Sun halos sundogs and sun pillars A

number of unusual phenomena are related to the interaction of the sun's rays with ice crystals in the upper atmosphere. A ring of light that circles and extends outward from the sun, or the Moo^ is known as a halo. The halo forms when ice crystals in high-level cirriform clouds refract the sun's or moon's rays. most sun halos form at an angle of 22° from the sun because randomly oriented small ice particles refract the light at this angle. occasionally a 46° halo is visible, formed when subhorizontally oriented columnar ice crystals refract the light at this higher angle. most sun halos are simply bright bands of light but some exhibit rainbowlike zones of color. These form when the light is dispersed by the ice crystals and light of different wavelengths (colors) is refracted by different amounts depending on its speed

Atmospheric crystals sometimes cause hexagonal or platy ice crystals to fall slowly through the atmosphere, and this vertical motion causes the crystals to become uniformly oriented with their long dimensions in a horizontal direction. This orientation prevents light that is refracted through the ice crystals from forming a halo, but when the sun approaches the horizon it causes two bright spots or colored bright spots to appear on either side of the sun. These spots are commonly called sundogs, or parhelia.

sun pillars are a similar phenomenon but are formed by light that is reflected off the ice crystals instead of refracted through them. in this case, usually at sunset or sunrise, the sun's rays reflect off the subhorizontally oriented ice crystals and form a long column of light extending downward from the sun.

Rainbows are a somewhat related phenomena. Rainbows are translucent concentric arcs of colored bands that are visible in the air under certain conditions when rain or mist is present in the air and the sun is at the observer's back. Rainbows form where sunlight enters the rain or water drops in the air, and a small portion of this light is reflected off the back of the raindrops and directed back to the observer. When the sunlight enters the rain drops, it is bent and slows, and as in a prism, violet light is refracted the most and red the least. The amount of light that is reflected off the back of each raindrop is small compared to the amount that enters each drop, and only the rays that hit the back of the drop at angles greater than the critical angle are reflected. Since the sun's rays are refracted and split by color when they enter the water drops, each color hits the back of the raindrop at a slightly different angle, and the reflected light emerges from the raindrop at different angles for each color. Red light emerges at 42° from the incoming beam, whereas violet light emerges at 40°. An observer sees only one color from each drop, but with millions of drops in the sky an observer is able to see a range of colors formed from different raindrops with light reflected at slightly different angles to the observer. Rainbows appear to move as an observer moves, since each ray of light is entering the observer's eyes from a single raindrop, and as the observer moves, light from different drops enters the observer's eyes.

See also atmosphere; Sun.

FURTHER READING

Ahrens, C. Donald. Meteorology Today. 7th ed. Pacific

Grove, Calif.: Brooks/Cole, 2002. Ashworth, William, and Charles E. Little. Encyclopedia of Environmental Studies. New ed. New York: Facts On File, 2001.

supercontinent cycles Supercontinent cycles are semiregular groupings of the planet's landmasses into single or large continents that remain stable for a period of time, then disperse, and eventually come back together as new amalgamated landmasses with a different distribution. At several times in Earth history, the continents have joined together forming one large supercontinent, with the last supercontinent, Pangaea (meaning all land), breaking up approximately 160 million years ago. This process of supercontinent formation, dispersal, and reamalgamation seems to be grossly cyclic, perhaps reflecting mantle convection patterns, but also influencing climate and biological evolution. Early investigators noted global "peaks" in age distributions of igneous and metamor-phic rocks and suggested that these represent global orogenic or mountain building episodes, related to supercontinent amalgamation.

The basic idea of the supercontinent cycle is that continents drift about on the surface until they all collide, stay together, and come to rest relative to the mantle in a place where the gravitational potential surface (geoid) has a global low. The continents are only one-half as efficient at conducting heat as oceans, so after the continents are joined together, heat accumulates at their base, causing doming and breakup of the continent. For small continents, heat can flow sideways and not heat up the base of the plate, but for large continents the lateral distance is too great for the heat to be transported sideways. The heat rising from within the Earth therefore breaks up the supercontinent after a heating period of several tens or hundreds of millions of years. The heat then disperses and is transferred to the ocean/atmosphere system, and continents move apart until they come back together to form a new supercontinent.

The supercontinent cycle greatly affects other Earth systems. The breakup of continents causes sudden bursts of heat release, associated with periods of increased, intense magmatism. It also explains some of the large-scale sea level changes, episodes of rapid and widespread orogeny, episodes of glaciation, and many of the changes in life on Earth.

Compilations of Precambrian isotopic ages of metamorphism and tectonic activity suggest that the Earth experiences a periodicity of global orogeny of 400 million years. Peaks have been noted at time periods including 3.5, 3.1, 2.9, 2.6, 2.1, 1.8, 1.6, and 1.1 billion years ago, as well as at 650 and 250 million years ago. One hundred million years after these periods of convergent tectonism and metamorphism, rifting is common and widespread. Geologist A. H. Sutton (1963) proposed the term chelogenic cycle, in which continents assemble and desegregate in antipodal supercontinents.

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