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Rich,
I use a hand-held IR thermometer with ~10 degree FOV. I have not relied on it, but I record its readings on imaging nights.
I have found no hard and fast rules.
However, if the zenith temperature is below -40 C, the sky is generally clear. No absolute guarrantee, as I have also recorded -40 C when there is definitely high cirrus.
On the other hand, if the measured temp is >-30 C the image is almost always noticeably compromised.
If there's low cloud in the air, the IRT will usually read between -20 and -5.
You can get a reasonably good idea of these indicators in daylight (obviously, don't aim near the Sun!) so that you can see what you're pointing at.
Sorry I can't say that the use of the IRT is anything more than a rough guide to the cloud state.....
Mark
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