While these oil studies were done in small format like his pastels, they differ slightly in several ways. The sky has almost always been the sole subject of the work. Boudin opted for a very high frame and the sea does not appear in the compositions. At most, the faint outline of a low horizon (dunes? shrubs or bushes? windmill?) can be seen in some of them. But more often than not, the clouds are the only motifs. At times, the subject becomes so fine or abstract that Boudin specified its meaning on the back of the work.
 
The main difference is in the execution. Boudin plays with the paintwork: sometimes he uses sweeping, energetic brushstrokes to apply fluid paint like a wash, sometimes he draws the paint out in long strokes or, just the opposite, uses pure colour with vigorous impasto. This thick paint is where he occasionally scrawled his "meteorological" observations with the tip of his brush handle.
One can sense the curiosity of an artist, who, in addition to his more significant production of pastels, endlessly searched and tested to paint the sky as the Dutch masters had done long before him.
 
A quote from Boudin's personal diary sheds remarkable light on this small group of sky studies: "Tuesday, December 3 [1856] To swim in the open sky. To achieve the tenderness of clouds. To suspend these masses in the distance, very far away in the grey mist, make the blue explode. I feel all this coming, dawning in my intentions. What joy and what torment! If the bottom were still, perhaps I would never reach these depths. Did they do better in the past? Did the Dutch achieve the poetry of clouds I seek? That tenderness of the sky which even extends to admiration, to worship: it is no exaggeration."