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Blue Clouds
We have all looked up at the sky and seen that odd cloud…you know the one, the color is amazing. It could be an hue of red, white, gray, purple, or blue. We are going to look into blue clouds in this article.
The color of a cloud depends on the color of the light that illuminates it. At sunset or sunrise the color of sunlight can be yellow or deep red due to the scattering of the blue component of sunlight as the light travels a longer path through the atmosphere. Blue light is scattered by the atmosphere and reflected by clouds making them appear to be blue.
A very similar process makes the sky blue. First, to understand why the sky is blue, you have to know about the composition of the atmosphere. The atmosphere is mostly gases and a few other molecules. The most common gasses are nitrogen(78%) and oxygen(21%). The remaining 1% is made up of trace gasses, like argon, and water vapor. Let’s not forget the many small solid particles, like dust, soot and ashes, pollen, and salt from the oceans. Next, look at the habits of light waves and the color of light. Light is energy that travels(radiates) in waves. Visible light is the part of the electromagnetic spectrum that our eyes can see. Light from a light source may look white, but it is actually a combination of many colors. Nature is splitting sunlight into its different colors when you see a rainbow in the sky. The colors of the spectrum blend continuously into one another. The colors have different wavelengths, frequencies, and energies. Violet has the shortest wavelength in the visible spectrum. That means it has the highest frequency and energy. Red has the longest wavelength, and lowest frequency and energy.
As light moves through the atmosphere, it goes in a straight line until it bumps into a bit of dust or a gas molecule. Depending on the wave length of the light and the size of the thing that it hits, it gets reflected, in different directions. The different colors of light are all reflected by the particle in the same way. The reflected light appears white because it still contains all of the same colors. On the other hand, gas molecules are smaller than the wavelength of visible light. If light bumps into them, it acts differently. When light hits a gas molecule, some of it may get absorbed. After awhile, the molecule radiates the light in a different direction. The color that is radiated is the same color that was absorbed. The different colors of light are affected differently. All of the colors can be absorbed. But the higher frequencies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering. Rayleigh scattering is the reason that blue clouds are able to appear. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Very little of the red, orange and yellow light is affected by the air; however, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in throughout the cloud.
We have written many articles about clouds for Universe Today. Here’s an article about the different types of clouds, and here’s an article about cloud formation.
If you’d like more info on clouds, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.
We’ve also recorded an episode of Astronomy Cast all about the Atmosphere. Listen here, Episode 151: Atmospheres.
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