Valenciennes holds a position of considerable importance in the history of landscape painting of the late eighteenth and the nineteenth centuries. In 1800 he published the influential treatise on landscape painting Elémens de perspective pratique, à l'usage des artistes, suivis de Réflexions et conseils à un élève sur la peinture, et particulièrement sur le genre du paysage (which was still read by Camille Pissarro in the 1860s). In this book, Valenciennes recommended an almost systematic program of study by painting oil sketches out-of-doors, the better for the young artist to understand nature's myriad appearances and to train his hand and eye in capturing them in paint. This theory was based on Valenciennes' own practice: since the early 1780s he had been painting a variety of oil studies in the open air, including a notable series executed during a period of study in Rome, between 1782 and 1785, most of which are now in the Louvre, Paris.

Study of Clouds over the Roman Campagna is related in style and subject to several oil sketches from that series, for instance At the Villa Borghese: White Clouds (Louvre, Paris). The artist adopted an elevated site, with just a suggestion of the brown rolling slopes of the Roman countryside indicated along the bottom of the image. His true subject is the blue sky above, streaked with silvery white clouds. The sketch was painted rapidly, with a delicately applied and lively impasto that captures the changing cloud formations. Valenciennes' observation was exact and scientific, yet at the same time highly poetic in spirit. In his treatise, Valenciennes placed great importance on the study of the sky because it is the main source of light in landscape painting. He recommended that the artist should paint such studies of the sky and its cloud formations in order to learn the different ways light modifies the appearance of nature, and to train his hand and eye in capturing a variety of natural effects.

These sketches were not made for sale or exhibition, but for purposes of study, as part of the long process that would lead to the creation in the artist's studio of more finished exhibition pictures. Valenciennes also employed them to teach his students how to paint rapidly, and how to select and simplify the complex forms of nature into the limited pictorial compass of a few square inches. Although such works were not normally sold during the artist's lifetime, they were sometimes exchanged among painters and were often acquired by fellow artists in estate sales of studio effects.

(Text by Philip Conisbee, published in the National Gallery of Art exhibition catalogue, Art for the Nation, 2000)