"En plein air is a French expression which means 'in the open air', and is particularly used to describe the act of painting outdoors."
- Wikipedia

The experience of painting outside, on location, is really an enjoyable one. The shifting light of day and/or from weather pattern changes, keeps a painter attentive. The natural light also reveals colors as they are, which can seem flat when painting from a photograph.

Since you are immersed in your painting's environment, the experiences you have come through in the work. For example, the sensation of painting in the rain, or in the hot sun of a field, or even in an Italian piazza next to a group of old men arguing and laughing while smoking cigars, will influence your work in some way. When the studio begins to feel claustrophobic, going outside to paint is a great way to liven up one's work.