“Plein Air” means “open air” and signifies outdoor painting that captures nature’s light. Plein Air painting became famous in France in the 19th century. The invention of paints in tubes enabled impressionist artists, such as Van Gogh and Monet, to paint outdoors, en plein air.

"Not until the late 1860s, with the work of Claude Monet, Pierre-Auguste Renoir, and Camille Pissarro, the leaders of Impressionism, did painting en plein air become more popular. This change came about from 1881, when Monet, in his efforts to capture the true effects of light on the colour of landscape at any given moment, began to carry several canvases at once into the out-of-doors. On each he began a painting of the same subject at a different time of day; on subsequent days, he continued to work on each canvas in succession as the appropriate light appeared." Britannica