pseudocode for cloud shader. if you are billboarding the cloud quad, basically takes the direction the cloud is facing and changes the amount sampled per RGB channel

anubis-cloud-shader

// passed in variables
float3 billboard_normal; // defined - (where billboard is facing), [-1,1] range
float2 uv; // defined - the UV of the quad [0,1] range
float time; // defined - passed in
texsampler tex_cloud_0; // cloud tex, each channel in RGB stores different monochrome texture
texsampler tex_cloud_1;


// normalize to [0,1] range
float3 billboard_normal_normalized = billboard_normal * 0.5f + float3(0.5f);

// sample from the texture
// (you can do things like animating the uv slowly)
float4 tex_sample_0 = tex2D( tex_cloud_0, uv );
float4 tex_sample_1 = tex2D( tex_cloud_1, uv );

// this part basically uses the normal and weights each channel
float cloud_val_0 = dot( tex_sample_0, billboard_normal_normalized ) / 3.0f;
float cloud_val_1 = dot( tex_sample_1, billboard_normal_normalized ) / 3.0f;

float cloud_val_final = lerp( cloud_val_0, cloud_val_1, sin(time)*0.5+0.5 );

basically, store a different cloud texture in each of the three RGB channels!