Finally we wanted to add some simple textures and play around with different settings such as camera position. For the background we decided to go with a simple gradient generated inside the shader prior to rendering the clouds:
/* Generate blue sky */
float blue = scale( v_position.y, -1, 1, 0.4, 0.6 );
out_color = vec4( vec3( 0.39, 0.30, 0.65 ) - blue, 1 );