As of Saturday, July 12, 2014
With summer weather and clear skies back again, I’ve been thinking about a timeless childhood question: Why is the sky blue?
The short answer is the sky looks blue because of light scattering in the atmosphere. But a longer answer adds important details and is more interesting.
Our local star showers the Earth with a constant flow of energy, in the form of both heat and light. About half this radiation arrives as visible light, moving at over 186,000 miles per second and reaching us in just over 8 minutes. Visible light, the spectrum human eyes can see, travels as electromagnetic waves with wavelengths roughly between 400 and 750 nanometers. Different wavelengths correspond to different colors, from violet blue at the short end of the spectrum to orange red at longer wavelengths.
At least that’s how we and other animals with color vision perceive this light. Specialized cells in the retina of the eye capture light energy, most effectively in the red, green and blue wavelengths, and convert it to electrical signals that get sent to the brain for interpretation. Our eyes evolved in response to light within this spectrum, and when all the wavelengths reach our eyes together we see it as white (or colorless) light.
If our planet lacked an atmosphere, sunlight would travel to our eyes in a straight uninterrupted stream. The sun would look like an extremely bright white star, with the surrounding sky black in all directions.
Fortunately, a thin envelope of protective gases and suspended particles surrounds the Earth. Nitrogen and oxygen make up the vast majority of the atmosphere, along with small amounts of other gases, dust, ash, pollen and even salt from the oceans.
Sunlight enters the atmosphere as white light, with all the wavelengths traveling together. When the light begins to collide and interact with increasing numbers of gas molecules and particles, it gets absorbed, reflected, dispersed and generally bounced in every direction imaginable. But different wavelengths and their corresponding colors aren’t all affected the same way.
Early scientists thought dust and haze in the air split sunlight into separate colors, giving the sky its color. But further study revealed that when white light hits these large particles, it simply bounces off without being changed.
It turns out that when sunlight strikes much smaller particles, not all the light bounces off. Shorter blue wavelengths are temporarily absorbed by oxygen molecules, while the rest of the colors continue on. When this blue light is then released in a different direction (very) shortly afterward, it travels on its own. Your eyes are therefore constantly bombarded from all sides by blue light being scattered by the abundant oxygen in the air, making the sky look blue.
The same effect explains why sunrises and sunsets look red. However, with the sun near the horizon, sunlight has to travel much further through the atmosphere. Most of the blue light gets scattered away before ever reaching our eyes, while longer wavelength red light passes through without interruption. Heavy smoke or dust blocks the blue light’s pathway even more, further highlighting red colors.
Clouds appear white because they consist of closely packed water droplets or ice crystals. All the color wavelengths bounce off these large particles together, reaching our eyes still combined as white light. Gray or black storm clouds result from shadows, when heavy clouds higher in the sky block sunlight from reaching the clouds beneath.
Not only these colors of the sky, but all colors we perceive arise from the properties of light combined with our brain’s interpretation of what reaches the eye. If Earth’s atmosphere was composed of other gases, or our eyes were adapted to sense other wavelengths better, the science would remain the same but the colors we “see” could be different. In fact, this article might be explaining why the sky is violet.
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