En plein air is a French expression which means “in the open air”, and is used to describe the act of painting outdoors. Artists have long painted outdoors, but in the mid-19th century, working in natural light became particularly important to the Barbizon school, Hudson River School, and impressionists.
The popularity of painting “en plen air” increased in the mid 1800s with the introduction of paints in tubes. Previously, painters made their own paints by grinding and mixing dry pigment powders with linseed oil. It was also during this period that the French Box Easel was invented. These developments increased the ease and portability of paint and art supplies, making hikes into the forest and up the hillsides less intimidating and more appealing to those looking to paint new landscapes.
French impressionist painters such as Claude Monet, Camille Pissarro, and Pierre-Auguste Renoir advocated plein air painting, and much of their work was done outdoors in the diffuse light of a large white umbrella.
The act of outdoor painting from observation has endured throughout the 20th century and into the 21st century.