Oil on Canvas Panel
8 x 10 in/20.3 25.4 cm
To me, it's the glue, so to speak, that holds the landscape together. It allows the painter to render the light correctly for the conditions he or she is trying to portray.
Most importantly, it adds to the mood and makes the painting believable. You achieve it by giving depth with a foreground, a middle ground, and by showing the distance somewhat bluer and less distinct. You can also show atmosphere by painting lost and found edges, which gives the illusion of moisture in the atmosphere.
Humans can't live without oxygen, and landscapes can't live without atmosphere.