Plein Air Painters have this to say...

Plein air painting is still an interpretation of reality, even though I am seeing the scene first hand. My "truth" is far more interesting to me than the scene in reality. (Linda Blondheim)

To steep oneself in the sky. To capture the tenderness of the clouds. To let the cloud masses float in the background, far off in the gray mist, and then make the blue blaze forth. (Eugene Boudin)

Plein air paintings are life, and without them the rest of my work would die. Without it, I would have nothing to say in the studio, because without real-life experience, art is impossible. (Scott Burdick)

Working outdoors or from life puts you in direct contact with the life force, not just the light and the landscape, but also the vitality of the world around you. (George Carlson)

All pictures painted inside in the studio will never be as good as the things done outside. (Paul Cezanne)

I strive to capture the moment, that fleeting light or atmospheric effect, tackled with a sense of urgency and an awareness that the prevailing conditions are transient and will not be precisely repeated. (Trevor Chamberlain)

I'm constantly painting the landscape in my head as I drive along, and when I see something that refuses to give way to the next scene, I stop the car and turn around. (Gay Falkenberry)

Plein air painting and painting from a reference: The first is like going to Paris for two weeks with your girl friend, the second is like reading a book about Paris at the local library. (Sylvio Gagnon)

After a few plein air attempts, I found the fast and often half finished and underdone pieces were giving me more pleasure than laboriously ended paintings. So I am left with many "unfinished" works done for my soul and not for sale. (George Kubac)

When you're an artist - especially a plein air artist, where you're working outside - you see the best of life all the time. (Tom Nichols)

Artists who battle the elements, the extreme effort and the exhaustive study to be able to paint a high-quality work in one sitting, wet-on-wet while on location, have something very special to offer. (B. Eric Rhoads)

I would not encourage young artists today to paint in retrospect, but rather enjoy painting directly from life, where all the answers are looking right at you. (John Stobart)

Monet, Manet, Sisley, Renoir, Van Gogh and others went outside to paint for one simple reason - it looks different outside. (Mike Svob)

source... Plein Air Painting-Resources for Artists by Jan Blencowe