Drawing cloud formations - Conte and oil pastel

In the first study done in conte, it was early afternoon on a bright but cold day where the sun would come out occasionally.  The cloud was very bright white at the top edge creating a strongly defined edge against the sky.  I achieved the brighter lines of white by rubbing off the conte with a putty rubber.  The cloud was darker underneath.  I attempted to get the smokey effect of the thin cloud in and around the cloud formation. 

The study below was done in oil pastel and is a second attempt at a sky with clouds full of snow.  In order to get the uniformity of blue colour needed for this sky, I first lay down an undercoat of white oil pastel and rubbed it into the paper.  Then I added a blue, apricot and grey colour with a lighter touch.  This enabled me to get a variety of effects, eg smooth blue for the sky and more variegated effects to get the different tones in the cloud with the help of adding and scraping off colour.