Pearl clouds (Nacreous clouds) are clouds that occur in winter and are best seen at dusk in the morning and evening when the Sun is below the horizon.
Although the name of pearl clouds refers to spectral colors, colored (type II) pearls are rare in Finland. We mainly see colorless, pale (type I) pearls. A striking feature related to pearl clouds in Finland is also the strong brown, which makes the landscape bathe in intense red or purple light.
The Sky Watch has categories for type I and II nacreous clouds, as well as the brown subtype. This selection reports pearl clouds representing type II spectral colors.
While ordinary clouds are located in the lowest layer of the atmosphere in the troposphere, nacreous clouds form in the stratosphere above this at a height of 15 to 25 km above the ground. They occur when the stratosphere is exceptionally cold, about -75 ...- 85 C.
The particles that cause pearl clouds are either pure water ice (type II) or chemically different crystals, all of which contain nitric acid (type I) as an ingredient.
Changes in stratospheric thermal conditions are quite sluggish, which is why pearl clouds are seen continuously for at least a few days unless the lower clouds obscure the view. Nacreous clouds can be extensive in their occurrence and can occur simultaneously throughout Finland. However, the focus of the performances is in Lapland.
The appearance of nacreous clouds in the sky can be predicted by stratospheric temperature predictions. Pearl cloud observations made in Finland from 1996 to 2014 show that they had been seen from December to March. Most occurred in December-January, in March pearl clouds were reported in only one year.
The particles responsible for the pearl clouds can also give rise to the Bishop ring. The Bishop’s ring may be a clear sign of nacreous clouds when the Sun is on the horizon. The pearl clouds themselves usually stand out when the Sun is on the horizon, but in this case they are usually very ghostly cloud fibers and easily go unnoticed.
In the winter of 2012-2013, Finland experienced an exceptionally long 13-day pearl cloud streak. On the second to last day of the episode, rare spectral colors also appeared in the clouds. Photo by Matti Helin.