I've been using the MeteoBlue clear sky clock[1], which is reasonably accurate for me, but doesn't predict transparency (just seeing, cloud cover, temperature, and jet-stream speed).
I just found another that doesn't have any site near me, but might be useful to others. They do predict transparency. (I might watch the Carlow and Isle of Man sites and see if either matches my conditions very closely.)
csc.lamost.org/eng/clocks/query.htm
-- Jeff.
[1]
pages.unibas.ch/geo/mcr/3d/meteo/
-- click on "Astronomy" at the top, and then "European Seeing Prediction".
