Climate Methodology

1. Data sources, licence, and last update

The climate section of a Home-Checker report is generated at the moment you run the report. We do not pre-compute climate profiles — each report fetches the full daily history for the resolved coordinates of the postcode you entered.

• Primary data: the Open-Meteo Historical Weather API, which serves daily historical weather records derived from the ECMWF ERA5 reanalysis and ERA5-Land datasets. ERA5 is the European Centre for Medium-Range Weather Forecasts' fifth-generation atmospheric reanalysis — the international scientific reference dataset for historical atmospheric conditions.
• Licence: ERA5 source data is published by the Copernicus Climate Change Service (C3S) under the Copernicus licence. Open-Meteo's redistribution licence permits free use including for commercial purposes. Home-Checker uses the Pro tier for production reports.
• Window: we request the most recent ten years of daily records, ending today. For each report we calculate annual averages, monthly extremes, and a first-half vs second-half comparison to surface a directional trend.
• National comparators: our England-average reference values are calibrated to the Open-Meteo / WMO methodology used in our pipeline (annual rainfall ~860 mm; annual sunshine ~2,200 hours under the Open-Meteo satellite-derived sunshine definition; annual mean temperature ~10 °C). See the “Known limitations” section for why our sunshine reference is higher than the figure you may have seen elsewhere.
• Last methodology update: May 2026. Data freshness: ERA5 daily values are typically available within a few days of real time. The 10-year window slides forward as you run new reports.

2. What we do

For the lat/lng resolved from your postcode, the Home-Checker climate module fetches ten years of daily ERA5 records and derives the following:

• Annual rainfall. Total mm averaged across the window, with the wettest and driest months identified from monthly averages, plus a comparison to the England national average.
• Temperature range. Average daily high and low (°C) across the window; the warmest month (by mean daily maximum); the coldest month (by mean daily minimum); the average number of frost days per year (daily minimum below 0 °C); and the average number of heatwave-band days per year (daily maximum above 30 °C).
• Sunshine. Average annual sunshine hours, the sunniest month, and a comparison to the England national average (using the Open-Meteo / WMO definition — see limitations below).
• Wind exposure. Average daily maximum wind speed (km/h), highest gust on record across the 10-year window, and a categorical exposure level (sheltered / moderate / exposed) using thresholds tuned to UK conditions.
• Growing season. The average number of days per year with a mean daily temperature at or above 5.6 °C — the long-standing UK Met Office threshold for plant growth.
• Climate trend. The change in mean daily temperature and total rainfall between the first half of the window and the second half. Direction (warming / cooling / stable) is reported with a 0.3 °C threshold to avoid spurious flags from year-to-year noise.
• National comparison percentiles. Indicative percentile bands for the location's rainfall, mean temperature and sunshine vs the England national average, derived from a simple z-score against assumed standard deviations. The label is "indicative" deliberately — see limitations.

3. What we don't

We are deliberate about what falls outside the scope of this section so you can fill in the gaps with the right professional instrument when needed:

• We do not produce future climate projections. Nothing in the climate section is a forecast of conditions in 2030, 2050, 2080, or any other future date. The data is historical reanalysis only.
• We do not apply UK Climate Projections (UKCP18 / UKCP Local) scenarios, RCP/SSP pathways, or any other forward-looking climate model output.
• We do not assess physical climate risk on a TCFD or CDSB-aligned basis, or model the property's exposure to acute climate hazards under a specified warming scenario.
• We do not evaluate flood risk, surface water risk, coastal erosion, or sea-level rise in this section — flood is a separate section of the report with its own dedicated data sources, and coastal / sea-level data is not within the scope of the report at all.
• We do not resolve micro-climates: ERA5 does not see frost pockets, urban heat-island intensity at street level, valley fog, leeward rain shadows below the model's grid scale, or shelter from individual buildings.
• We do not render climate-risk-adjusted insurance premiums, mortgageability assessments, or asset-value impacts. None of those are derivable from the data we have.

4. Known limitations

You should weight the climate section with the following caveats in mind:

• Resolution. ERA5 is published on a global grid of roughly ~30 km native resolution; ERA5-Land downscales the land-surface variables to ~9 km. Either way, two postcodes within the same valley or town will often share the same grid cell. The figures are representative of the broader local climate, not the specific garden or street.
• Sunshine definition. Open-Meteo derives sunshine duration from satellite-derived direct irradiance above a 120 W/m² threshold (a WMO definition). The UK Met Office historical averages are produced from Campbell-Stokes recorders, which use a different threshold and historically yield lower hours per year for the same location. We compare like with like — both your figure and our national comparator use the Open-Meteo / WMO method — but you should not directly compare our figure against a Met Office sunshine value.
• Reanalysis is not measurement. ERA5 is a numerical reanalysis that assimilates observations into a physical model. It is the best-available global historical climatology, but the figure you see for a specific location is a model estimate, not a station reading.
• 10-year window. A decade is long enough to surface seasonality but short enough that volcanic, solar or ENSO variability can move the average. The "trend" comparison (first 5 years vs last 5 years) is directional, not statistically significant; treat it as a rough check, not a finding.
• Heatwave threshold. "Days above 30 °C" is a conservative band. The official UK definition of a heatwave is a multi-day run above a county-specific threshold. We deliberately use a single, stricter cutoff to avoid the impression of forecasting a heatwave.
• Percentile estimates. The national comparison percentiles use a simple sigmoid against assumed standard deviations. They are presented as indicative bands to help you place the location, not as rigorously calibrated statistics. The underlying figures (annual rainfall, average temperature, annual sunshine) are the authoritative numbers.

5. What this is NOT a substitute for

6. Related

• Methodology overview — how the whole report is built
• Report Disclaimer — the full disclaimer
• Data Sources & Attribution — full list of upstream data sources