Star Gazing / Astrophotography
This app is focused on atmospheric data more than any other app I’ve ever seen. It’s just brilliant!
One thing that I think would fit well into this app would be some sort of indicator of how transparent the night sky is. In addition to cloud cover, it would have to take into consideration humidity as well as atmospheric instability and other factors like maybe pollution (light pollution, ozone, etc).
There is a website called cleardarksky.com that publishes a chart giving just the sort of information I’m thinking of. Perhaps there’s a way to integrate their data into the Windy.com app, except it wouldn't need to be their whole chart, just a simple rating with maybe a map overlay with different colors being assigned to each rating.
I know that such a feature would be extremely popular among anyone doing any sort of astronomy.