A Great Sunset Takes A Few Clouds
Although the twilight sky can certainly inspire awe even when it is devoid of clouds, the most memorable sunsets tend to be those with at least a few clouds. 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. Why? To produce vivid sunset colors, a cloud must be high enough to intercept 'unadulterated' sunlight (i.e., light which has not suffered attenuation and/or color loss by passing through the atmospheric boundary layer. The boundary layer is the layer near the surface which contains most of the atmosphere's dust and haze). This explains why spectacular shades of scarlet, orange and red often grace high level 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. Skies like these are often associated with well-defined mid- and upper-level jet streams (i.e., they mark the zones of transition between large-scale regions of atmospheric ascent and descent). When viewed at sunrise, a sky of this type implies that the weather is likely to deteriorate as the mid- and upper-level moisture continues eastward. At sunset, of course, the opposite is true, hence the saying 'Red sky at night, traveler's delight; Red sky in morning, traveler take warning.' Sunsets like this are perhaps most notable for the 'bathed in red' effect which they produce. The entire landscape takes on a surreal saffron hue as the cloud deck reflects the fading sun's red and orange glow, allowing very little blue (scattered) light from the upper levels of the atmosphere to reach the ground.
