En plein air (French: 'in the open air') describes painting outdoors, directly observing the subject in natural light rather than from memory or sketches in a studio. The practice became central to Impressionism in the 1860s and 1870s, when Monet, Sisley, Pissarro and others took advantage of portable paint tubes (invented in 1841) to work by rivers, in fields, and on beaches. The portable tube made spontaneous outdoor work possible for the first time — previously, pigments had to be ground fresh and transported in pig bladders. Plein air painting changed what painting could capture: the momentary quality of light, the way shadows shift in minutes, the atmosphere of a specific place at a specific hour. It remains a practice of artists worldwide.