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Hi Stephen,
All of the commercial cloud detectors measure the ambient and sky temperature. When the sky is clear, the sky temperature is about 20 degC below the ambient temperature. When there is cloud cover, the sky temperature approaches the ambient temperature (ie the delta T reduces). I initially set up my Arduino project on this basis however I noticed the delta T vs cloud cover relationship wasn't constant such that the model was predicting cloud when it was actually clear. I went searching for any modelling that had been done and found a paper that compared models used for prediction of sky temperatures. Some of those models use cloud cover in the prediction and some use water partial pressure (which is a function of relative humidity). Since I'm directly measuring the sky temperature but want to predict the cloud cover, I rearranged the equations in a couple of those models and that is what my cloud prediction model uses.
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