The sky is a spectrum of colors—most commonly blue, white, or gray—determined by how sunlight interacts with Earth’s atmosphere. The color we perceive is a direct result of atmospheric optics, specifically the scattering of light waves by gases and particles suspended in the air. This process transforms the sun’s white light into the familiar canvas of the sky.
Why the Sky is Typically Blue
The deep blue color of a clear daytime sky is governed by Rayleigh scattering. This process involves sunlight colliding with the atmosphere’s smallest components, primarily nitrogen and oxygen molecules. Because these molecules are much smaller than the wavelength of visible light, they scatter the shorter, higher-energy wavelengths—blue and violet—far more effectively than longer, lower-energy wavelengths like red.
This preferential scattering diffuses blue light across the entire sky, making it appear blue when looking away from the sun. Although violet light is scattered the most due to its shorter wavelength, the sky does not appear violet for two main reasons. The sun emits slightly less violet light than blue light, and the human eye’s sensitivity peaks in the blue and green regions. This visual limitation, coupled with the dominance of blue light, results in the sky’s characteristic azure hue.
How Atmospheric Conditions Change Sky Color
The sky’s color shifts drastically when sunlight travels a greater distance through the atmosphere, such as at sunrise and sunset. When the sun is near the horizon, its light passes through a path of air up to 11 times longer than when it is directly overhead. This extended journey means that nearly all the short-wavelength light—the blues and greens—is scattered away before reaching the observer.
The remaining light that penetrates this thick atmospheric layer consists mainly of the longer wavelengths: reds, oranges, and yellows. This results in the warm, saturated colors that dominate the sky at sunrise and sunset. When the atmosphere contains larger particles, such as water droplets in clouds or pollution, a different process called Mie scattering takes over. Because these particles are larger than the light’s wavelength, they scatter all colors of light almost equally. When all wavelengths are scattered uniformly, the combined light appears white, which is why clouds and hazy skies often have a white or gray appearance.
Specific Phenomena That Display Green Hues
Although a general green sky is not typical, a few rare atmospheric events display green light. One such phenomenon is the Green Flash, a brief optical effect occurring for only a second or two as the sun disappears below a clear horizon. The atmosphere acts like a prism, separating the sun’s light into distinct colors. Refraction then bends the green light to the observer’s eye just after the red and yellow components have dropped below the horizon.
A green hue can sometimes be seen beneath severe thunderstorms, particularly those carrying large amounts of water and hail. This color results from a specific light interaction within the dense cloud structure. When the reddish-yellow light of a low sun illuminates a massive storm cloud scattering blue light, the combination produces a faint, eerie green tint. A different source of green is the aurora borealis, where the most common color is a bright green. This light is emitted at a specific wavelength of 557.7 nanometers when charged particles from the sun collide with excited oxygen atoms at altitudes between 100 and 150 kilometers.