Weather Almanac for October 2008

HOW MANY COLORS HAS THE SKY?

In the opening line of one of Rod McKuen's poems in his album The Sky, he asks the question: "How many colors of blue make up the sky? Seven, ten maybe?" While McKuen's question is more observant in its scope than one of the most commonly-asked questions of meteorologists: Why is the sky blue? (This is often followed by: Then, why is the sky red at sunrise and sunset?), it is way off the mark in its number. The question of "how many?" is, however, both easy and difficult to answer. Theoretically, there should be an infinite number of blue colors as we parse the wavelengths in the blue region of the visible spectrum more and more finely. But the human eye has its limits of discrimination, so the answer is more likely: Too many to count.

However, McKuen's question is also too limited in its choice of hues. In the last month, I have seen photographs taken across North America and Europe showing sky colors, attributed to the eruption of Alaska's Kasatochi volcano containing many more shades and hues in the sky than just blue. So while we and McKuen usually think of the sky as blue — as it generally is — we might legitimately ask: "How many colors can the sky be?" Indeed, the painter often mixes several hues when depicting a sky.

Begin with the Sun

To look for the answer to these and similar questions, we need to begin with our Sun. The nuclear furnaces burning within the Sun produce an average surface heat of about 6000^oC. Now, all objects emit radiant energy across a range of wavelengths depending on their temperature. One of the basic laws of physics (Wien's Law) states that the radiant energy of any object has a characteristic peak wavelength in the electromagnetic radiation spectrum dependent on its radiating temperature. For the Sun, the peak region occurs in the visible wavelengths (so-called because they are visible to our human eyes) between 400 and 700 nanometres (nm where a billion nanometres comprise a metre). These wavelengths encompass the visible colors from violet to red. We humans all radiate heat energy, but do so weakly in the infrared wavelengths (wavelengths above the red wavelengths) which are invisible to each other, though we shine with a light seen by other animals, such as mosquitos.

Solar Light Distribution

When the light leaves the Sun, it traverse the nearly empty expanse through space until it reaches our atmosphere. There, sunlight encounters a much denser mixture of atoms and molecules of air with assorted dust particles suspended within it. A portion of the solar beam reflects right back into space off clouds and the larger suspended particles in the air. Other parts of the beam are absorbed and sent either re-radiated back to space or toward the surface, but a major portion continues its journey through the atmosphere toward the planetary surface.

Through the Atmosphere

During the split-second it takes for the solar beam to penetrate the atmosphere and reach our eyes, various atmospheric constituents have had enough time to affect the path of the incoming solar rays. Many are bumped off the direct path, scattered in the physicist's jargon, and, after many further bumps, continue their downward journey on a different angle from the original solar beam. This scattering is actually comprised of two different scattering processes. One called Rayleigh scattering is more effective at short wavelengths: the blue end of the visible spectrum. The other Mie scattering, which is not very wavelength dependent, produces the almost white coloration we see surrounding the sun.

We find answers to our questions about sky color within the nature of this scattered portion of the solar beam. Scattering treats the wavelengths of the white light solar beam (which contains all the spectral colors) differently, and the effects depend on the size of the matter doing the scattering. Air atoms and molecules, being relatively small compared to the other particles in the atmosphere, scatter the shorter solar wavelengths, the violets and blues, more readily than the longer visible wavelengths, the yellows and reds. The scattering from molecules and very tiny particles (diameters< 1/10 the light's wavelength) is predominantly Rayleigh scattering.

The scattered light rays scatter (hence the term) in all directions while a portion of the direct solar beam continues downward on it original course. As the scattered rays continue on, they are further scattered by other atmospheric constituents so that light rays reach our eyes coming from all directions away from the direct solar beam. These scattered wavelengths are called skylight which is richer in blue due to Rayleigh scattering as the angle away from the direct solar beam increases, separating it from the Mie-scattered portion. (Light reflected off clouds is also included in skylight.)

Meteorologists refer to the volume of atmosphere that solar light rays traverse as the optical air mass. In the cleanest, driest optical air mass, the scattered solar light is predominantly in the blue end of the visible spectrum, although it does contain some wavelengths of all colors. Rayleigh scattering due to air molecules scatters violet light about ten times more efficiently than red light, and blue about five times more efficiently, in the middle of the color's spectral band. As a result, we see an all-encompassing blue sky rather than the deep black sky seen by the Apollo astronauts on the airless Moon. The more air molecules lying between the observer and the incident solar beam, the thicker the optical air mass and, therefore, the greater the light scattering. As a result, the sky appears darker when we ascend a high mountain or fly to high altitudes in an aircraft than it is at the surface. This is because fewer air molecules are between us and the incoming solar beam at altitude to scatter the light around. The direct beam is also brighter at higher altitudes since less of its original brightness has been scattered or absorbed.

