How to Draw Clouds - Building Darks and Lifting Lights
Shading with a sharp B pencil builds up value in the darker areas of the drawing. Shade carefully up to the edges of very crisp areas of highlight. Shadows in the clouds are shaded, then a kneadable eraser used to soften edges. Once the values are well established, use sweeps of a clean kneadable eraser to lift out cirrus cloud.