Guess it was my turn to draw this picture too :-)
Anyway, the last tutorials i watched for sketching clouds proved really very useful, it's easy to draw shadows on a face exactly the same way, so i'll share the links here if anyone is interested... basically it's just sketch - soften & smudge - repeat until you're happy :-)...
https://www.youtube.com/watch?v=YdfPFMx_clQ
https://www.youtube.com/watch?v=qZQ7XUKwFtc
As usual, comments & suggestions are very welcome, and thank you all for taking a look!!!