EnvironmentalVisualEnhancements (a.k.a. EVE) is a mod for KSP that adds clouds and city lights. It is highly configurable and a number of other mods provide configurations for it with improved clouds not just for the stock planets, but for other planets provided by other mods.
To use EVE, install the plugin for your version of KSP and a Config.
The sample config in the Files section works for all plugin/KSP versions.