Boudin played a critical role in promoting plein-air (outdoor) painting in France: in addition to his own artistic contributions, he was the first to introduce the young Claude Monet (1840 – 1926) to the pleasures and challenges of painting directly from nature. As Monet would much later comment, "If I have become a painter, I owe it to Eugène Boudin." Their friendship was long and fruitful; Monet even invited his mentor to participate in the first impressionist exhibition of 1874. Like the impressionists, Boudin had a keen interest in depicting the changing character of contemporary life and used lively brushwork to animate his scenes. He likewise shared the impressionists' concern with light and atmospheric effects, which is evident in the sea breezes that seem to blow through many of his pictures, snapping flags and ruffling skirts and parasols. Boudin's mastery at conveying the nuances of midday sunshine, brilliant sunsets, and menacing storm clouds led Camille Corot (1796 – 1875) to praise him as "the king of the skies."
Open-air painting was already an established tradition by the early nineteenth century. An influential treatise published in 1800 by Pierre-Henri de Valenciennes stressed the value of outdoor studies done quickly and with a broad sweep: "all études (studies) from Nature should be done within two hours at the outside, and if your effect is sunrise or a sunset, you should take no more than half an hour." Studies were a vital part of a young painter's training and continued to play an important role in his or her preparation. Many artists made regular expeditions to paint in scenic locations, sharpening their eyes and recording effects that could be employed in later compositions; however, at midcentury neither artists nor audiences—and certainly not the official art establishment—afforded these plein-air sketches equal standing with a finished work painted in the studio. Although sketches were exhibited at the Salon, they were always identified as studies.
Several factors helped erode this distinction. In 1841 an American artist invented collapsible metal tubes for oil paints, which made working outdoors much easier (previously, pig bladders were used to store pigments). Eventually, artist suppliers offered portable paint boxes (boîtes de campagne), some with built-in easels and parasols. These innovations coincided with the expansion of railways, which made it much easier to reach the countryside in the first place. The sheer number of people painting outdoors increased. Artists' colonies grew up in picturesque locales, and many of them would become tourist destinations. By the mid-1800s a new mindset was also at work. Realism, in art and literature, emphasized "truth" and "sincerity" rather than contrived constructions, prompting progressive artists to paint nature with unembellished directness. These bolder painting styles also started to blur the difference between sketches and fully realized studio works.
Boudin's understanding of light and weather effects depended on plein-air studies, and he began to paint finished works almost entirely outdoors. In 1864 Charles-François Daubigny exhibited as a Salon picture a landscape created entirely on the spot. By the 1870s plein-air painting had become a touchstone for impressionists, who placed a premium on direct observation, speed, and spontaneity as they tried to capture the look of changing weather, seasons, and times of day.