Optical air mass (oam) as measured from the zenith (Z) and the horizon (H).
(scale exaggerated)

The angle of the sun as it traverses the heavens from dawn through noon to dusk also becomes a factor in determining the level of scattering. When the sun is directly overhead, the amount of atmosphere the solar beam must traverse is at a relative minimum for the day. But as the sun approaches the horizon, that optical air mass increases dramatically, reaching thirty-eight times longer when the sun is on the horizon than at its zenith. We notice the impact of increasing the optical air mass depth when we compare the sky color around the sun around midday with that at dusk or dawn. With the sun near the horizon, the greater optical depth has scattered away nearly all the blue coloration in the original sunlight, and the sun appears to be from yellow to red in color, as does the surrounding sky. This high level of scattering gives us the fiery reddish sunrise and sunset skies, which can be further enhanced when some of that light reflects off cloud surfaces. In fact, we can generalize a rule for the change in color with optical depth: The more air a light beam must penetrate, the redder it gets.

The colors of twilight.
For more on twilight skies, see the article
Heavenly Shades of Nighttime Falling: It's Twilight Time

The same argument can be applied to seasonal variations outside the equatorial zones. The noon summer sun traverses less atmosphere than the noon winter sun, and is therefore brighter. In the higher latitudes where or when there is a winter noon sun, it often seen as a reddish disk just above the horizon.

The above description assumes the atmosphere consists mainly of the gaseous diatomic molecules of nitrogen and oxygen with a little argon tossed in. Water vapor and other gas molecules may make up a small fraction of the atmospheric gases, but they can have a significant influence on sky color. Water vapour molecules are smaller than the nitrogen and oxygen molecules, but they have a great affinity to cling to or combine with other constituents. For example, if water vapour and sulphur dioxide gas combine, they form sulphuric acid aerosols — larger molecules that scatter the longer wavelength colors. Other larger gaseous molecules may be emitted from vegetation and industrial and transportation sources that also scatter light at the longer wavelengths. As a result, the sum of the scattered wavelengths in the full complement of scattering produces alternate hues to the dominant blues ranging from the murky greys and browns common in polluted urban atmospheres to vivid oranges and reds at low sun angles.

Larger particles than gaseous molecules are also found in the atmosphere as liquids or solids, and often in large concentrations. Wind-blown dust is one such source of particles, as are emissions from human-made sources such as transportation and industry. Violent volcanic explosions, such as Krakatoa and Mount Pinatubo, have hurled dust and gases high into the stratosphere. These dusts and gases remain suspended in the atmosphere for many years and may alter the color the global skies. An atmosphere filled with dust will appear reddish because the scattering and absorption of the shorter wavelengths would only leave the red in the direct and scattered beams. Perhaps some of the myths and folk beliefs related to blood-red skies grew out of the effects of volcanic dusts on sky color. On Mars, as we have seen in the NASA mission photographs from Spirit and Opportunity, children might ask their parents: "Why is the sky red?"

Martian horizon as seen from NASA rover Spirit.
Photo courtesy NASA/JPL

Scattering is not the only process that affects the color of the incoming solar beam. Light rays may also be absorbed by the atoms, molecules, and dusts they strike. By absorbing light in specific wavelengths, these constituents can change the atmosphere's color. An atmosphere thick with smoke, for example, might appear brownish or even black as the smoke particles absorb, reflect and scatter the daylight. When a number of atmospheric constituents scatter and absorb different wavelengths in a different way, the combined result leaves the sky a murky brown or gray.

Clouds, which reflect, scatter, and absorb sunlight, will also alter sky color, as can falling rain and snow. Mie scattering, which is not strongly dependent on the light's wavelength, produces the whitish color we see when viewing the sun through fog or mist. The color of surfaces, natural and human-altered, may tinge the horizon with reflected color. And more and more, the lights, particularly the yellowish mercury lamps, of cities and other large infrastructures alter the sky color. City street and parking lot lighting often casts a colored pall on the underside of low-lying clouds and fogs.

A Final Word: The Artist's View

Pick up any book on painting skies, whether in oil, watercolor, or acrylic, and you will be instructed to paint a lightening gradient of blue starting with the darkest blues at the top of your canvas but from the horizon level, bring up a yellow-brown gradient that will merge with the blue somewhere above the horizon. An example would be to use an ultramarine blue in the blue gradient and a weak raw sienna wash near the horizon.

