Sudden change in UV (CL calibration applied)

Today the sky unit reported a sudden jump in UV value as seen in the image. There is no such change in brightness value (added my production from the solar panels just for fun).
The obvious answer would be that this is the continuous calibration doing its job. But that should have been running for a very long time now so I expect only tiny, almost invisible changes. What might be causing this? Did the algorithm change overnight?


interesting enough, the RIVM in our country measured (I guess around 150km away from my station) a maximum of 3.5. But it also goes down way faster. so at the latest measurement Sky unit reports 3.4 (down from a max of 3.7) where as the official measurement is already down to 2.7, down from the max of 3.5.

Hi sunny,

The CL system applied this calibration correction around 10:25 UTC. Your UV calibrations have remained consistent for over a week but the latest calibration was a bit higher.

good to know that it is the correction mechanism.Thanks. I’ll keep an eye on it, as I don’t expect changes of 30% after months of running.

Agreed, it will be interesting to see what the calibration looks like tomorrow.

Untitled-1 Today I probably received another big callibration adjustment in UV index. But I think the current values are bigger than I expect on the official RVIM measurements in bilthoven, which today reach a high of 4.1. My sky unit reported 4.8.

uv values are still too high. at 8.30 in the morning it already showed 1.9. That’s more than double the max expected value. There is some scattering in the clouds, but that doesn’t justify a doubling of the measurement.

@WFmarketing @WFstaff still way too high. My uv max so far today is 4.3. The national ‘official’ value at this point in time is 2.6 ( ). Constant Calibration is doing something, but it isn’t particularly good.

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Since the calibration is done on readings at Solar Noon, it would help if you reported the values measured at that time.

That does not look like a WeatherFlow graph.

Today it is already two units too high. My observation would be that the applied automatic calibration (26 days ago) was a step in the right direction, but overshoot the target in a big way. I think it would be better if calibration was done in smaller steps. Perhaps max 5% per calibration. If the result still isn’t good, the system could always apply another one at the next opportunity.
Steps of 30% are perhaps only good for the very first calibration. But as my station has been online for a long time, it’s unlikely that that would be the case here. @WFmarketing

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Same results here I reach sometimes 8 and on most days a max of 7 ( 4,8 ) should be normal I think it’s become worse lately. My sensor is free for sunlight most of the day and sure round noon

Hi @sunny . Thanks for the heads up. A quick glance our system shows last completely clear day was April 23 and the expected maz UV was 4.3. At your location, current expected max UV is 4.5, but your data is showing greater than that based on the calibration applied on April 23. Have asked our data scientists to take a closer look. Will advise. Cheers.

just to make sure… today’s expected max (according to rivm website) was 5.8, not 4.5 as you just mentioned. That might be extra high due to northerly winds, bringing in very clean air (weatherman in the news mentioned the extra blue skies today)

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@dsj for station 6962.

Solar noon was at 12:58 and a reading of 9.6. Form 12:50 to 13:00 readings were 9.6 to 9.9. Even though the curve shows moments of cloud cover, do these readings qualify for calibration?

What would you base the calibration on?


Hi Gary. Yes, it looks a little low. Your SKY hasn’t had enough clear days for an automatic CL adjustment but we have manually bumped it up in the meantime. Should look better tomorrow (today).

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I think you actually adjusted the Sun. I’m starting to understand why some complain about UV.

It actually hurts being in direct sun for 10 minutes.