They are available in HDR format and both in full 16K and in 4K for previews. All sky files may be used commercially.
Here you can download sky 1, here sky 2 and here sky 3. Watch out: The zip files are each 200 to 250 MB in size. So probably not suitable for downloading on a mobile device. ;)
More sky footage material is available here: hdri-skies.com
Archive notice: This article was originally published on 2016-10-24. Links and embedded videos are preserved as part of the historical record.
