Nacreous clouds (mother-of-pearl 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, colorless, pale nacreous clouds are mainly observed in Finland. 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 colorless elections (type I) and colored (type II) pearl clouds, as well as the brown subtype.

This selection reports type I colorless pearl clouds. While ordinary clouds are located in the lowest layer of the atmosphere in the troposphere, pearls 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 pearls 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.

Nacreous clouds 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 nacreous 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

Type I nacreous clouds. Image by Panu Lahtinen.

Noctilucent clouds like nacreous clouds of type I. Image by Mikko Peussa.

Half an hour before sunset, wavy nacreous clouds. These nacreous clouds stood out exceptionally well from the daytime sky. Image by Marko Riikonen.

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.

Pearlescent brown is a brownish sunset associated with pearl clouds that appears at dusk in the morning and evening when the Sun is below the horizon.

It is usually remarkably powerful and makes the landscape bathe in red or purple light. Most often, some degree of pearlescent is observed in the context of pearl clouds. Pearl brown can also be the dominant element in which the actual pearl clouds stand out poorly if at all.

The pearl clouds themselves are reported in the Sky Watch in the categories of colorless light (type I) and spectral color (type II) clouds.

Parel clouds are often also associated with the Bishop ring. The Bishop’s ring may be a clear sign of pearls when the Sun is on the horizon. In addition, pearlescent cloud fibers are visible, which, however, are often very inconspicuous when the Sun is on the horizon.

The Pearl Bishop gradually turns into a pearl brown in the evening as the sun continues to sink below the horizon. In the morning the opposite. However, the Bishop ring is not always visible, even if pearl clouds are observed in the morning or evening. In this case, the pearlescent brown would also appear to be weak or non-existent.

Sunset colors with pearl clouds Eetu Saarti.

Close up of mother of pearl brown. It also shows type I pale pearl clouds. Photo by Mika Aho.

Pearlescent sunset by Veikko Mäkelä. 

The rocket contrails are colorful clouds that appear when the sun is below the horizon. The contrails can float so high in the atmosphere that the sun shines on them even if it is already completely dark on the ground. They stay visible noticeable quite long after the launch.

The colors are created by the scattering of sunlight in the small ice crystals. In Finland, rocket phenomena from two different locations have mainly been observed. One of them is the missile launches from the submarines from the White Sea and the Arctic Ocean. They use solid fuel that creates colorful clouds.

Another source of rocket phenomena is the Plesetsk Cosmodrome Area in Russia. There are a few launches from there every year. These fires usually use liquid fuels, making the clouds less spectacular.

The visibility of the rocket launch is affected by the time at which the launch is made and where the winds blow. The best of all is around 3-5 o'clock Finnish time, when the cloud has time to spread out a bit and shine in the morning sun coming from the east when it is still dark or dark in Finland.

During the first decade of the 2000s, rocket phenomena have been observed about twice per year in Finland. Since then, however, it has been quieter.

Rocket contrails from Oulunsalo, Photo by Jarmo Moilanen.

Barium clouds have very rarely been seen in the sky during rocket launches. Typically a space organization has done an experiment in which a barium cloud is released into the upper atmosphere.