Odd drop in UV/Brightness measurement

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.

Here’s what the drop looks like on my station:

Here are 4 photos of my sky unit (from 10:30am to 11:20am):

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.

Any thoughts, comments, suggestions?


If you can figure out which skewer may be shadowing at that time, remove that one and see what happens.

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Scary stuff… that’s going to involve climbing up on the hot tile roof during the heat of day. Times like this I wish I had a 30’ ladder.

How large in diameter are the skewers? I just posted pictures of the WF BDA in the announcements post.

Zooming in on the picture and comparing to the one you posted, I’m guessing the skewers are no more than 1.5 times as wide the ones on the WF BDA.

Maybe I can test this with a Sky I have.

More details about the spikes here.

When I get home this evening I’ll pull out a spare bamboo skewer and measure it for comparison.

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 ///


So my skewers are 2.8mm in diameter so about a 1mm wider than the “official” spikes. :slight_smile:

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