What Color is Your Sky?

Science Project:  What Color is Your Sky? A dust storm in West Texas transforms the sky from blue to brown.

Image courtesy Forrest M. Mims III
Web Id: P12

Have you looked at the sky recently? If so, what color was it? If you don’t know, you’re not alone, for it’s common for people not to notice the sky, much less its color. This activity will explain why the sky is not always blue. It will also provide some ideas for science projects that will give you a better appreciation of the sky.

Age Range: 11 to adult
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.

The GLOBE Program has developed a general classification guide for sky color:

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.

Project Links:

The GLOBE Program has a very nice protocol for
Observing Visibility and Sky Color.

Newton’s Apple has a great video on Why is the sky blue?

Newton’s Apple also has a nice
activity page on sky color.

NASA’s Space Place has a page on Why the sky is blue

Sun and Sky includes a variety of unusual sky color photographs.

For practical information about color blindness, check out this excellent
site by two colorblind scientists

Real Time Data Source:

MODIS Rapid Response
LANDSAT Image Viewer


Aerosols and Carbon Monoxide

Project Ideas:

1. Make a Sky Photo Calendar. A digital camera is ideal for making a daily record of sky color. A photographic record of sky color has real scientific value, especially if it covers a long period of time.

For best results, each day photograph the same area of the sky at about the same time. It’s best to make the photographs looking away from the sun. If you live in the northern hemisphere and make the photos around noon, the best region of the sky to photograph is the sky over the north horizon. If possible, be sure to include a strip of ground with some trees or other vegetation in each photo. If you live in the temperate latitudes, this will allow you to keep track of leaf fall in autumn, snow during winter and leaf arrival during spring. take vertical photos to capture a larger slice of the sky.

When you save the photos in a computer, assign each one with its date. For example, a camera might assign the file name of a photo made on June 22, 2007, as IMG1642. Change the name of the file to: 06242007 IMG1642. (In a Microsoft environment, place the cursor over the photo’s file name and right click the mouse. A window will open, and near the bottom will be the ‘Rename’ feature. Left click on ‘Rename.’ The name of the file will now be on a blue field. Place the cursor at the left side of the blue field, left click the mouse and type the new name in front of the name assigned by the camera. Leave a space between the new name and the camera-assigned name.)

Save all your sky photos in a folder labeled with the year you made them. After a month or so, you can fill the screen of your computer with a display of daily sky photos. You don’t need special software to make a calendar, but various calendar programs are widely available. You can paste your photos into daily cells in these calendars to display your photos in an authentic sky calendar. The sky calendar will reveal much about air pollution and haze–if they are present. Be sure to photograph contrails and unusual clouds. If you take photos on all days, even those when it is overcast and no sky is visible through the clouds, your calendar will be more complete.

Tip: Try to use a camera with a time stamp feature that places the date and time on each photo.
Related Picture Post Project

2. Make an Artificial Sky. Pour 1-2 inches of water into a clear plastic or glass tray to form a clean, artificial sky. Place the tray on a dark surface in a partially darkened room. Shine the beam from a flashlight through the side of the container and straight into the water. The light will not form a distinct pattern in the water. Next, place a teaspoon of milk in the tray and stir gently. Now when you shine the flashlight through the water its beam should be visible. You should also notice that the beam of light has a bluish tint near the flashlight. The molecules of milk scatter the blue color of the flashlight beam better than the red color, just as the molecules in the sky scatter the blue color of sunlight.

You can easily prove that the blue light in the flashlight beam is scattered more than the red light by following the beam of the flashlight as it travels through the milky water. The beam begins with a bluish tint. Farther away, the beam looks orange. If you look directly through the tray of water toward the flashlight, the light will be deep orange or even red, much like the sun looks when it sets.

You can use a white card to follow the beam from the flashlight side of the tray to the opposite side. Insert the card into the water in the tray near the flashlight. The beam will form a white spot on the card. As you move the card away, the spot of light will become bluish. As you continue to move the card away from the flashlight, the spot of light will become orange or brownish orange.

Tip: An older flashlight with an incandescent bulb works better than a newer flashlight with a light-emitting diode (LED). This is because the LED emits much more blue than red.

3. Capture Photos of Unusual Sky Events. The color of the sky can be greatly altered by major air pollution events, dust storms and smoke from agricultural fires and forest fires. Considerable dust and smoke from East Asia often reaches the Northern Hemisphere in the spring. Dust from North Africa’s Sahara Desert often arrives in the Northern Hemisphere during late summer and fall. Dust can fade a blue sky and give it a yellowish or brownish tint. Smoke can cause the sky to appear gray or dark gray. These color changes are sometimes much more obvious near the horizon. Sometimes the layers of dust or smoke are so thick that they completely block the sun before it sets. Photographs of the sky during these events will have scientific value.

4. Learn How the Sky Affects Shadows. Shadows require a direct source of light. The sky controls how much sunlight arrives at the ground directly from the sun and how much is scattered toward the ground by molecules and particles in the sky. Therefore, the sky controls how shadows appear. On a day when the sky is clear and deep blue, shadows appear dark and very sharply outlined. This is because most sunlight arrives in a direct path from the sun. On a hazy day, shadows are fuzzy and not as dark. This is because tiny particles and droplets in the sky scatter some of the sunlight so that it arrives from many different directions. Shadows disappear entirely when clouds block the sun.

You can see how the sky affects shadows by looking at your shadow on days when the sky is clear and blue and on days when the sky is hazy. Your shadow will be very dark and sharply outlined on clear, blue days. It will be lighter and have a fuzzy outline on hazy days.

You can get a better idea about how haze affects shadows with the help of a narrow, pointed object like a pencil. Hold the pencil so that the shadow of its point falls on a white card or piece of paper. On a clear, blue day, the shadow of the point will be very sharp and distinct. On a hazy day the shadow will be more gray than black. The outline of the point of the pencil or pen will be fuzzy.

Note: Mention of commercial products does not imply that they are endorsed by NASA.

Analysis Ideas:

Using your Sky Calendar or records of Unusual Sky events, think about
how the sky has changed over a week or a month’s time. What are some
factors that could explain these changes? Can you pinpoint the cause
of the unusual events? Are there multiple different causes for these?
Do they result in differences in the sky?

Related Projects:



1. A grayish-colored sky might indicate the presence of what?

2. A yellowish or brownish-colored sky might indicate the presence of what?

3. Air pollution can be (a) gases and vapors, (b) mists and droplets, (c) tiny particles, (d) all the above.

4. Who explained why a clean sky is blue?

Going Further:

An ideal way to share sky photos is through a web site. You can use a group web site, but for best results, try to use a school or personal site. The photos can be displayed in sequence down the left side of the page with descriptions to the right side of each photo. Or display the photos in rows with brief descriptions under each one.

A sky photo web site is an ideal science fair project. It will also contribute to the scientific understanding of pollution events over your area. To better understand your results you can consult forecasts about dust, smoke and sulfate (from burning coal) over your region. See the Naval Research Laboratory’s Aerosol Page. Scroll down to the second box of products labeled Aerosol Modeling. You can then select forecasts for many regions around the earth. You can select the current forecast or animated forecasts for the next 120 hours. Like weather forecasts, aerosol forecasts are not 100 per cent accurate. But they often provide a very good idea of what is influencing the color of the sky.

Project ideas contributed by Forrest M. Mims III