A red sun at sunset is a fascinating atmospheric phenomenon that can have several different causes. In general, it occurs when the sun’s light passes through more of the earth’s atmosphere near the horizon, scattering blue light and allowing more red light to reach the viewer. A red sunset may indicate high humidity, dust particles, pollution, or smoke in the air which preferentially scatters blue and green light, leaving more long wavelength red light to give the sun its dramatic crimson hue. Let’s explore some of the main factors that can lead to remarkably red sunsets.
Dust and Pollution
One of the most common causes of vivid red sunsets is the presence of small particles like dust, soot, ash, and industrial pollution in the atmosphere. These aerosol particles increase the amount of light that gets scattered as direct sunlight passes sideways through the atmosphere near sunset. Shorter wavelength blue and green light tend to bounce off the particles more, while longer red and orange wavelengths travel better through dusty air. This process, called Mie scattering, results in more vivid reds and oranges reaching the observer, intensifying the red colors of the setting sun.
High levels of air pollution, from sources like factories, fires, and automobile exhaust can dramatically increase the number of particles in the air, leading to extremely red sunsets. Major volcanic eruptions that eject large amounts of ash into the upper atmosphere are also known for creating memorable blood-red sunsets noticeable across entire continents. The Krakatoa eruption of 1883 and Mount Pinatubo in 1991 produced such striking global sunsets for months or even years after the initial eruption.
Moisture and Humidity
Increased moisture and humidity in the air can also intensify the red colors of sunsets. Water vapor molecules in the atmosphere scatter light efficiently, especially at shorter blue and green wavelengths. With more water vapor near the horizon as temperature decreases, sunlight passing at a shallow angle encounters more scattering of blue light, leaving relatively more red light to beam through.
Humid conditions like those over tropical oceans or during summer weather over land create the ideal environment for vivid red sunsets. Locations known for extremely humid, hazy air such as Hawaii and Singapore regularly experience spectacular red and orange evening skies thanks to all the moisture and suspended water droplets diffusing the sunset light. The humidity of late summer is a major reason for some of the most dramatic sunsets over places like the Southeastern United States.
Cloud Conditions
Certain cloud conditions can also enhance the intensity of red sunsets. Low level clouds with horizontal structure like stratocumulus sheets often produce bright red colors along the cloud bottoms at sunset. These low clouds provide an ideal elevated platform for sunlight to be diffused and scattered as it passes sideways through the atmosphere. Low visibility conditions with high thin clouds covering much of the sky can also lead to remarkable sunset views, as the veil of clouds scatters a significant amount of light.
High cirrus clouds in the upper atmosphere may produce bright red twilights as well, since these icy clouds contain millions of tiny ice crystal prisms. Each prism can split white sunlight into a miniature rainbow spectrum and preferentially scatter blue light, acting together to filter the sun’s rays into vivid reds and oranges. Different combinations of low, middle, and high cloud decks can interact in complex ways to determine the final sunset colors.
Rayleigh Scattering
The fundamental process that makes sunsets red is called Rayleigh scattering. This natural optical phenomenon causes shorter wavelengths of sunlight like blue and violet to scatter more easily in the atmosphere than longer red and orange wavelengths.
When the sun is high overhead, sunlight passes through less air mass and minimal scattering so the sun appears white. But near sunrise and sunset, sunlight passes through more atmosphere and the blue components scatter away, leaving the longer “redder” wavelengths to come straight through to the observer. This selective scattering of short wavelengths happens due to the size of air molecules relative to light wavelengths. The effect intensifies as the sun nears the horizon, producing those dramatic orange-red sunsets.
Conclusion
Vibrant red sunsets result from a combination of all these atmospheric effects. Dust and humidity cause extra diffusion and preferential scattering of short blue wavelengths. Cloud conditions add reflection platforms to scatter more light. And Rayleigh scattering continually filters out blue components as sunlight passes obliquely through greater atmospheric depth near the horizon. Next time you witness a spectacular sunset, you can appreciate how the interplay of all these factors creates a truly stunning sight of nature.
