En Plein Air

En plein air (pronounced [ɑ̃ plɛ.n‿ɛʁ]; French for "outdoors"), or plein air painting, is the act of painting outdoors. This method contrasts with studio painting or academic rules that might create a predetermined look. The theory of ‘En plein air’ painting is credited to Pierre-Henri de Valenciennes (1750–1819) first expounded in a treatise entitled Reflections and Advice to a Student on Painting, Particularly on Landscape where he developed the concept of landscape portraiture by which the artist paints directly onto canvas in situ within the landscape. It enabled the artist to better capture the changing details of weather and light. The invention of portable canvases and easels allowed the practice to develop particularly in France and in the early 1830s.
-Wikipedia