Ground truth
Satellite and forecasts against RIVM Bilthoven
06/ 10/ 2026
Every UV number on this site is modelled: from satellites, or from weather forecasts. The Netherlands has one place where UV is measured from the ground with a spectroradiometer, on the RIVM roof in Bilthoven. So we check ourselves against it, every day, in public.
Today: satellite against the ground
No data for today yet. RIVM publishes once a day; KNMI needs an API key on this server.
Peak, ground
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Peak, satellite
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Typical gap
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Mean absolute difference, UVI
Bias
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Satellite minus ground
0 paired readings within 8 minutes, ignoring values below 0.5. Correlation –.
Forecast scoreboard
The scoreboard starts filling the morning after alerts and the daily job are switched on.
Each morning we record what every service forecasts for Bilthoven's peak, then score it against the peak RIVM measured that day (3-reading running median, so one cloud gap doesn't count). Cloud-free sources are expected to overshoot: that gap is the cloud effect.
Method
Ground: RIVM spectroradiometers (Dilor, Brewer) and radiometers, unvalidated near-real-time data. Satellite: KNMI cloud-modified UV index for the Benelux, nearest grid cell to the station. Data: JSON, CSV. Ground measurements: RIVM/opendata. Satellite UV: KNMI, CC BY 4.0.
Source status: ground ok, satellite not_configured.