Negative space is typically defined as the area in between or around the “positive” elements of a subject. … In landscape, the sky comes closest to behaving in this way, sitting like a vast backdrop behind all the land-based elements. But there are other large spaces … that behave similarly, such as large bodies of water, fields … because they are so large and often uniformly coloured …
Skies can … be activated by using closed negative space. When parts of the subject–such as a tree, a telephone pole, or a rooftop–touch or nearly touch the edge of the painting, it breaks up the negative space into segments. Two or three segments of negative space are visually more interesting than a single space.Mitchell Albala, “The Landscape Painter’s Workbook“, page 70-72
In the painting by Vincent van Gogh below, “Cypresses and Two Women“, Van Gogh hasn’t one large area of sky. He has used the largest of the trees to cut the sky into two sections of different sizes. Notice how the negative space of the sky on the left echoes the shape of the tree adjacent to it, tall and narrow, whilst on the right the negative space of the sky and the shape of the greenery below it are triangular.
The two areas of sky are in turn broken up by the shapes of the clouds, with a different number of clouds on either side of the tree. Notice too how the clouds are angled towards the top of the tallest tree, adding to its sense of height and leading your eye.
Each shape of “blue sky” between the clouds then has variation in colour, along with Van Gogh’s characteristic strong, directional brushmarks.
Don’t be afraid to let elements go off the edges, leaving the viewer’s mind to fill in what’s “cut off”. How different this painting would be if the sky were a uniform colour, and if it were painted around the top of the tree.