The familiar blue daytime sky and deep black night sky are common visual experiences. However, the atmosphere can produce a spectrum of dramatic colors, including intense reds, vibrant pinks, and rare flashes of green or purple. These unusual atmospheric events are governed by specific physical laws and temporary conditions. Understanding how light interacts with air molecules, dust, and water vapor explains the science behind these spectacular and fleeting color displays.
The Physics of Typical Sky Colors
The daytime sky appears blue due to a principle known as Rayleigh scattering, which describes how light is dispersed by particles much smaller than its wavelength. The Earth’s atmosphere is composed primarily of nitrogen and oxygen molecules, which are the perfect size to scatter the shorter wavelengths of visible light most effectively. Since violet and blue light have the shortest wavelengths, they are scattered in all directions across the sky.
The human eye is more sensitive to blue than violet, causing the sky to appear blue. When the sun is high overhead, its light travels the shortest path through the atmosphere, resulting in little scattering. This is why the sun appears nearly white or pale yellow, and the blue light reaching our eyes seems to come from all directions.
Conditions for Purple and Pink Skies
The appearance of purple and pink hues is a phenomenon tied directly to the angle of the sun and the resulting change in the path length of light through the atmosphere. At sunrise or sunset, the sun’s rays must travel a much greater distance to reach an observer, forcing the light through a significantly thicker column of air. This extended path ensures that almost all the short-wavelength blue and green light is scattered away from the direct line of sight.
The remaining light reaching the lower atmosphere is dominated by longer red and orange wavelengths. Pink and purple skies occur when this intense, reddened light illuminates airborne particles or high-altitude clouds. The resulting purple is an optical effect created by the long-wavelength red light from the setting sun mixing with remaining short-wavelength blue light. This composite color is frequently seen illuminating the undersides of clouds or in the direction opposite the sunset.
Extreme Red and Orange Hues
The most dramatic red and orange skies are caused by the presence of larger particles in the atmosphere, often seen during intense sunsets or following major environmental events. This type of light scattering is known as Mie scattering, which occurs when light interacts with particles roughly the same size as the light’s wavelength, such as dust, smoke, or volcanic ash. Unlike Rayleigh scattering, Mie scattering is not strongly dependent on the wavelength of light, meaning it tends to scatter all colors more equally.
When a dense concentration of these larger aerosols is present, they act as an effective filter. They remove virtually all the shorter-wavelength blue and green light from the sun’s path. This leaves only the longest wavelengths—red and orange—to penetrate the atmospheric layer and reach the observer’s eye. Major events like large-scale wildfires or volcanic eruptions inject significant amounts of smoke and aerosols, often leading to exceptionally vivid and prolonged red and orange sky displays.
The Science Behind Green and Other Rare Colors
The “Green Flash” is a rare and fleeting phenomenon, visible briefly as the sun disappears or begins to rise. This event is caused by atmospheric refraction, not scattering, where the atmosphere acts like a weak prism. As sunlight passes through the dense air at a low angle, the atmosphere bends the light, separating the colors of the spectrum.
Blue and green light rays are bent more than red and orange rays, creating offset color images of the sun. The red image sets first, and the blue light is scattered away, leaving the green image visible for a second or two on the sun’s upper rim. Other rare colors emerge from atmospheric emissions, such as the faint green “airglow” caused by excited oxygen atoms releasing photons high in the atmosphere.