Painting the Sky

Painting the sky with pivot irrigation in the Buena Vista Grasslands of Portage County, Wisconsin.

Ever stopped to wonder why the sky seems to be painted in streaks of color at sunrise and sunset?  One day, while driving in the Buena Vista Grasslands along Taft Avenue (just south of Plover Wisconsin) I found the answer; farmers paint the sky in the early evening using pivot irrigation.  The paint then gently settles from the sky at night ripening and coloring various crops.  During the day, these same systems are used to irrigate crops.

Pivot irrigation systems are a common sight in the Buena Vista Grasslands of central Wisconsin.  This area is the former bottom of Glacial Lake Wisconsin formed from meltwater of glaciers that extended to just east and north of Stevens Point some 15,000 years ago.  While the lake was being formed, meltwater silt and sand settled to the lake bottom resulting in today’s sandy soil.  As sandy soil does not hold moisture, irrigation has become a necessary part of agriculture.  Overhead high-pressure irrigation is becoming less common as the industry is converting to more efficient low-pressure sprayers hanging beneath the superstructure.

Glacial Lake Wisconsin was formed because meltwater was prevented from draining down the Wisconsin River due to a huge ice dam at the Baraboo Hills.  When the ice dam eventually failed, Glacial Lake Wisconsin drained over several days forming Wisconsin Dells.

Of course, nature also plays an important role in coloring the sky.  Unless we are looking directly at the Sun, we only see objects and perceive colors based on the reflecting or scattering of light.  If there is no scattering or reflecting of light, we do not see it.

Unadulterated sunlight is white composed of the rainbow family of colors. We can see the constituent colors of sunlight when their various wavelengths are differentially scattered while passing through a prism. Reds are scattered the least while blues the most. The result is the familiar rainbow distribution of colors.
Air molecules in our atmosphere are small, about the size of the wavelength blue light. Sunlight streaming through air molecules preferentially scatter blue light significantly more than other wavelengths allowing red, orange and yellow light to continue close to their original path relatively unscattered. It is the scattered blue light that reaches our eyes giving the sky it’s perceived blue color during the daytime and the yellows, oranges and reds of sunrise/sunset.
Clouds are composed of simple and complex molecules that on average scatter all wavelengths of light in every direction. Their perceived color matches the color of light illuminating them; white during the day, a combination of colors at sunrise/sunset. Daytime clouds that are thick or dense will also display various shades of grey.
Can you guess what the color of the sky would be on the Moon? There is no Lunar atmosphere so there are no molecules to scatter sunlight or redirect the light to our eyes. The sky would be black. Yes, you would still see faint light of distant stars and you would be able to see the surface of the Moon as sunlight is reflected off it. Of interest, you don’t see stars in photographs taken from the Moon (such as this NASA image of Buzz Aldrin during the Apollo 11 mission) as reflected but unscattered light off the landscape is so bright, the very dim light of distant stars is not bright enough to be visible when the film is exposed to image the Lunar surface.

I find it fascinating to contemplate everyone on Earth who sees sunlight at a moment in time, see the same sun beam at virtually the same instant. How we perceive the light is dependent on what types of particles it encounters on the  path to our eyes. 

Our atmosphere preferentially scatters blue light. The more atmosphere a ray passes through, the more blue light is scattered allowing red, orange and yellow light to continue close to their original path. The same light beam colors the sky blue when the Sun is high in the sky but increasingly red where the Sun appears lower.

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