Shimmering, colorful clouds are a natural optical phenomenon often resembling oil slicks on water. Understanding this spectacle requires knowing the specific name for the event and the atmospheric physics that creates the colors. The scientific explanation reveals that the colors result from how sunlight interacts with microscopic particles suspended within the clouds.
Identifying Cloud Iridescence
The most common name for these colorful displays is cloud iridescence, or simply iridescent clouds, a term derived from Iris, the Greek goddess of the rainbow. This phenomenon is characterized by patches of color that appear in the general vicinity of the Sun or Moon, looking like soft, pastel shades or sometimes intensely vivid, mingled tones. Unlike the organized arc of a rainbow, the colors of iridescence are patchy and appear to follow the contours of the cloud itself.
Iridescence is most often observed in high-altitude, thin clouds that are composed of small water droplets or ice crystals. Cloud types such as Altocumulus, Cirrocumulus, and Lenticular clouds frequently exhibit this effect, especially along their thinner edges or when they are newly forming. The cloud must be optically thin, meaning the light only encounters a single layer of particles, allowing the colors to be seen clearly. To view the colors most distinctly, it is often necessary to shield the bright light source, such as the Sun, by hiding it behind a building or using a hand.
The Physics Behind the Colors
The mechanism responsible for cloud iridescence is called diffraction, which is the bending of light waves as they pass close to the edges of tiny particles. Iridescence is a direct result of light waves bending around the microscopic water droplets or small ice crystals within the cloud. This bending causes the light waves to spread out and overlap, a process known as interference.
For the colors to appear vibrant and distinct, the water droplets or ice crystals within the cloud must be nearly uniform in size across a significant area. The droplets must be roughly the same size as the wavelength of visible light for the diffraction effect to be noticeable. When light waves of different wavelengths diffract and interfere with each other, they either reinforce or cancel out certain colors. This constructive and destructive interference creates the spectrum of colors seen in the cloud, with the specific colors depending on the particle size and the angle of the light.
Nacreous Clouds: The Polar Stratospheric Phenomenon
A much rarer and more vivid form of colorful cloud is the nacreous cloud, also known scientifically as Polar Stratospheric Cloud (PSC). These clouds are distinct from standard iridescent clouds because they form at extremely high altitudes, typically in the stratosphere between 15 and 25 kilometers (9 to 15 miles) above the Earth’s surface. Most common clouds form much lower, in the troposphere, below about 10 kilometers.
Nacreous clouds require exceptionally cold temperatures, often colder than minus 78 degrees Celsius (minus 108 degrees Fahrenheit), to form the tiny ice crystals from the sparse moisture in the stratosphere. These frigid conditions typically occur during the polar winter in high-latitude regions, such as Scandinavia, Canada, and Antarctica. Their name, nacreous, comes from “nacre” or mother-of-pearl, which describes their intense, pearlescent sheen.
Because of their great height, these clouds are often illuminated by the Sun even when it is below the horizon for observers on the ground. This makes them glow brightly before dawn and after dusk. The ice particles in nacreous clouds are often composed of supercooled liquid and nitric acid. Their uniform size contributes to the intense, extensive iridescence, appearing brighter and more widespread than common tropospheric iridescence.