I cannot answer your question but for what it is worth, I use the Aurora cloud sensor which does exactly that - it measures the sky temperature to work out whether it is cloudy or not. It has settings which you can adjust which change whether it reports cloudy or clear. It also reports what it calls sky 'clarity' which I suspect would be what you are referring to - the difference between ambient and sky temps. I have only been playing with it for a few weeks by comparing what it is reporting against what I see in the sky and it works quite well. The unit also has a rain sensor which can operate a relay which can then be used for other actions such as closing the observatory roof etc.
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