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September 2020

September 2020

Cloud at sundown

What is it with sunsets and sunrises? Why do we like them without exception? One reason has to be the colours. At daybreak and day’s end, they gild the world in ruby hues reminiscent of the warmth from a campfire. These colours are because the light from a low Sun reaches us by travelling sidelong through the low, dense atmosphere. Since the atmosphere is denser near to the surface, the light’s journey when the Sun is low takes it through much more dense air than when it reaches us the middle of the day. Since the atmosphere scatters the shorter, blue-looking, wavelengths of the visible spectrum more than the longer, red-looking, ones most of the blue and green light is scattered away en route. Only the yellows, oranges and reds survive the journey, and they bathe the landscape in the magic of the golden hour.

But the colours aren’t the only appeal of these times of day. There is also the special appearance of the clouds. Take these Stratocumulus and Altocumulus at sunset over Jersey, in the Channel Islands (between Britain and France). They were spotted by Annie Queree (Member 17,961), and they show how clouds at sunset and sunrise act as canvases onto which are painted dramatic scenes. The drama comes partly from the angle of the light. This ensures the cloud shadows stretch across the sky. The shadows visible here are cast by tendril-like cloud features known as virga. These are streaks that hang from the undersides of clouds like Altocumulus. They’re falling ice crystals or raindrops that evaporate away in the drier air below, long before reaching the ground. These virga would barely be noticeable in the middle of the day. At sunset, they’re unmissable.

Another reason for the drama of sunsets and sunrises is that these are the only times in the day when clouds are lit from below. So long as the Sun is unobscured with clear skies off on the horizon, its light can shine up onto the cloud bases. This brief reversal of the lighting conditions transforms the clouds. Every subtle contour, every undulation of the undersides is picked out. Clouds that look dull and dark grey in the middle of the day on account of being dense enough to block out much of the sunlight suddenly appear radiant when the sun shines up on them from below.

But mostly we love a sunset or a sunrise surely because we know it can’t last. Within minutes, the Sun will either have climbed above the cloud layer or dipped below the horizon. Either way, that brief transformation of the sky will have passed, and thanks to the ever-changing clouds, it will never be repeated in quite the same way again.

Stratocumulus and Altocumulus with virga spotted over Jersey, Channel Islands by Annie Queree (Member 17,961).

1 Comment
  • Joan Packer avatar

    torchlite

    October 13, 2020at9:19 pm

    Another reason to love sunrise and sunset is simply the time of day. At sunrise, I feel alive, rejuvenated, and eager to greet the blessings of a new day. At sunset, I feel satisfaction and a sense of completeness at a job well done. The colors in the sky enhance these feelings.

    Joan Packer
    Bellingham, WA
    USA

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