Back in the 1920s the Bauhaus artists like Johannes Itten and later followers like Faber Birren tried to quantify a recipe for luminosity. Birren would eventually describe the condition in his book ” Principles of Color” as the result of dissolved edges along areas of contrasting or complementary colors with close values. As you can see in the Sanford R. Gifford painting “Kaaterskill Clove”(in the Metropolitan Museum in NYC), the light is diffused and luminous. It owes its luminosity to slow almost edgeless transitions of color and value. The sky moves from a light yellow into a light violet of almost equal value at the horizon. These edgeless transitions of complementary color and close value provide Gifford with his goal, luminosity. In my own painting notice that the clouds at the top of the picture have soft diffused edges and their color is complementary to the cerulean sky. Therefore, the clouds acquire some luminosity and, they are suspended in space in front of sky. At my horizon, the sky dissolves upward but finds a harder and more contrasting edge as it meets the horizon (terra firma). This arrangement pushes the sky back and the ground forward.
Observe that in both paintings a rubbed in burnt siena (iron oxide) base was first applied to the canvas. It can be seen most prominently in the dark foreground thresholds of the two pictures. The standard of a dark threshold has been a fundamental aspect of European landscape painting since the development of the ideal Italian landscape in the 1600s.