Historical Data to Analyze US Observing Sites

I hope this is not off-topic for this forum, but I struggled to find the right one.

I am interested in analyzing various sites in the US for astronomical observations.  I would like to use multiple publicly available datasets.  I am aware of the METAR dataset for weather including visibility and sky conditions at many airports.  I know that cleardarksky.com provides historical summaries of their FORECASTS.  I also see that the US National Weather Service provides sky cover summaries in various historical climate summaries for US locations.

I wonder what other sources of data people can suggest?

Clear skies,

Cliff