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Altitude: Can appear for all altitudes of the Sun

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Glory

CloudSpotters must gain some elevation to add a glory to their collection of cloud optical effects, for this striking phenomenon is seen only with the Sun directly behind you, as it casts your shadow on to a layer of cloud. The glory, which looks like a series of rainbow rings around the shadow, is produced by cloud droplets reflecting, refracting and diffracting sunlight, although the exact optics are still not fully understood.

One of the easiest places to spot a glory is from that great cloudspotting location, the window of a plane. It can sometimes appear around the plane’s shadow, cast on to a nearby layer of cloud or fog. When the cloud’s some distance away, the shadow is absent and you just see the coloured rings.

The most eerie form of glory, the ‘Brocken spectre’, is when the rings appear around your own shadow as you look at cloud from a mountain ridge. The perspective can make the legs of your shadow flare out. What with the multicoloured halo, it looks like a ghost from the 1970s.

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Crepuscular rays

Even if they don’t know the name, most CloudSpotters will have plenty of opportunities to add crepuscular rays to their collection of cloud optical effects. They’re the familiar Sunbeams that appear to burst from behind a Cumulus cloud, or shine down through a hole in a Stratocumulus.

Crepuscular rays appear when the path of sunlight is made visible by tiny atmospheric particles, too scarce to appear as cloud, but plentiful enough to scatter the light noticeably. Like fingers through a torch beam, the cloud shadows give edges to the rays. In spite of being almost parallel, these rays seem to radiate outwards from behind the cloud. This is just the same perspective effect as railway tracks seeming to widen as they get nearer.

Whenever you notice crepuscular rays from a low Sun, look to the opposite horizon for the far less obvious ‘anti-crepuscular rays’. Appearing to emanate from a point directly opposite the Sun, these are the shadows cast by clouds behind you, like the shadow of someone shuffling behind you in a dusty cinema. Perspective makes them appear to converge in the distance. Few people ever notice anti-crepuscular rays – except vampire CloudSpotters, eager for the arrival of night.

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