That reason for painting clear skies this way only came to light in the past few hundred years, though many early artists got it right by painting what they saw. According to the very informative manuscript by Professor Stanley David Gedzelman, The Soul of All Scenery: A History of the Sky in Art, it was the Romans two millennia ago that first truly depicted sky in their murals. (I cannot understand why no publisher has put this book in print. It truly bridges a gap between science and art. On the other side of the coin, their loss is our gain as Gedzelman has kindly posted the complete manuscript on-line in pdf-format. You can download it here.) The first to look at the sky with both an artist's and scientist's eye was likely Leonardo da Vinci. In his Treatise on Painting, he appears to have understood the physics to go with his observations. He explains:

"The air tinges with its own colour more or less in proportion to the quantity of intervening air between it and the eye...."

He also realized that the greater thickness of the air near the horizon resulting in the sky being whiter near the horizon.

Sunset J.M.W. Turner	Hampstead Heath John Constable	Largeness Caspar David Friedrich

Though many great painters and paintings depicted realistic skies before the late-18th century, a full appreciation for the sky itself bloomed with the Romantic painters, notably J.M.W. Turner, John Constable and Caspar David Friedrich. All three were youths during a time when the eruptions of major volcanoes — Laki in Iceland and Asama in Japan in 1783 — altered sky color and increased optical phenomena for several years. These eruptions not only brought the sky to the attention of these future painters, but also had an impact on several major poets (Coleridge, Goethe). They were also active in 1815 when the great eruption of Tambora not only altered the visible nature of the sky but also produced the extremely cold summer of 1816 in many areas of the Northern Hemisphere. (This was also about the time that Howard's work was published for general distribution.)

Indeed, the colorful skies also influenced a young Luke Howard, who would take his fascination into the sciences. At the turn of the century, Howard would devise a system for naming the clouds that is almost unchanged today. His paper on the topic was translated into German and thus read by Johann Wolfgang von Goethe, a poet–philosopher– scientist, who was thoroughly impressed. He, in turn, commissioned Friedrich to do a series of cloud studies to illuminate Howard's naming scheme. Constable also set out to paint a watercolor cloud series. Turner turned his talents into depictions of sky-dominated scenes in a manner that would be called impressionistic or abstract today.

Turner's works brought the full admiration of John Ruskin who produced a five-volume set Modern Painter which Gedzelman noted: "This is the first work after Leonardo's Treatise on Painting to carefully inform painters what to look for when painting the sky." One chapter in Volume I was titled "On the Truth of Skies" another in Volume V, "On the Beauty of Skies."

John Constable, another of Howard's converts, wrote in his Lectures on Landscape Art that the sky is the "keynote, the standard of scale, and the chief organ of sentiment" for a landscape work. Constable's studies of sky have been important resources for both scientists and artists as he wrote detailed meteorological notes on the back of each painting of the conditions when the sketch was rendered.

Toilers of the Sea Albert Pinkham Ryder	Twilight in the Wilderness Frederic Church	The Scream Edvard Munch

Constable's "organ of sentiment" could easily be an organ of emotion in an art work. We know that many striking paintings have been based on the mood engendered by the sky. "Toilers of the Sea" by Albert Pinkham Ryder and Frederic Church's "Twilight in the Wilderness" are two that come to mind, but likely the most famous is Edvard Munch's "The Scream" which he envisioned when enveloped in a blood-red sunset.

McKuen perhaps anticipate more that his initial question stated. In a later line in that poem, he asked "If you look beyond blue, what do you see?"

Learn More From These Relevant Books
Chosen by The Weather Doctor

• Heidorn, Keith C.:And Now...The Weather, 2005, Fifth House, ISBN 1894856651
• Lynch, David K. and Livingston, William: Color and Light in Nature, 2nd edition, 2001, Cambridge University Press; ISBN: 0521775043 (PB).
• Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.

Written by
Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
October 1, 2008

The Weather Doctor's Weather Almanac: How Many Colors Has The Sky?
©2008, Keith C. Heidorn, PhD. All Rights Reserved.
Correspondence may be sent via email to: see@islandnet.com.

For More Weather Doctor articles, go to our Site Map.

I have recently added many of my lifetime collection of photographs and art works to an on-line shop where you can purchase notecards, posters, and greeting cards, etc. of my best images.

To Purchase Notecard, Greeting Cards and Posters featuring my images, visit The Weather Doctor's Nature Gallery

Now Available! Order Today!
Now Available in the US! And Now...The Weather by Keith C. Heidorn To Order in Canada: And Now...The Weather by Keith C. Heidorn	The BC Weather Book: From the Sunshine Coast to Storm Mountain by Keith C. Heidorn Now Available in the US! The BC Weather Book: From the Sunshine Coast to Storm Mountain