Nature is, as John F. Carlson said, an overloaded property room, and often as landscape painters we find it challenging to create a painting that has a sense of unity, as opposed to being a composite of parts.
There are many “picture-making” strategies for simplifying and unifying nature into a work of art, such as looking for “big shapes” and creating a coherent range of values.
Carlson admonishes us not to copy Nature “tone for tone” i.e., value for value, and to keep the simplicity of the large masses. But it is Birge Harrison, the author of Landscape Painting (1909) and Carlson’s teacher, who shows us the way to do that: break temperature not value. By using temperature shifts of the same value, rather than value changes, we can add variety and interest to the large shapes. By doing this, those big shapes are not broken up and the large masses and the painting retain a sense of unity.
The technique can be done in a number of ways. Harrison describes one way in which the artist creates a warm underpainting, essentially a full value study and the paints over it, matching tone for tone but using cooler colors in the overprinted layer. Allowing some of the underpainting to show through creates vibration.
Another method would be to paint opaquely and juxtapose warm and cool touches in the same mass.
Here is an example by William Langson Lathrop (1859-1938), a well known painter, etcher and draftsman and one of the founders of the New Hope School of Painting. In the foreground of this painting there are relatively few value changes in the ground plane yet there is a great deal of variety accomplished by the use of warm and cool colors of the same or very close value.
Here is an example by Peder Mork Monsted ( Danish, 1859-1941). If you squint you can see the light and shadow parts of the snow hold together, but when you really look into them you can see numerous temperature changes which he uses to add variety AND to suggest information. The warm and cool notes on the snow bank to the right of the road is a great example of that.
In this second example, we see contemporary landscape painter Clyde Aspevig doing the same thing in the shadow area of the snow and in the sky.
Here is another example by contemporary American landscape painter Eric Merrill. He paints stunning nocturnes of the desert. As you can see from this painting and the greyscale of it, color is used here not only to create variety but actually to create both form and pictorial space.
In this painting by Marc Dalessio, another contemporary American Landscape painter, the street is depicted with both warm and cool colors of the same value creating lively vibration and interest without breaking up the mass.
In all of these examples there are just a few values shifts, but lots of temperature changes.
So, to create variety AND unity: break temperature not value!