Rainbow clouds are a relatively rare optical phenomenon. It can be seen in all seasons, but especially often in autumn. These clouds can be colored in all colors of the spectrum.
They are made up of small water droplets of almost the same size.
So, when the sun occupies a certain position in the sky, and at the same time is hidden behind sufficiently dense clouds, then any (transparent) cloud located near it can be colored in spectral colors. This phenomenon is explained by the fact that beams of light of different wavelengths are deflected in different ways, which means that the light of these waves comes to the observer from different directions.
The cloud can become rainbow colored completely or only at the edges, can have dull colors or very bright. In the latter case, the cloud droplets need to be the same size. Only then will it have rich colors.
This phenomenon is best seen on Altocumulus (especially Altocumulus lenticular) and Cirrocumulus.
And now in more detail
The period of the late XIX - early XX centuries gave mankind a whole galaxy of great scientists in the field of nuclear physics, genetics, research of polar regions. For example, the goal of Robert Scott's expedition on the Terra Nova to Antarctica in 1910–1912 was not only a sports rush to the South Pole, but also comprehensive geophysical studies of the southernmost continent of the Earth. So, George Simpson, a staff meteorologist of the expedition, based on the results of observations of optical effects in the clouds, published in 1912 the first article devoted to such a phenomenon as irisation in clouds (from the Greek Iris, Iρις - rainbow), also called "rainbow clouds".
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Rainbow clouds are a rather rare optical phenomenon in which very thin clouds near the Sun are colored in spectral colors. Usually these colors are pastel, pale, but under certain conditions they can be very bright. Simpson rightly pointed out that irisation is the most common type of crowns - an optical phenomenon associated with the diffraction of light by drops of supercooled water in clouds and the formation of colored circles in a cloudy veil around the Sun.
At their core, rainbow clouds are part of a failed crown. And if full-fledged crowns in the atmosphere are extremely rare, then almost everyone can see rainbow clouds, the main thing is to be careful! It is best to observe rainbow clouds in dark glasses, so as not to go blind, because they appear only near the Sun, at a distance of about 3-15 °, in some cases up to 30 °. But if the star is hidden behind something (behind another cloud, behind a mountain, etc.), then the iridescence can be seen with the naked eye.
There is iridescence usually at the edges of cirrus, cirrocumulus and altocumulus clouds. The source of light, by the way, can be not only the Sun, but also the Moon. Iris can be seen on airplane condensation trails, and also on the top of cumulonimbus clouds (on the so-called veil or anvil). True, such rainbow clouds do not bode well, on the contrary, they speak of an imminent deterioration of the weather! And most often, iridescence occurs in altocumulus lenticular (lenticular) clouds characteristic of mountainous areas. The air in the mountains is cleaner, practically free of impurities; as a result, it is much more difficult for water droplets to transform into crystals. The fact is that supercooled water is preferable to ice crystals for the appearance of iridescence.
Sunlight striking a cloudy droplet or ice crystal deflects from propagation in a straight line. In this case, the magnitude of the deflection of light depends on the wavelength, therefore the diffraction of sunlight always leads to its decomposition into a spectrum. Colored circles are formed around each drop due to this single scattering. Their brightness is very low and is visible only as a result of superposition. The size of the color circles depends not only on the wavelength, but also on the size of the obstacle (by the way, the radius of cloud particles can be calculated quite accurately from the angular distance of circles of the same color in the crowns from the Sun).
In a cloud with a large dispersion of particles in size, the color circles will overlap each other and the iridescence will disappear. In optically dense clouds, the effect associated with multiple scattering increases, which is also “fatal” for the iridescence effect. Thus, optically thin clouds (or parts of clouds) with a monodisperse distribution of cloud particles in size and shape are ideal for iridescence. The higher the uniformity of cloud particles, the brighter the colors of the rainbow cloud. And it is higher in water drops. And they are much more successful in size than their ice counterparts.
