OpenGL 4 Shading Language Cookbook - Third Edition

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If you desire less clouds and more sky, you could translate and clamp the value of t prior to using it to mix the cloud and sky colors. For example, you could use the following code:

float t = (cos( noise.g * PI ) + 1.0 ) / 2.0; 
t = clamp( t - 0.25, 0.0, 1.0 ); 

This causes the cosine term to shift down (toward negative values), and the clamp function sets all negative values to zero. This has the effect of increasing the amount of sky and decreasing the size and intensity of the clouds.