The spectacle of clouds displaying pink, orange, and red hues at dawn and dusk is a universally admired atmospheric event. The scientific reason behind these striking displays is rooted entirely in how sunlight interacts with the Earth’s atmosphere at specific angles. Understanding this phenomenon requires knowing the basic physics of how the atmosphere handles light throughout the day.
Understanding Why the Sky is Blue
Sunlight, which appears white to the human eye, is actually composed of all the colors of the visible spectrum, each corresponding to a different wavelength. When this light enters the atmosphere, it encounters tiny gas molecules, primarily nitrogen and oxygen. Since these molecules are much smaller than the wavelength of visible light, they cause a process known as Rayleigh scattering.
This type of scattering affects shorter wavelengths—like blue and violet light—much more strongly than the longer wavelengths, such as red and orange. Blue light is scattered across the entire sky four times more effectively than red light. The scattered blue light reaches our eyes from every direction, which is why the sky appears blue when the sun is high overhead.
The direct light from the sun has much of its blue component removed by this scattering, leaving the sun itself with a slightly yellowish tint. This efficient scattering of short-wavelength light is why blue dominates the sky during the day.
The Physics of Red Light at Sunrise and Sunset
The appearance of red and pink clouds is a direct result of the sun’s position low on the horizon during sunrise and sunset. At these times, the sunlight must travel through a significantly greater thickness of the Earth’s atmosphere before reaching an observer. This extended journey means the light interacts with many more air molecules than it does when the sun is directly overhead.
As the light travels this much longer path, the process of Rayleigh scattering is greatly amplified. Nearly all the shorter-wavelength light—the violet, blue, and green—is scattered away and filtered out from the direct path of the sun’s rays. Only the longest wavelengths, which are the least scattered, successfully penetrate the thick atmospheric layer.
The light that finally reaches the lower atmosphere is therefore dominated by the long-wavelength colors: red, orange, and pink. This atmospheric filtering effectively turns the sun into a source of intensely colored light, rather than a source of white light.
Clouds as Reflective Canvases
The clouds themselves do not generate the color; instead, they act as highly effective surfaces that reflect the already-filtered reddish light. Clouds are composed of water droplets or ice crystals, which are significantly larger than the nitrogen and oxygen molecules responsible for Rayleigh scattering. Because of their size, these particles scatter all wavelengths of light equally, a process described by Mie scattering.
When the red, orange, and pink light from the low sun strikes the clouds, the water droplets scatter this light uniformly. Since all remaining colors are scattered equally, the clouds take on the exact hue of the light hitting them, making them appear pink or red.
The altitude of the clouds also plays a role in the intensity and duration of the color display. High-altitude clouds, such as cirrus clouds, can catch the sun’s filtered light long after the sun has set below the local horizon for the observer.