When a thin veil of clouds covers the sky, sunlight is diffused and indirect.
The sky appears light grey or white. It gives a soft overall illumination that lightens all the upward-facing planes. As planes face progressively downward, they get darker. But there’s no sharp demarcation between light and shadow, and there aren’t any definite cast shadows.
Overcast light is ideal for complicated outdoor scenes. It was a favorite with the French academic painters, like Edouard Detaille (1847-1912), above. One of its virtues is that it allows you to paint forms in their true local color with a minimum of modeling (description of form using light and dark values).
Ernest Meissonier (1815-1891) was a leading academic painter who frequently made use of this kind of light. With overcast light his picture is simpler than it would have been with direct sunlight.
For example, here’s a painting by Meissonier in the full sunlight. You can see how the shapes of the cast shadows become new elements that he has to manage in the composition.
Technically speaking, this painting by Meissonier is set in open shade—note the sunny window on the other side of the house—but it’s the same basic effect as overcast light.
I believe Meissonier was a huge inspiration for Howard Pyle (above). Meissonier was the artist that every history painter was talking about during Pyle’s formative years, but he’s still in eclipse among most American art historians, unfortunately.
Pascal-Adolphe-Jean Dagnan-Bouveret (1852-1929) used overcast light to beautiful effect in this painting of Breton women. The light allowed him to reduce modeling. The white headdresses appear as simple silhouettes first.
Dagnan-Bouveret used shape welding in both the light and dark areas to unify the tonal masses even further. Painting a scene with nine major figures and still keeping it simple is very, very difficult to do.
You can survey more work by these artists, some in very large files, at the website artrenewal.org. Links to: Detaille, Meissonier, Pyle, Dagnan-Bouveret.