When a child colors a picture of the sky, the sky is blue and the sun is yellow. Why isn’t the sky green and the sun purple?
The sky appears more blue in the mountains because there are fewer aerosols to scatter white light
The color of the sky is determined by what is in the atmosphere. Without an atmosphere the sky would be black, like what astronauts see from the moon.
Sunlight consists of a broad spectrum of electromagnetic radiation traveling at different wavelengths. We see the visible light portion of the spectrum (violet, indigo, blue, green, yellow, orange and red). The sunlight travels in a straight line until something reflects it (mirror), bends it (prism) or scatters it (gas molecule).
The atmosphere is filled with different components, such as gas molecules, water droplets and dust, and each component scatters different wavelengths of light depending on the component’s size.
Our eyes perceive clouds as white because water droplets in clouds scatter white light
Aerosols (such as soot, water droplets and dust) are larger components in the atmosphere and scatter all wavelengths of light. Gas molecules are smaller and scatter shorter wavelengths and blue light has a short wavelength. When blue light is scattered it goes in all directions and that is the light that strikes our eyes.
Other colors of light pass through the atmosphere without scattering and aren’t visible until they reflect off an object. Then we see the yellow daffodils, green grass and purple plums.
If blue light scatters in all directions, why isn’t the sky uniformly blue on a clear day? You may have noticed that closer to the horizon the sky is a lighter blue or even whitish.
The sky near the horizon often appears a lighter blue
When the sun is directly overhead, sunlight passes through the least amount of atmosphere. The more atmosphere sunlight passes through, the more chances there are for the light to be scattered and rescattered in all directions. This scattering mixes the scattered colors together again so we see more white and less blue.
The same reason applies to why clouds are white and hazy days are whitish. On cloudy or hazy days, there are more aerosols in the atmosphere (such as water droplets) and these aerosols scatter all wavelengths of light, causing them scatter, rescatter and mix back together.
Scattering also influences the color of sunsets. Since the sun is low on the horizon, sunlight has to travel a greater distance to reach our eyes. This extra distance provides more opportunities for blue light to scatter in every direction. So much scattering happens that red and yellow light is all that is left to reach our eyes.
Light traveling a longer distance causes sunsets to be red, orange and yellow
Sometimes the whole western sky appears red, especially if it is smoky from a forest fire, because large particles (like soot) reflect and scatter more of the red and yellow light. If there are fewer particles in the atmosphere, the sunset will be more yellow or orange.
After the sun sets, the sky sometimes turns a purple hue. The purple hue is a result of red light reflecting off aerosols higher in the atmosphere and mixing with scattered blue light to reach our eyes as purple.
However, during and right after sunset, the entire sky doesn’t turn red, orange or yellow–the sky overhead is still blue. The sky is still blue because of the ozone. Ozone absorbs the longer wavelengths of light (red, yellow and orange) and leaves the shorter wavelengths (blue). At sunset the light travels its longest distance in the atmosphere and the ozone filters out most of the wavelengths, leaving the sky blue overhead.
The color of the sky can change color for a variety of reasons. But we can be certain the sky will be blue on a clear day and the sun yellow, just as kids depict it.
