When the sun appears red, it is not due to any change in the sun itself. This altered appearance is a result of how sunlight interacts with Earth’s atmosphere. The color we perceive depends on the atmospheric conditions between the sun and our eyes.
Understanding Light and the Atmosphere
Sunlight, which appears white to our eyes, is composed of various colors, each with a different wavelength. These colors span the spectrum from violet and blue, which have shorter wavelengths, to red and orange, which have longer wavelengths. As sunlight travels through Earth’s atmosphere, it encounters tiny gas molecules like nitrogen and oxygen.
These molecules cause Rayleigh scattering, where shorter wavelengths of light, such as blue and violet, are scattered more efficiently than longer wavelengths. This scattering explains why the sky typically appears blue; the abundant blue light is dispersed across the atmosphere. Conversely, longer wavelength red and yellow light is scattered less and passes straight through. When the sun is high in the sky, its light travels through a relatively thin layer of atmosphere, allowing most colors to reach our eyes, making the sun appear white or slightly yellow.
Atmospheric Conditions Causing Red Suns
A red sun occurs when sunlight travels through a greater amount of atmosphere or when there are more particles present that scatter light. At sunrise and sunset, the sun’s rays pass through a much thicker section of the atmosphere due to the low angle of the sun. This extended path means more of the blue and violet light is scattered away, leaving primarily the longer wavelength red, orange, and yellow light to reach our eyes. This is why sunsets and sunrises often display vibrant red and orange colors.
Beyond the natural occurrence of sunrise and sunset, various atmospheric particles can intensify this reddening effect. Wildfire smoke, for instance, contains soot, ash, and other organic aerosols. These particles scatter blue and green light more effectively, allowing the red and orange wavelengths to pass through, causing the sun to appear distinctly red.
Dust storms and volcanic ash plumes also introduce a high concentration of particles into the atmosphere. Volcanic eruptions can inject dust and sulfur dioxide into the stratosphere, creating intense red and yellow hues. Air pollution, including smog and aerosols from industrial and vehicle emissions, also contributes to increased scattering of blue light, leading to pronounced red suns.
What a Red Sun Can Indicate
While a red sun at dawn or dusk is a natural and beautiful phenomenon, its appearance at other times of the day, or with unusual intensity, can signal changes in air quality. The presence of smoke from wildfires, dust from storms, or elevated levels of pollution introduces fine particulate matter into the air. These particles can have health implications.
Poor air quality, often indicated by a hazy sky and a red sun, can lead to respiratory issues. Fine particles can irritate the eyes and nose, and for individuals with pre-existing lung or heart conditions, exposure can aggravate their symptoms, making breathing more difficult.
When a red sun is observed outside of typical sunrise or sunset hours, especially if accompanied by a noticeable haze or the smell of smoke, checking local air quality alerts is advisable. These alerts offer information on pollutant levels and suggest precautions, such as limiting outdoor exertion, to protect health.