Dynamically Fading One Texture Into Another
I'm not sure if this is what you're looking for, but it is a very interesting technique I got from Andrew Price's tutorial on how to make an earth. Andrew's technique uses the following node tree to have one texture (the day image) dynamically fade into a second texture (the night image), depending on which part of the earth mesh is being lit by the Sun lamp as the earth rotates. Here's the node tree:
Here, wherever the Separator material is not receiving light from the Sun lamp and is thus black, the night texture map is used, and wherever the Separator is white due to the light of the Sun, the day texture is used. By adjusting the position of the second color stop on the Color Ramp node, you can adjust how the two textures mix together.
For your purpose, instead of using a light from a lamp on a Separator material to make some part of your mesh light, you could instead use something like a black & white Cloud texture to act as the light and dark input.
Here is the blend file -- the node tree above is found on the Earth mesh.
Here is a Andrew's tutorial on his website Blender Guru. Also, notice that on Andrew's site, below the tutorial, you can download Andrew's finished Earth blend file.