Acutance is a term generally applied to photography; I've never seen it used in reference to landscape painting. But the word can be pressed into service in describing the characteristics of the daylight sky as one's attention descends from its zenith to its horizon.
Here is a motif I painted this afternoon. While it doesn't pay to get too formulaic about things, there is one characteristic of a cloudy sky which, in my experience, has never been violated — not by nature, and not by painters seeking to replicate nature. It involves today's magic word. Acutance can be defined as the sharpness of an image, although that doesn't really capture the essence of the word. It's not necessarily sharpness of edge, although that can be involved. Nor is it necessarily a matter of high contrast, although that also can be involved.
Acutance is more a matter of detail in the various values which make up an image. A black and white photograph with high acutance will show a full range of values, rather than generalizing them into one or two values.
Back in the days of silver nitrate emulsions, the film most renowned for its acutance was Eastman Kodak's Panatonic-X. It was terribly slow, with an ASA rating of 40, as opposed to Tri-X Pan's 400. No photojournalist would mess with it. But its acutance was a wonder. You could liken it to what Jimi Hendrix could do with a guitar, hitting twenty notes in between each note on the scale. With Panatonic-X's range of value and crispness, it was the Rolls Royce of black and white film, if you didn't mind having to use very slow shutter speeds or wide apertures to compensate for its slow speed. Not very contrasty, but you could bump up contrast when you printed. Acutance was the stock in trade of the great black and white landscape and portrait photographers: David Vestal, Ansel Adams, and so on. They'd use Panatonic-X, or the midrange Plus-X Pan or, in a pinch, the speedy Tri-X Pan, but holding its 400 ISO rating back a stop to 200 by cutting its developing time to 3/4. Acutance was a commodity weighed against other factors. If you wanted to capture action in a dark-lit situation, the smart move was to open your lens wide, use fast film, and if necessary, give it a few minutes extra in the developer. Whatever acutance strategy you chose, however, it held for the entire image. Having low acutance in one zone and high acutance in another was beyond the range of the material, and its technology. And here lay a key distinction between what can be captured by the photographer, his processor and his printer, and that of which is seen by the human eye, and painted by the human hand. Look at anything with your own two eyes, and particularly any scene which takes in a wide range of vista, and any kind of equality of acutance becomes a pipe dream. In fact, the whole concept of homogenous acutance finds itself at variance to human perception, and to any picture making scheme intended to resemble human perception. Acutance, like seating on buses in the 1950s South, is segregated. Like it or lump it, the rules change the higher in the sky one gazes. You can ignore that reality, on philosophical grounds, and you may be right as a humanitarian, but your pictures will not resemble what we see in the sky. What you choose to observe and record is your own business, but it's hardly a good example of nature depicting homogeneity.
In the sky, acutance is not equally parceled out. The zones of the sky are separate and unequal. The lower middle area of the sky tends to hog the acutance, leaving the rest of the sky a mush. This asymmetry of acutance is one of the salient characteristics of the sky. The painter ignores it at his own peril.
For purposes of discussion, we'll divide the sky into three zones. The upper zone goes from the zenith to maybe 60º above the horizon. Here it is on the photograph:
The upper portion has some contrast, but not a whole lot of acutance. As one looks upward to see the clouds of the upper portion, what is visible is their gray undersides. There is not a great range of value in the clouds viewed from below. The blue of the sky tends to be darker, as you are looking through less atmosphere than one sees in the other two views. On the whole, the upper portion has little acutance and a range of values that hugs the darker side of the sky scale. The blue sky and the gray undersides of clouds are often just about the same value, which makes this area even more nondescript in comparison to what one sees looking lower.
The bottom portion of the sky, from the horizon upward to about twenty degrees, has still less acutance, and much less contrast. Viewing it, one is looking through the maximum quantity of atmosphere; you're seeing clouds, even if they're so distant that you can't make out their shape. The color of clouds and of sky are very close to each other. It's a mush, although some very subtle and beautiful rosy hues can often be found just above the horizon.
But it's in between these two zones, somewhere in between twenty and sixty degrees above the horizon, where the real excitement is.
This middle portion gets the acutance, all of it, which translates into clearly visible cloud shapes and the highest contrast. At these middle levels, you're likely to see one or two clouds catching the full blast of the sun, producing a stark contrast with the cloud's undersides. It is best to remember that even within this middle area, acutance is not equally rationed. One cloud is likely to be the most prominent. Like the queen bee rules the hive, that one cloud rules the sky. Identify it, get its shape and edge right, and let everything else go.
Another view of the same sky, with my easel perched on the hill. That hill covers the lower portion, but you can see the unevenly rationed out acutance. If there are just a couple of clearly defined clouds, lying perhaps 30º above the horizon, this is the characteristic display of acutance in the sky.
Here's a sketch I attempted of the scene and its sky. I should paint a few hundred of these this year, just to get chummier with the sky and its proclivities. But what acutance the painted sky offers is all concentrated in its middle levels. As I said, formulas aren't how pictures are made, but some things just happen all the time, and one ought to take note of them.