This reduces the amount of light which reaches the ground, robbing sunrises and sunsets of brilliance and intensity. Thus, twilight colors at the surface on dusty or hazy days tend to be muted and subdued, even though purer oranges and reds persist in the cleaner air aloft. When the pollution layer is shallow, a similar effect is sometimes evident at the surface, as shown in Figure 2. The photographs show a sheet of altocumulus erupting into a blaze of color once the sun has sunk low enough that it no longer directly illuminates the thin veil of haze below the clouds. The haze shows up as a dark layer along the horizon in the second view.
Clouds catch the last red-orange rays of the setting sun and the first light of the dawn.
But certain types of clouds are more closely associated with eye-catching sunsets than others. This explains why spectacular shades of scarlet, orange and red often grace cirrus and altocumulus layers, but only rarely low clouds such as stratus or stratocumulus. When low clouds do take on vivid hues, it is a clue that the lower atmosphere is very clean and therefore more transparent than usual. Some of the most beautiful sunrises and sunsets feature a solid deck of middle or high clouds that covers the entire sky except for a narrow strip near the horizon. An example is shown in Figure 4.