Pink sunsets occur when the light from the setting sun passes through more atmosphere than usual before reaching your eyes. This causes the shorter wavelengths of light to be scattered away, leaving more of the longer wavelengths of red and pink light to reach your eyes. There are a few different factors that can lead to more dramatic pink sunsets:
More particulates in the atmosphere
Particles like dust, smoke, pollution, and moisture in the air can increase the scattering of light. This filters out more of the shorter blue/green wavelengths and allows more pink light through. Things like dust storms, wildfire smoke, volcanic ash, and air pollution will enhance those vibrant pinks in the sunset sky.
The angle of the sun
When the sun is lower on the horizon, its light has to travel through more atmosphere to reach your eyes. This leads to more scattering and filtering of the shorter blue light, bringing out those reds and pinks. Sunsets during winter often appear more pink, as the sun angles lower across the sky compared to summer.
Cloud cover and composition
Clouds can also enhance those dramatic pink sunsets. High cirrus clouds are made of ice crystals that effectively scatter light. Lower clouds like altocumulus are made of water droplets that also scatter sunset light. Having some clouds strategically placed near the setting sun can amplify its pink shades.
Why does light scattering cause pink sunsets?
To understand why light scattering leads to pink sunsets, we need to know a bit about the visible light spectrum. Daylight appears white to our eyes, but it is actually made up of a spectrum of colored light waves of different wavelengths. Violet and blue light have the shortest wavelengths, while red and orange light waves are longer.
Shorter wavelength violet and blue light are more easily scattered by particles in the air. This scattered light appears blue across the rest of the daytime sky. As sunlight passes through more atmosphere near sunset, more of that violet and blue light is scattered away. This leaves the longer wavelengths of red and orange to come through, creating those vivid pink and orange hues.
Other factors that enhance pink sunsets
In addition to increased scattering, there are some other atmospheric factors that can enhance those brilliant pink sunsets:
Ozone gas in the upper atmosphere absorbs more of the violet and blue light waves. Higher ozone levels block more of that short wavelength light, allowing more pink, orange, and red light to shine through.
Airborne dust and water droplets
Tiny particles in the air, from sources like dust, smoke, smog, humidity, and cloud droplets, increase scattering of light. The more particles in the atmosphere near sunset, the pinker the sky appears.
Normally temperature decreases with height in the lower atmosphere. Inversions happen when this is flipped, and cooler air is trapped below a layer of warmer air. This acts like a lid, trapping more particles and enhancing scattering.
Geography and surface features
Areas surrounded by mountains can also see enhanced pink sunsets. The sloping terrain means sunlight interacts with more atmosphere and particles near the surface at sunset. Proximity to bodies of water, like oceans and large lakes, can also amplify scattering from water vapor.
Famous examples of remarkably pink sunsets
Some exceptionally vivid pink sunsets have made history. Here are a few interesting examples:
The Pink Sky of England, September 1950
In September 1950, strong red sunsets blazed across the country for nearly a month. It was theorized to be caused by smoke particulates still trapped in the atmosphere after major forest fires in Canada earlier that year.
Krakatoa Volcanic Eruption, 1883
One of the most dramatic pink sky events followed the enormous Krakatoa volcanic eruption in Indonesia in 1883.Ash and sulfuric gases spread around the globe, causing vivid red and orange sunsets for 3+ years afterwards.
Mount Pinatubo Eruption, 1991
The powerful eruption of Mount Pinatubo in the Philippines in 1991 injected massive amounts of sulfur dioxide gas into the stratosphere. This led to exceptionally colorful sunsets and sunrises around the world for about 3 years.
California Wildfires, 2020
Intense wildfires in California in 2020 lofted huge plumes of smoke and haze into the atmosphere. This yielded remarkable orange and pink sunsets viewable across North America.
In summary, pink sunsets are caused by increased scattering of shorter wavelength violet and blue light in the atmosphere near sunset or sunrise. Particulates like dust and smoke, clouds, ozone concentration, temperature inversions, and geography can all contribute to enhanced violet/blue light scattering and brilliant pink skies at dusk and dawn. The next time you see a dramatic pink sunset, you can appreciate the interesting atmospheric physics that produce this colorful phenomenon.