Ambient light is defined as how sunlight is dispersed in the atmosphere and how it affects visibility.
When air is illuminated with light from the sun, it becomes brigther, making it more difficult to get the desired contrast between the atmosphere and the distant object.
As we can see, darker skies allow distant objects to became more evident in the horizon, and this is the effect we have to look for.
This process is very related with the amount of clouds on the different layers in the atmosphere.
When the light from the sun encounters a cloud, it blocks part of it, so the air below the clouds will be darker, and better for distant sights.
Best results happen with middle to high clouds, while low clouds will also block direct visibility with distant objects.
Beyond Horizons 