From The Art of Michael Whelan:

I actually saw these clouds from my backyard late one summer evening, and they inspired me to come right inside and paint the scene. They didn't look entirely real when I saw them, and the myserious quality of the light made me want to document them in a larger project.

As an experiment, I used a homemade medium that included plenty of poppyseed oil, for its nonyellowing properties. It's also an extremely slow drier--it must have taken four weeks for the painting to dry to the touch.