Sunsets are very dramatic and often highly anticipated. With their vibrant oranges, yellows and reds, they tint the clouds and light up the sky to the delight of romancing couples and photographers alike. These stunning light displays may make you wonder what exactly makes them so colorful. What causes a pink sunset, and why are sunsets different colors sometimes? It comes down to a mixture of natural and human-derived phenomena.
A Scattered Approach to Creating a Sunset
Sunsets are caused by scattered light waves breaking up sunlight in the atmosphere. Light from the sun, though white in appearance, is actually made of up of all colors of the spectrum — from red, which has a longer wavelength, to shorter-wavelength colors like blue and violet.
During the day, particles in the Earth’s atmosphere — such as water vapor, dust and ash, for example — mostly reflect back short-wavelength blue light from the sun’s spectrum. As Vox notes, this is what causes the blue color of sky during the day. Molecules of nitrogen and oxygen in the air reflect the shorter wavelength blue and violet light more than any other color to form the familiar blue dome.
However, at the end of the day, the angle of the sun relative to the horizon forces its light to pass through a greater distance of atmosphere than it does when the sun is overhead. Due to a phenomenon known as Rayleigh Scattering, this means that primarily red, yellow and orange light — which all have longer wavelengths — will get through to light up any clouds.
What Causes a Pink Sunset?
Similar to a rainbow, sunlight contains a full spectrum of colors. However, unlike rainbows — which the Science Education Center of UCAR notes are caused by the refraction of sunlight in water droplets — sunset colors come from light being scattered or reflected as it passes through our atmosphere.
According to Earth Eclipse, the pink color comes from mixing the red part of the spectrum with additional white light. This happens when there are more aerosols or fine particulate particles in the air to scatter and reflect the sunlight spectrum. Shorter wavelengths of light such as blue and violet don’t get through as much as the longer-wavelength reds and orange, which are in turn reflected and scattered more.
Why Are Sunsets Different Colors?
Sunsets form with different color palates each evening, and these reflect the atmospheric conditions that the sunlight must pass through to reach our eyes. These conditions also affect the intensity of the colors seen. Since the Rayleigh Scattering depends on aerosols in the atmosphere, conditions that alter these fine particles will impact the sunset you see each evening.
For example, sunsets are much more intense in winter months. Not only does the sun take longer to set, but the atmosphere contains less water vapor. When humidity is low, aerosol particles have fewer water droplets to interact with, and they don’t interact as much with the sunlight passing through. With less reflection, the sunlight spectra are less diffused, and the colors are purer, appearing more vibrant.
Another reason for why sunsets are different colors? The level of pollution in the atmosphere. This can be man-made from burning fossil fuels, or from natural events such as hurricanes or even volcanic action throwing dust plumes into the sky. With each of these, sunsets take on a more blue or purple appearance. Grist points out that the effects of volcanic activity in the past is often seen in older art. Industrial smog results in sunsets of dark red, purple, or pink.
Atmospheric composition also explains why sunsets seen on other worlds are different. Live Science notes that due to quantities of iron-rich dust found in the Martian atmosphere — which is around 1% the density of that on Earth — sunsets appear blue. On Uranus, with hydrogen, helium and methane in abundance, sunset is blue or turquoise. And on Titan, it is yellow, orange or brown due to this moon’s atmospheric composition.
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