objective, independently sourced, location-specific meteorological data.
What counts as exceptionally adverse?
Under JCT contracts, the relevant event is "exceptionally adverse weather conditions" — weather that is adverse beyond what could reasonably be expected at that time of year and in that location. The test is comparative: you must show that the weather was worse than the historical norm.
Under NEC contracts, the compensation event at clause 60.1(13) covers weather measurements that occur less frequently than once in ten years. This is a more precise test: the contractor must show that the weather measurement (rainfall, temperature, wind speed, snow depth) exceeded the 1-in-10-year threshold for that calendar month at the location of the works.
NEC 60.1(13): "A weather measurement is recorded — within a calendar month — before the Completion Date for the whole of the works — at the place stated in the Contract Data — which is shown by the weather data to occur on average less than once in ten years."
The data you need
1. Historical weather data
The Met Office (UK), Bureau of Meteorology (Australia) and NOAA (USA) all publish historical weather records. For NEC contracts, the 1-in-10-year thresholds must be established from the data source stated in the Contract Data — usually the nearest weather station to the site. If no weather station is specified, use the closest station with a sufficiently long record (at least 20 years).
2. Contemporaneous site records
Historical data alone is not sufficient. You must also have contemporaneous records from the site itself: site diaries, supervisors' daily reports, rainfall gauges, temperature logs. These corroborate the meteorological data and show the operational impact at your specific location.
3. Programme impact evidence
The third element is the link between the weather event and the delay to the critical path. This requires a programme showing which activities were affected, why they could not proceed in those conditions, and why they were on the critical path at the time.
Obtaining meteorological evidence
For most disputes, the best source of independent meteorological evidence is the national meteorological service. In the UK, the Met Office provides historical weather data for any location in the UK, including daily temperature, precipitation and wind speed. For international projects, the ERA5 reanalysis dataset — produced by the European Centre for Medium-Range Weather Forecasts — provides hourly data globally from 1940 to the present.
PlanLens integrates directly with the Open-Meteo API, which draws on ERA5 data, to produce a site-specific weather report for any project location and date range. The report identifies each day on which weather measurements exceeded the relevant thresholds — providing the objective meteorological evidence that adjudicators require.
Presenting the claim in adjudication
A well-presented adverse weather EoT claim has five components, presented in the following order:
- Contractual basis — which clause entitles the contractor to an EoT for adverse weather, and what the threshold is
- Meteorological evidence — objective data showing that the threshold was exceeded, sourced from an independent provider
- Contemporaneous site records — corroborating the meteorological data with evidence from the site itself
- Programme impact analysis — a TIA or equivalent showing the impact on the critical path for each weather event
- Notice evidence — proof that notice was given in accordance with the contract
Automatic weather evidence
PlanLens fetches site-specific historical weather data and identifies adverse days automatically — producing the meteorological evidence your claim needs.