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Interesting thread.. and thanks Ray for the comments.
I'd concur that I don't think there's a simple correlation with sky temperature. I'm still using an AAG.. getting the parameters for clear sky right are a little tricky.. humid nights definitely confuse it but not related to transparency.
In general transparency indexes are based on humidity/precipitable water. The idea being that if you have lots of pollutants (aerosols) then in humid atmosphere they help condense moisture and degrade transparency. That might work ok as a correlation in Texas, but in Oz in southerly airflow you can have high humidity but very clear air off southern ocean. So humidity does not equal poor transparency here. Conversely you could have very dry northerly air flow with dust/smoke and poor transparency but very low humidity.
Hope that helps confuse the situation even more..
Phil
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