Rainbow clouds: what, where, when?

Alexander Chernokulsky, cand. physical. Sciences, N.S. Institute of Physics of Atmosphere. Am Obukhov RAS, tells about the clouds that delight our eyes.

Oleg Bartunov from Gaish kindly provided his photos for the article. On February 6, 2014, Alexander Timazhev, Alexander Gabil-Ogly and Alexander Timazhaev, was awarded to Alexander, along with his colleagues at the Mirsseid Gabil-Ogli Institute. The award in the nomination “Science of Earth” noted a study of circulation and cloud regimes in the regions of Russia and the Arctic and their role in the formation of extreme weather and climatic phenomena.

The period of the end of the XIX - early XX centuries gave humanity a whole galaxy of great scientists in the field of nuclear physics, genetics, and studies of polar areas. For example, the purpose of the expedition of Robert Scott to Terra Nova in Antarctica in 1910-1912 was not only a sports throw to the South Pole, but also the complex geophysical studies of the Southern Earth Continent. So, George Simpson, the standard meteorologist of the expedition, according to the results of observations of optical effects in the clouds, published the first article in 1912 [1], dedicated to such a phenomenon as irration in the clouds (from Greek Iris, Iρις - rainbow), also called “iris”.

What?

Rainbow clouds are a rather rare optical phenomenon, in which very thin clouds located near the Sun are painted in spectral colors. Usually these colors are pastel, pale, but under certain conditions it can be very bright. Simpson rightly pointed out that irration is the most common type of crowns - an optical phenomenon associated with the diffraction of light on drops of hypothermia in the clouds and the formation of colored circles in a cloud veil around the Sun.

At its core, rainbow clouds are part of the failed crowns [2]. And if the full -fledged crowns in the atmosphere are extremely rare, then almost everyone can be seen rainbow clouds, the main thing is to be attentive! It is best to observe the rainbow clouds in dark glasses so as not to blind, because they appear only near the sun, at a distance of about 3-15 °, in some cases up to 30 °. But if the luminary is hidden behind something (after another cloud, behind the mountain, etc.), then the irrigation can be seen with the naked eye.

Where?

Irization is usually observed at the edges of cirrus, cirrus-dos and high-dough clouds. The source of light, by the way, can be not only the Sun, but also the Moon [3]. Irization can be seen on condensation traces of aircraft [4], and also on the upper part of the cumen -removal clouds (on the so -called veil or an anvil). True, such rainbow clouds do not portend anything rainbow, on the contrary, they talk about the imminent deterioration of the weather! And most often, irration is found in high-dick linzine (lenticular) clouds characteristic of mountainous areas [5]. The air in the mountains is cleaner, almost without impurities, as a result, water drops are much more difficult to go to crystals. The fact is that for the appearance of irrigation, hypothermias are preferable to ice crystals.

Sunlight, falling on a cloud drop or ice crystalline, deviates from distribution in a straight line. In this case, the value of the deviation of light depends on the wavelength, so the diffraction of sunlight always leads to its decomposition into a spectrum. Around each drop, due to such a single scattering, colored circles are formed. Their brightness is very small and 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, along the angular distance of circles of the same color in the crown of the sun, you can accurately calculate the radius of cloud particles).

In a cloud with a large spread of particles in size, color circles will be overlapping each other and irrigation will disappear. In optically dense clouds, the effect associated with multiple scattering increases, which is also “deadly” for the effect of rainfulness. Thus, optically thin clouds (or parts of clouds) with a monodiste distribution of cloud particles in size and shape are ideal for irritation. The higher such a uniformity of cloud particles, the brighter the color of the rainbow cloud. And it is above at water drops. Yes, and the size of them are much more successful than their icy brothers.

