Optical phenomena and other features

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© E.A. Ripley
© H.O.F. Robbins

HALO PHENOMENA  The small halo of 22 degrees radius centred on the sun or moon is most frequently seen. In the top photograph the white horizontal line at the same elevation as the sun is called the parhelic circle and the two bright spots on this are called mock suns. Mock suns appear farther from the sun when it is higher in the sky. The bright spot above the sun is part of an arc of contact. Pillars of light may appear vertically above or below the sun or moon (bottom photograph) and are most frequently seen at sunrise and sunset. These, coupled with a portion of the parhelic circle, may form a cross. A large, less-bright halo of 46 degrees radius is sometimes seen, its arc of contact perhaps showing strong colouration. Rarely other arcs may occur, but usually only part of the display is seen. Halo phenomena are usually associated with Cs; sometimes Ci (mock suns or pillars are sometimes seen in Ac).

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© R. Fleet

CORONA   A series of coloured rings surrounding the sun or moon. The space next to the luminary is bluish-white, while this region is bounded on the outside by a brownish-red ring, the two forming the ‘aureole’. In most cases the aureole alone appears but a complete corona has a set of coloured rings surrounding the aureole - violet inside, followed by blue, green and on the outside yellow to red. The series may be repeated more than once, but the colours are merely greenish and pinkish tints. The example shown closely follows this description. Corona are most frequently associated with Ac, but sometimes occurs with Cc, Cs, As, Sc and St.

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© S.W. Gallier

IRISATION   Irisation consists of colours, predominately green and pink, often with pastel shades that sometimes appear on Cc, Ac or Sc. The colours may appear as bands nearly parallel to the margins of the clouds, or as a mosaic pattern.

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© R.K. Pilsbury

VIRGA   Virga consists of trails of precipitation (fallstreaks), that do not reach the earth's surface, attached to the under surface of a cloud. Mainly associated with Cc (small trails), Ac, As (may be clearly visible), Ns, Sc (especially at very low temperatures), Cu and Cb.

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© R.A.R. Tricker

MAMMA   Mamma highlights downdraughts which can sometimes cause udder-like protuberances to form on the under surface of Ci, Cc, Ac, As, Sc (irregular and ragged) and Cb (bulbous). The protuberances may appear prominent when the sun is low in the sky.

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© N. Elkins

SMOKE   City smoke and industrial pollution causes the sun to look very red at sunrise/sunset and to have an orange tint when high in the sky. From a distance such pollution may be confused with a bank of cloud on the horizon, but pollution generally appears light grey or to have a bluish hue.

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© R.N. Hughes

RAINBOW   Appears on a screen of raindrops when the observer has their back to the sun. A less bright secondary bow, with colour sequence reversed, sometimes occurs, with darker sky between the two. They may be bordered by fainter bows. When produced by the moon the colours are much weaker or are absent. The rainbow indicates Cb or precipitating large Cu.

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© G. Giles

CREPUSCULAR RAYS   Crepuscular rays take the form of pale blue or white rays diverging from the sun when it is behind Cu or Cb. Sunbeams piercing small gaps in cloud layers (sometimes called 'sun drawing water') and shadows cast by clouds near the horizon at twilight are also called crepuscular rays.

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© C.J. Richards

LIGHTNING   Cloud accompanied by lightning is reported as Cb.

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© J.H. Golden

SPOUT   An often violent whirlwind, revealed by (a) the presence of a cloud column or inverted cloud cone (funnel cloud) protruding from the base of a Cb and (b) a 'bush' composed of water droplets (raised from the surface of the sea) or of dust, sand or litter (raised from the ground). It is not uncommon for the funnel to meet the 'bush'. Spouts occur under newly formed parts of Cb, not from where the rain is falling. Weak spouts are occasionally seen beneath Cu.

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© R.K. Pilsbury

VELUM/PILEUS   These are accessory clouds close to the top or attached to the upper part of cumuliform cloud (Cu or Cb), which often penetrates it. Velum is of great horizontal extent, whilst Pileus has a much less horizontal extent. The example shown is Velum, with CL9 (C9).