A pink sun is a striking atmospheric event. This dramatic color shift from the typical yellow or white signals a significant change in the composition of Earth’s atmosphere between the sun and the observer. The phenomenon is a direct visual indicator that a vast quantity of specific particles has been introduced into the air column.
The Physics of Normal Sunlight
The familiar blue color of the daytime sky results from Rayleigh scattering, which involves the atmosphere’s smallest components. Sunlight, containing all colors of the visible spectrum, travels through the air and encounters tiny gas molecules, primarily nitrogen and oxygen. These molecules are significantly smaller than the wavelength of visible light.
This small particle size causes them to preferentially scatter shorter, higher-energy wavelengths, specifically blue and violet light, in all directions. Longer wavelengths, such as red and orange, are scattered much less and pass more directly to the observer. When the sun is high overhead, it appears yellow or white because a large portion of the blue light has been scattered away.
How Atmospheric Particles Filter Light
The appearance of a pink, orange, or deep red sun is caused by the introduction of much larger particles into the atmosphere, a process described by Mie scattering. Unlike tiny air molecules, these new particles, known as aerosols, are comparable in size to the wavelengths of visible light. Aerosols include dust, smoke, and volcanic ash.
When sunlight encounters these larger particles, the light is scattered more evenly across all wavelengths. However, a dense layer of these larger particles acts as a selective filter, effectively blocking the shorter blue and green wavelengths from reaching the observer’s eye. This filtering allows only the longest wavelengths—the reds, oranges, and pinks—to penetrate the atmosphere and be seen. The pink hue is often a combination of filtered red light and white light scattered by the larger particles.
Sources of Sky-Coloring Aerosols
The large concentration of aerosols required to turn the sun pink originates from major natural events or widespread industrial activity.
Wildfires are a common cause, injecting enormous plumes of smoke, soot, and fine ash particles high into the atmosphere. The smoke can travel thousands of miles from the source, causing vivid sun coloration in distant regions.
Volcanic eruptions are another powerful source, capable of lofting massive amounts of sulfur dioxide and fine silicate ash into the stratosphere, where they can persist for months or even years. Major dust storms, especially those originating from arid regions, also contribute significantly by suspending fine mineral particles. Industrial pollution, in the form of sulfates and nitrates, can create a persistent haze of larger particles that shifts the perceived color of the sun.
Air Quality and Health Implications
The visual spectacle of a pink sun is a warning sign of diminished air quality due to the high concentration of particulate matter. The aerosols responsible for the color change often include fine inhalable particles, specifically PM 2.5 and PM 10. These particles are small enough to be inhaled deep into the lungs.
Exposure to these high concentrations can lead to immediate respiratory irritation, triggering or worsening conditions such as asthma and bronchitis. Prolonged exposure is also linked to more serious health outcomes, including increased risk of cardiovascular events like heart attacks and strokes. When this atmospheric phenomenon occurs, it is prudent to check local air quality indices, which provide data on the concentration of these harmful particulates.