Astronomical seeing and bortle information
Astronomical seeing is key for amateur astronomers, a forecast exists in sites like meteoblue. Would be a great info as a layer or as a parameter for alerts. It's measured in arcsec, as the size of the distortion a puntual light suffers by the air mass movements.
Also light pollution is forecasted in some sites on the bortle or NELM scale, based on haze or pollution I suppose. It states the brightness of the sky background. Darker is better as it lets see fainter deep sky objects.