This mod by RohitXL5 interchanges the default sky textures of London and San Francisco. Now you can play San Francisco with the skies of London, and play London with the skies of San Francisco. Also, not just the skies, the fog and shadings are also swapped.
Note: The mod may be used only in absence of any other graphic mod. Any other graphic mod which affects sky textures should be disabled before using this one.