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What Color is Your Sky?

A dust storm in West Texas transforms the sky from blue to brown.

Time Required: You can learn the basic facts about sky color in half an hour. You can then enhance your knowledge by observing the sky every time you have the opportunity. You can share with family members and friends what you have learned about sky color while looking out a window or walking outside. And you can teach students the basic facts about sky color in two brief sessions of 5 to 10 minutes each. For best results, one session should be indoors and the other outdoors or in a room with a window or windows having an unobstructed view of the sky. For best results, spend time with your students so they can more fully appreciate, understands and discuss the appearance and color of the sky.


As rainbows and prisms demonstrate, sunlight is comprised of the full range of colors from violet to red. When viewed together, the violet, blue, green, yellow and red colors of sunlight appear white. Isaac Newton was the first person to explain this.

Sometimes thick pollution causes the sky to appear white, but a clean sky is blue. Why?

Air is made mainly from molecules of nitrogen and oxygen with a dose of argon, water vapor, carbon dioxide and traces of many other gases. Together, as Lord Rayleigh explained, the molecules of these gases scatter the blue colors of sunlight much more effectively than the green and red colors. Therefore, a clean sky appears blue.

In many places air pollution causes haze that causes the sky to appear pale blue or even milky white. Layers of air pollution can cause the sky over the horizon to appear brown or gray. Air pollution can take many forms. It can be gases and vapors, mists and droplets or tiny particles of carbon or other materials. It can even be all the above!


The color of the sky provides valuable clues about its condition. Learning about the significance of sky color will allow you to make educated guesses about the presence of natural haze and air pollution simply by looking out a window.


The best way to appreciate subtle differences in sky color is to make sky watching a regular activity. This is best done outdoors, but sky watching can also be done through a clean window, preferably when the sun is away from the window.

A deep blue color means a very clean sky. A deep blue sky can occur when a cold front brings in clean, unpolluted air from the north. A deep blue sky can also occur when clean air from over the ocean is pushed over the land.

A medium blue sky suggests there might be plenty of water vapor in the sky. It can also suggest the presence of sulfur from coal burning power plants. In some regions a medium blue or even pale

blue sky can be caused by emissions from plants and trees. The Smoky Mountains of North Carolina and Tennessee are famous for their bluish haze caused by tiny particles in the air formed by terpenes, a chemical emitted by some trees and plants.

A pale or milky white sky suggests the possibility of considerable air pollution in the form of sulfur from coal-burning power plants or certain chemical plants. In some areas this condition occurs mainly in summer when the air is still and pollution accumulates. There are also natural sources of sulfur dioxide, including volcanoes and ocean plankton. Large portions of the island of Hawaii have often been blanketed by a thick layer of hazy sulfur dioxide from the Kilauea Volcano.

Diffuse smoke from forest fires and agricultural burns can cause the sky over the horizon to look gray or dark gray.

Emissions from cars and trucks can cause a layer of brown or brownish orange pollution over the horizon. The major component of this pollution is nitrogen dioxide.

Major volcano eruptions can cause a hazy sky that can last for several years and cause the sky to appear brown.

The reason for a particular sky color is not always obvious, especially when there are multiple reasons. For example. in Texas there have been times when the sky was filled with dust from Africa, smoke from agriculture fires in Texas and neighboring States and sulfur dioxide from local coal-burning power plants.

Deep blue (unusually clear)

Blue (clear)

Light blue (somewhat hazy)

Pale blue (very hazy)

Milky (extremely hazy)

These descriptions are the most common. But severe air pollution, including smoke plumes from agricultural and forest fires and giant volcano eruptions that may occur every 10-20 years, may alter these colors.

Color blindness might alter how some people perceive the colors of the sky. Blue color blindness is very rare, so most color blind males can perceive the various hues of a blue sky. However, some color blind males may not perceive a polluted sky in the same way as children and adults with normal vision. According to Masataka Okabe and Kei Ito, ‘The frequency of colorblindness is fairly high. One in twelve Caucasian (8%), one in 20 Asian (5%), and one in 25 African (4%) males are so-called ‘red-green’ colorblind.’ (From How to make figures and presentations that are friendly to color blind people.)

Caution: Always be safe and alert to your surroundings when sky watching, especially when walking along a sidewalk or trail. The sky scatters much of the sun’s ultraviolet rays, so sky watchers should wear sunglasses if at all possible, especially during the summer. Never look at or near the sun! Instead, watch the sky away from the sun.

The projects that follow might make good science fair projects, so keep that in mind as you try them.

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