Real Sky is an add-on that allows you to create sky and clouds with physically based lighting
Based on physical formulas, it gives you the right lighting from the most common environment: the sky. Real Sky accurately simulates the scattering of the sun’s rays through the Earth’s atmosphere at any given time of the year. There are 2 sky method: our sky colors method and the traditional sky texture method. Just by changing the month and day values you will get the accurate position of the sun in the sky, so you can create any kind of condition.
Not only the sky itself but also the four procedural clouds are based on real clouds shapes. The clouds themselves have physically accurate materials to suit any kind of climate. There are 4 of the most common clouds, and they are fully procedural: Cirrus, Cirrocumulus, Altostratus and Cumulus.