Clouds can be a nightmare to reproduce on canvas because of several reasons. The important thing is that one avoids the appearance of a solid object because they are not solid - yet they have more form (shape) than any landscape image. I have used a palette knife and a Q tip (cotton bud) and a very inexpensive small fan brush to get this far.
These are 'towering cumulous' which often develop into a late afternoon thunderstorm in summer months. This layer is not yet complete but the building blocks are going in the right direction.