Every day between 10am and 11am, I see a drop in the UV/Brightness on my Sky unit. I was thinking it was a shadow from somewhere though I couldn’t think of where. I’ve been meaning to post about this for a while but I never seemed to be home (or, more importantly, remember) to take some pictures of the station while the drop was happening… but this week I finally remembered and now I’m a bit more puzzled.
And for grins and giggles and to show that this happens most every non-cloudy day, here’s what it’s looked like over the last 4 months (you can see it in that darker orange stripe that goes across):
Based on the photos, the only thing that I can figure could be casting any kind of shadow on the sensor is one of the bamboo skewers I put on to keep the birds away, but those seem awfully slender to cause such a measurable drop.
the sensor itself is probably just a very very tiny chip. but there is a diffuser on top of the unit. That window is - just guessing - 20 mm wide. I guess that one of your anti bird spikes is just darkening the window. This pattern matches that of a small shadow passing over a round window. At first little shadow then in the center the most, and going back to a little and zero influence. If it were a very broad shadow, the influence would be constant for some time when the window is fully in shadow.
/// just some guestimates… radius window 10 mm, so surface area = 314 mm2, your drop in uv is about 25%, so shadow should block at least 78mm2 at is max. which would be a rectangle of 20mm (the width of the window) by 3.9 mm. Looking at your picture that 3.9 mm might come very close to the width of your spike ///