Sunset boat at Key West.
In Key West, Florida, tourists flock to Mallory Square at the end of the day to watch the sun set. Street performers entertain waiting crowds with magic and vendors sell souvenirs of the daily sunset celebration. Flashes click as tourists try to capture the beautiful orange sun as it disappears behind the sparkling blue ocean.
In almost every location around the globe, the sky appears orange at sunrise and sunset. What causes this colorful phenomenon?
Sunlight is composed of a multicolored spectrum, just like a rainbow, explains Jon Nese, senior lecturer in meteorology at Penn State. Combined together, its different wavelengths are perceived as white light when they enter the Earth's atmosphere.
That atmosphere is made up of a mixture of gaseous molecules, mostly nitrogen and oxygen, with some water vapor and trace gases thrown in. These molecules, clumped more densely close to Earth where the atmosphere is thickest, create tiny obstacles for traveling light waves to navigate.
The light at the longest wavelengths—red, orange, and yellow—sails more easily over these atmospheric speed bumps, while the shorter blue and violet rays get bounced left and right as they journey towards us, in a process called "scattering."
At "solar noon," when the sun appears at its highest point in the daytime sky, light reaches us most directly, passing through less atmosphere on the way, reducing the scattering effect. When the spectrum remains together, the light we see is the familiar yellowish-white look of sunshine.
But as the Earth turns during the day and the sun drops toward the horizon, sunbeams enter the atmosphere at a slant and pass through a denser swath of air before they reach us. "When the short rays at the violet and blue end of the spectrum are deflected out in all directions, they can't get to our eyes," Nese notes, "while the orange and red wavelengths dominate our perception of the sky's color."
While people in Key West almost always view orange sunsets, residents in heavily populated cities often see red. That's due to pollution in the air, says Nese. Older residents of Donora, Pennsylvania, a town on the outskirts of Pittsburgh, recall beautiful red sunsets at the height of coke production decades ago, due to the coal dust in the air.
Pollution particles are larger than the molecules of atmospheric gases, Nese explains. Even orange and yellow light waves have a hard time passing through. Red—which is composed of the longest wavelengths in the visible spectrum—is the most successful at streaming past the particles, creating a scarlet sky.
Volcanic activity can produce the same effect. In April of 1982, sunrises and sunsets were fiery red across most of the United States after the El Chichon volcano erupted in Mexico, spewing ash clear into the stratosphere. Red rays were the only visible light rays long enough to slice through the clouds of dense ash and sulfur dioxide.
While science has unlocked the secrets of the sky's many shades, "to many, the scientific explanation is secondary," Nese admits.
"There's something magical, even mysterious, about it because the colors only appear near sunrise and sunset, and few people really understand why."
Jon M. Nese, Ph.D. is senior lecturer in meteorology in the College of Earth and Mineral Sciences. He can be reached at firstname.lastname@example.org.