A red sun, a striking and often beautiful atmospheric phenomenon, captivates observers with its intense hues. This natural occurrence transforms the familiar solar disk into shades of orange, deep red, or even purple. The science behind this vibrant display involves complex interactions between sunlight and Earth’s atmosphere, primarily influenced by light scattering.
How Light Interacts with the Atmosphere
Sunlight, which appears white, is composed of various colors, each with a different wavelength. As this light enters Earth’s atmosphere, it encounters tiny molecules of gases, primarily nitrogen and oxygen. These molecules are much smaller than the wavelengths of visible light, leading to a process called Rayleigh scattering. This phenomenon, named after British physicist Lord Rayleigh, explains why the sky appears blue.
Shorter wavelengths of light, such as blue and violet, are scattered more efficiently and in all directions by these small atmospheric molecules. In contrast, longer wavelengths, like red and orange, pass through the atmosphere with less scattering. Our eyes are more sensitive to blue light than violet, which is why we perceive the sky as blue during the day.
Atmospheric Particles and Light Scattering
When the atmosphere contains larger particles, the interaction with sunlight changes significantly from Rayleigh scattering. These particles, which can be larger than the wavelengths of visible light, include substances like smoke, dust, and water droplets. Unlike small gas molecules that scatter blue light effectively, these larger particles scatter all wavelengths of light more uniformly or allow longer wavelengths to pass through more readily.
The presence of these larger atmospheric particles allows much of the blue and violet light to be scattered away or absorbed before it reaches the observer’s eyes. Consequently, longer wavelengths, specifically reds and oranges, are less affected and continue on a more direct path. This selective removal of shorter wavelengths leaves behind a greater proportion of red and orange light, causing the sun to appear distinctly red or orange. The composition and concentration of these particles determine the sun’s coloration.
Common Events Causing a Red Sun
Several common events introduce the types of atmospheric particles necessary for a red sun phenomenon. During sunrise and sunset, sunlight travels through a considerably greater thickness of Earth’s atmosphere compared to midday. This extended path length means the light encounters more air molecules and particles, leading to increased scattering of blue and violet light. As a result, more of the red and orange wavelengths are left to reach our eyes, creating the familiar warm hues of dawn and dusk.
- Wildfires: Smoke plumes, containing vast numbers of tiny ash and soot particles, can travel thousands of miles and linger in the upper atmosphere. These particles efficiently scatter blue light, allowing red and orange light to penetrate, making the sun appear deep red.
- Dust storms: Large dust storms can inject fine dust particles into the atmosphere, which scatter light, favoring the transmission of longer, redder wavelengths.
- Volcanic eruptions: Volcanic eruptions also play a role by releasing massive amounts of ash and sulfur dioxide into the stratosphere. These aerosols can remain suspended for months or even years, leading to widespread and persistent red sun effects.
- Atmospheric pollution: Fine particulate matter from industrial activities can also contribute, enhancing the red appearance of the sun, especially when combined with other factors.