Photographer and gallerist Alfred Stieglitz demonstrated the expressive potential of photography with his evocative images of clouds. More than portraits of the sky, the photographs are meant to reflect the emotions and states of mind Stieglitz experienced while finding their equivalents in the varied patterns of the clouds. Though the photographs were taken during the day, Stieglitz printed the images very dark, making it difficult to discern whether the clouds are lit by the sun or moon.
Additional text from Seeing Through Photographs online course, Coursera, 2016