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Halo – caused by refraction of light into its rainbow colours by randomly-orientated ice crystals in cirrus or cirrostratus cloud (often ahead of an approaching frontal system), haloes are have a radius of 22° and much larger than a corona. They can exist with sun dogs and indicate approaching bad weather. |
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Mock Sun (Sun Dog or Parhelion) – similar to haloes, these bright spots either side of the sun occur when the sun is low in the sky, and are caused by refraction of light into its rainbow colours by ice crystals in cirrostratus cloud. The red colours of the spectrum are seen closest to the sun and the blue colours furthest from it. In contrast to haloes, parhelia are produced by flat, horizontally aligned ice crystals. Moon dogs and lunar haloes are much rarer versions of the same effect. |
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Sun Pillar – these vertical streaks above and below the setting or rising sun are caused by reflection (as opposed to refraction with haloes and parhelia) of light from the horizontal surfaces of hexagonal ice crystals in cirrostratus cloud, and therefore do not exhibit rainbow colours. Rarely they can occur with sun dogs. |
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Circumzenithal Arc (CZA) – similar to sun dogs, refraction by flat, horizontally aligned ice crystals in cirrostratus gives rise to a bright inverted rainbow directly overhead. Comprising a quarter circle, blue colours of the spectrum are closest to the zenith and red colours on the outer edges. The brightest arcs occur when the sun is 22° above the horizon, and they cannot occur at all if the sun is above 32.2°. |
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Corona – a coloured ring or rings around the sun and moon caused by diffraction (scatter) of light by water droplets in thin low or medium level cloud. The colours range from blue-white in the centre towards red at the outer fringes. A similar effect caused by dust and sulphate particles after a volcanic eruption is known as a Bishop’s Ring. |
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Irisation – also caused by diffraction of light by water droplets causing irregular patches of predominantly green and pink mother of pearl shades. It usually occurs in thin layers of cirrocumulus, altocumulus and stratocumulus. |
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Glory – an effect caused when light is scattered back towards its source by a cloud of uniformly-sized water droplets. Often accompanied by a ‘Brocken Spectre’ (a magnified shadow of a person or object cast onto clouds below the observer), glories are seen as a sequence of coloured rings. These effects are most often observed from aircraft. |
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Rainbow – caused by the dispersion of light into its seven colours – red, orange, yellow, green, blue, indigo, and violet – rainbows occur when the sunlight is behind the observer with airborne water droplets directly in front. Like Brocken Spectres, rainbows are unique to the observer, and no two people see the same rainbow. Primary bows are always 42° in radius with red on its outer edge and violet nearest the centre. Secondary bows with the colours reversed can sometimes be seen with a radius of 52°. A pale arc known as a supernumerary bow can sometimes be seen inside the primary bow. |
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Crepuscular Rays – these are beams of sunlight illuminated by minute dust, gas and water droplets in the lower atmosphere. There are three types – 1. Those which emerge from gaps in low cloud, often known as ‘Jacob’s Ladder’; 2. Those which rise from behind a cumuliform cloud; and 3. Pinkish rays emanating from below the horizon. Anti-crepuscular rays appear to diverge away from the sun when the observer has his back towards the setting sun. |
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Lightning – positive and negative charges within cumulonimbus clouds become separated by violent updraughts inside the cloud. The lower parts of the cloud develop a negative charge whilst the upper parts develop a positive charge, causing electrical discharge between the two (in-cloud or sheet lightning). Cloud-to-ground (forked) lightning occurs between the negatively charged base of the cloud and the positively charged ground. Reaching temperatures of 28,000°C (hotter than the sun), the sudden expansion of the surrounding air causes the sound we hear as thunder. Thunder can be heard from 20 miles away as indistinct rumbles whereas closer lightning can produce a sudden crack. |
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Aurora – named after Aurora, the Roman goddess of the dawn, they occur 250km (60-150 miles) above the earth at polar latitudes and comprise ribbons or glowing patches of multicoloured light. They are formed as fast moving particles from the solar wind collide with gases in the Earth’s upper atmosphere, causing them to fluoresce. In the northern hemisphere, they are known as the aurora borealis (Northern Lights) and the aurora australis in the south. |