Hi Markus
This Hue outside motion sensors are marketed as weather proof and so far (9 months in use) I did not have any water issues. What is a bit strange, they report still 100% battery level. Either they have super efficient electronics (with solar charging?) or the battery level report is just wrong. We will see. In any case, 9+ months is not bad as they update the lux levels every 5 mins and even almost instantly, if there is a dramatic level change. I just chart the cloud index, but from the values I assume there must be Lux levels over 100k measured. I will check on a sunny day 
To calculate the cloud index I divide the lux level by “the clear sky radiation” (total radiation from the astro binding). I turned out to be precise until autumn. But when winter came (and sun was low at the sky) I was not happy anymore. I turned out that the Astro binding total radiation is for square meter on horizontal ground. But my sensors are vertical! So I had to calculate the clear sky radiation for a vertical surface. Unfortunately nothing in the Astro Binding, so I calculate them from scratch. I updated now this article with the new formulas. The last 2 months since the update I had very good result!
BR Thomas