How to use normal map on build-in skybox?

I want to add the cloud effect in build-in skybox.First,I add a cloud texture in the shader.And I want make some very easy lighting effects,like lambert, on the cloud .So I also use a normal map for the cloud.However when I write the shader for skybox,I find there is no normal and tangent information in the vertex of skybox.Therefore I can't find a way to use normal map on skybox shader.Does anyone have an idea how to solve the normal map problem with build-in skybox?

 

You are able to emulate the built in skybox to test your normal map theories simply by mapping the skybox to a cube that's got normals facing in.

 

If I consider the skybox as a sphere,I can calculate the normal by v.vertex in VS code.But I don't know how to deal with the tangent information.