I created these clouds and the mist and fog using my own photography of clouds and skies.
If you want to add some high-res clouds to your empty blue skies here is a big pack of cloud brushes for you.40 Cloud and Fog and Mist Brushes However I'd like to offer some tips concerning fog and mist brushes: - apply fog and mist using low opacity and flow settings, they appear much more realistic, if they are subtle and have some transparency - some of the fog brushes come with brush settings that I have tested them with, but their result depends on the size you chose, so you might have to play around a bit to suit your needs - in my experience you get the best results with fog and mist brushes, if you start with an opacity around 20% and a flow setting between 60% and 90%. THESE WILL ONLY WORK IN PHOTOSHOP CC!!!!! I am often in need of additional fog in my works, so I also included brushes that are perfect as a basis for the creation of ground mist, rising fog and so on - check out the two example images to get an idea how it works. I was very careful to avoid ugly fringes and distortions, so these are prefect for your art works.