For the formation of rainbow clouds, the size of the cloud particles should be 5-50 times larger than the length of the light wave [6, 7], that is, from 3.5 to 35 microns for red and from 2 to 20 microns for blue. Observations show that the brightest rainbow clouds are observed in clouds with particles of about 10 microns or less. And according to the latest data from satellite observations [8], the most common size of ice crystals in the clouds is about 30-40 microns, although there are ice crystals and smaller and larger (from 2-3 to 60-65 μm). The range of variability of water drops in the clouds already: from tenth shares to 30-40 microns, while the sizes of drops in the ranges of 2-3 microns and 10-15 microns are most often found. It is such supercooled drops that are ideal for the formation of rainbow clouds! By the way, another interesting fact: it was George Simpson in his article of 1912 on the basis of observations of rainbow clouds that the first confirmed (albeit indirectly) that the water in the clouds is in a hypothermia. Modern observations show that about -15 ° C of the clouds almost completely consist of drops of water, to a temperature of -40 ° C -both from water drops and from ice crystals, and only at lower temperatures the water in the liquid phase in the clouds is almost not found [9]. The works of the first half of the 20th century indicated that rainbow clouds can only be formed on drops of hypothermia, but in recent decades it was found that ice crystals can lead to the formation of rainbow clouds.

When?

Now the phenomenon of irritation of abnormally high and cold cirrus clouds consisting of ice crystals, which have an almost monodispersed distribution in size, is actively studied. These clouds are located near the tropopause (a narrow layer of the atmosphere that separates 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 have made the assumption that these ice crystals were formed as a result of lowering sulfuric acid particles from the stratosphere, which serve as a kind of condensation for water vapor.

Sure gets into the stratosphere during large eruptions in volcanoes, especially “good” tropical volcanoes for this. They can abandon the sulfur into the stratosphere to a height of up to 20 - 30 km, here the sulfur quickly spreads throughout the planet (thanks to the circulation of Bruser - Dobson, which transfers air to the stratosphere from the tropics into polar latitudes) and begins to slowly settle in the lower layers of the atmosphere. The settlement process can last up to 2-3 years.

Sulfate aerosols in the stratosphere cause various optical effects, ranging from colorful sunsets and dawn, ending with the so-called bishop rings-a variety of halo with a white-blue bright center and a dark red-brown outskirts. The last powerful eruption is the explosion of the Pinatubo volcano in 1991, the next year was marked by a real riot of light phenomena in the atmosphere.

So, in Holland, the Bishop rings were recorded almost every day [10], weather forecasters did not see them only on days with continuous low cloudiness. It is possible that rainbow clouds were observed more often, but there is no direct information about this: today there is no systematic assessment of climatology (spatial distribution, annual changes, inter-day changes, etc.) of this phenomenon. So to confirm the influence of volcanoes on the formation of rainbow clouds, it seems that you will have to wait for the next powerful eruption. In the meantime, you can simply enjoy the photos that are divided by lucky researchers of unusual natural phenomena.

1. Simpson GC, Coronae and Iridescent Clouds, Quarterly Journal of the Royal Meteorological Society, 38 (164), 291299, 1912.
2. BOHREN C. Simple Experiments in Atmosphereic Physics: A Serendipitous Iridescent Cloud, Weatherwise, 38 (5), 268-274, 1985.
3. SHAW JA and PUST NJ ICY WAVE-CLOUD LUNAR CIRONA and CIRRUS IRIDESENCE, Applied Optics, 50 (28), F6-F11, 2011. Sassen K. Iridescence in an Aircraft Contrain. The Optical Society of America, 69 (8), 1080-1083, 1979. Shaw Ja and Neiman Pj Coronas and Iridescence in Mountain Wave Clouds, Applied Optics, 42 (3), 476485, 2003, 2003.
4. Sassen K. Cirrus Cloud Iridescence: A Rare Case Study, Applied Optics, 42 (3), 486-491, 2003.
5. Lock Ja and Yang L. Mie theory of the Corona, Applied Optics 30 (24), 3408-3414, 1991. Stubenrauch CJ and Coauthors, Assessment of Global Cloud Datasets From Satellites: Project and Database Initiated by the Gewex Radiation Panel, Bulletin of the American Meteorological Society, 94 (7), 1031049, 2013.
6. Mazin I.P. and Khrgian A.Kh. Clouds and cloud atmosphere, Leningrad: Hydrometeoizdat, 648 pp., 1989.
7. Hattinga Verschure P.-P. Thirty Years of Observing and Documenting Sky Optical Phenomena, Applied Optics, 37 (9), 1585-1588, 1998.