For the formation of rainbow clouds, the size of cloud particles must be 5-50 times the wavelength of light, that is, from 3.5 to 35 µm for red and 2 to 20 µm for blue. Observations show that the brightest rainbow clouds are observed in clouds with a particle size of about 10 microns or less. And according to the latest satellite observation data [8], the most common size of ice crystals in clouds is about 30–40 µm, although ice crystals of both smaller and larger sizes (from 2–3 to 60–65 µm) are found. The range of variability of water droplets in clouds is narrower: from tenths to 30–40 µm, with the most common droplet sizes in the ranges of 2–3 µm and 10–15 µm. It is these supercooled drops that are ideal for the formation of rainbow clouds! By the way, another interesting fact:it was George Simpson, in his 1912 paper, based on observations of rainbow clouds, who first confirmed (albeit indirectly) that water in clouds is in a supercooled state. Modern observations show that up to a temperature of about -15 ° C, clouds are almost entirely composed of water droplets, to a temperature of -40 ° C - both water droplets and ice crystals, and only at a lower temperature is water in the liquid phase in clouds almost never occur. In the works of the first half of the 20th century, it was indicated that rainbow clouds can form only on drops of supercooled water, but in recent decades it was discovered that ice crystals can also lead to the formation of rainbow clouds. Modern observations show that up to a temperature of about -15 ° C, clouds are almost entirely composed of water droplets, to a temperature of -40 ° C - both water droplets and ice crystals, and only at a lower temperature is water in the liquid phase in clouds almost never occur. In the works of the first half of the 20th century, it was indicated that rainbow clouds can form only on drops of supercooled water, but in recent decades it was discovered that ice crystals can also lead to the formation of rainbow clouds. Modern observations show that up to a temperature of about -15 ° C, clouds are almost entirely composed of water droplets, to a temperature of -40 ° C - both water droplets and ice crystals, and only at a lower temperature is water in the liquid phase in clouds almost never occur. In the works of the first half of the 20th century, it was indicated that rainbow clouds can form only on drops of supercooled water, but in recent decades it was discovered that ice crystals can also lead to the formation of rainbow clouds.that rainbow clouds can only form on droplets of supercooled water, but in recent decades it has been discovered that ice crystals can also lead to the formation of rainbow clouds.that rainbow clouds can only form on droplets of supercooled water, but in recent decades it has been discovered that ice crystals can also lead to the formation of rainbow clouds.
The phenomenon of iridescence of abnormally high and cold cirrus clouds, consisting of ice crystals with an almost monodisperse size distribution, is being actively studied.
These clouds are located near the tropopause (a narrow layer of the atmosphere separating the troposphere and stratosphere), their temperature is about –70… –75 ° C, and the size of ice particles is only 2–5 microns. In one of the latest works, American scientists made the assumption that these ice crystals were formed as a result of the dropping of sulfuric acid particles from the stratosphere, which serve as a kind of condensation nuclei for water vapor.
Sulfur enters the stratosphere during large volcanic eruptions, tropical volcanoes are especially "good" for this. They can throw sulfur into the stratosphere to an altitude of 20-30 km, here sulfur quickly spreads across the planet (thanks to the Brewer-Dobson circulation, which transports air in the stratosphere from the tropics to polar latitudes) and begins to slowly settle into the lower atmosphere. The subsidence process can last up to 2-3 years.
Sulfate aerosols in the stratosphere cause various optical effects, ranging from colorful sunsets and sunrises to the so-called Bishop rings - a type of halo with a bright blue-white center and a dark red-brown margin. The last powerful eruption was the explosion of Mount Pinatubo in 1991, the next year was marked by a real riot of light phenomena in the atmosphere.
So, in Holland, Bishop's rings were recorded almost every day, forecasters did not see them only on days with continuous low clouds. It is possible that rainbow clouds were observed more often, but there is no direct information about this: to date, there is no systematic assessment of climatology (spatial distribution, annual variation, interannual changes, etc.) of this phenomenon. So to confirm the influence of volcanoes on the formation of rainbow clouds, it seems, will have to wait for the next powerful eruption. In the meantime, you can just enjoy the photos that lucky researchers of unusual natural phenomena share with us.