Landscape Paintings
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Plein Air Landscape Paintings
Strictly speaking plein air paintings can be complete or used as sketches for another version to be painted in the studio. 
Completed versions should be painted on site with very few adjustments made afterwards in the studio.  They are completed in as few hours as possible before the light changes very much and because of that they are lose and free as the artist is trying to capture the impression of the scene with the correct color notes.  

Studio Landscape Paintings
Studio paintings are preferably created by referencing plein air sketches or photo reference materials.  After years of outdoor painting, convincing color relationships can be accomplished when attempting a realistic representation.