Ihonpintapilvi

Ihonpintapilvi, or the *iridescent cloud*, is an optical phenomenon that occurs in certain types of clouds, most commonly altocumulus, cirrocumulus, or lenticular clouds. The name derives from Finnish, where *ihonpinta* means "skin surface," referring to the shimmering, pastel-colored appearance resembling skin or scales. This effect is caused by the diffraction of sunlight or moonlight by tiny water droplets or ice crystals within the cloud, similar to the way light is dispersed through a prism.

The colors in an ihonpintapilvi typically range from soft pinks and blues to greens and purples, often appearing as bands or patches across the cloud’s surface. Unlike rainbows, which form when sunlight refracts through larger water droplets, ihonpintapilvi arises from the interference of light waves scattered by much smaller particles—around 10 to 20 micrometers in diameter. The angle of the sun relative to the observer and the cloud’s structure determine the visibility and intensity of the colors.

This phenomenon is not unique to Finland but occurs worldwide under similar atmospheric conditions. It is sometimes confused with coronae, which are smaller, more circular halos around the sun or moon caused by similar diffraction effects. Unlike coronae, ihonpintapilvi often spans larger areas of the cloud and exhibits more varied, pastel hues.

While ihonpintapilvi is a natural optical effect, it has inspired cultural interest, particularly in Finland, where it is sometimes associated with folklore and natural beauty. The phenomenon is most frequently observed in temperate climates during times of stable atmospheric conditions, making it a sought-after sight for photographers and cloud enthusiasts.