Mirages!

Mirages (photos above and below by TheBrockenInaGlory) come in two varieties – inferior (like the one above) and superior. Inferior mirages are quite common, often showing an image of the sky that gives the illusion of a lake or pool of water. Inferior mirages result from refraction by hot air near the ground.

Superior mirages (like the one below) result from refraction by cold air near the ground, and are more commonly seen in the Arctic and Antarctic. Complex superior mirages (or Fata Morgana) can contain multiple images of the original object, often distorted, and both inverted and right side up.

The complex superior mirages seen near the poles are mentioned in H. P. Lovecraft’s classic horror novel At the Mountains of Madness:

Our early flights were disappointing … though they afforded us some magnificent examples of the richly fantastic and deceptive mirages of the polar regions, of which our sea voyage had given us some brief foretastes. Distant mountains floated in the sky as enchanted cities, and often the whole white world would dissolve into a gold, silver, and scarlet land of Dunsanian dreams and adventurous expectancy under the magic of the low midnight sun. … I had seen dozens of polar mirages during the preceding weeks, some of them quite as uncanny and fantastically vivid as the present sample; but this one had a wholly novel and obscure quality of menacing symbolism, and I shuddered as the seething labyrinth of fabulous walls and towers and minarets loomed out of the troubled ice-vapours above our heads.

The effect was that of a Cyclopean city of no architecture known to man or to human imagination, with vast aggregations of night-black masonry embodying monstrous perversions of geometrical laws and attaining the most grotesque extremes of sinister bizarrerie. There were truncated cones, sometimes terraced or fluted, surmounted by tall cylindrical shafts here and there bulbously enlarged and often capped with tiers of thinnish scalloped discs; and strange, beetling, table-like constructions suggesting piles of multitudinous rectangular slabs or circular plates or five-pointed stars with each one overlapping the one beneath. There were composite cones and pyramids either alone or surmounting cylinders or cubes or flatter truncated cones and pyramids, and occasional needle-like spires in curious clusters of five. All of these febrile structures seemed knit together by tubular bridges crossing from one to the other at various dizzy heights, and the implied scale of the whole was terrifying and oppressive in its sheer giganticism. The general type of mirage was not unlike some of the wilder forms observed and drawn by the Arctic whaler Scoresby in 1820; but … we all seemed to find in it a taint of latent malignity and infinitely evil portent. … I was glad when the mirage began to break up, though in the process the various nightmare turrets and cones assumed distorted temporary forms of even vaster hideousness.

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Rainbows!

Rainbows are one of the most frequently observed atmospheric phenomena, although double rainbows can still get a strong reaction.

Rainbows form when light is refracted and reflected in droplets of water from rain (or some other source) as shown below. The light emerges at angles of up to 42°, so that the primary rainbow forms a circular halo around the antisolar point, at an angle of 42° from it. For the secondary rainbow, light enters the droplet from below and is internally reflected twice, emerging at angles of 51° or more, thus forming a larger halo (with reversed colours) around the antisolar point.

No light is refracted into the region between the primary and secondary rainbow, and this dark region (shown below in a photo by L.T. Hunter) is called Alexander’s band, after Alexander of Aphrodisias, who first discussed it in around 200 AD, in his commentary on Aristotle’s Meteorology.

Morning Glory clouds!

The Brisbane Courier-Mail is running a story on Morning Glory clouds – sets of roll clouds which are visible around this time of year over the southern Gulf of Carpentaria.

The photograph above is by Mick Petroff (2009). See here for a technical explanation of the phenomenon, or check out the photos galore on Twitter.

Noctilucent clouds!

The beautiful Noctilucent clouds (photo above by Kevin Cho) are slightly misnamed. The name means “night-shining clouds,” but in fact they are only visible between the end of civil twilight and the end of astronomical twilight (and at latitudes north of 45°N or south of 45°S).

Noctilucent clouds are the visible form of polar mesospheric clouds, made up out of ice crystals in the normally very dry almost-vacuum at extremely high altitudes, around 80–90 km up (see NASA image of a polar mesospheric cloud above).

Noctilucent clouds were first identified as a distinct atmospheric phenomenon in 1885 (see the Google ngram below for uses of the phrase). The ice crystals from which noctilucent clouds are formed require both water vapour and dust for crystal growth nuclei. The sources of these ingredients is still mysterious, although at least some of the dust may come from meteors.

Yet another atmospheric phenomenon for the bucket list!

Crepuscular rays!

Unlike some other atmospheric phenomena, crepuscular rays (photo above by Michael Alumbaugh) are fairly easy to observe, and a truly wonderful sight.

Although crepuscular rays appear to radiate out from a single point, this is a perspective effect. They are actually almost parallel, resulting from clouds casting long morning shadows or evening shadows. This becomes clear when crepuscular rays are seen from above, as in this International Space Station photo:

See here for a detailed explanation of the phenomenon. The name comes from the Latin crepusculum (twilight, dusk).

The green flash!

The green flash (photo above by TheBrockenInaGlory) has been seen at sunset by a few lucky people. Under the right conditions, the atmosphere behaves like a prism, separating the last solar light via refraction, so that the final trace of the Sun is green (blue light is scattered too much to be visible). This rare atmospheric phenomenon forms the backdrop to Jules Verne’s 1882 novel The Green Ray:

All eyes were again turned towards the west. The sun seemed to sink with greater rapidity as it approached the sea; it threw a long trail of dazzling light over the trembling surface of the water; its disk soon changed from a shade of old gold, to fiery red, and, through their half-closed eyes, seemed to glitter with all the varying shades of a kaleidoscope. Faint, waving lines streaked the quivering trail of light cast on the surface of the water, like a spangled mass of glittering gems.

Not the faintest sign of cloud, haze, or mist was visible along the whole of the horizon, which was as clearly defined as a black line traced on white paper.

Motionless, and with intense excitement, they watched the fiery globe as it sank nearer and nearer the horizon, and, for an instant, hung suspended over the abyss. Then, through the refraction of the rays, its disk seemed to change till it looked like an Etruscan vase, with bulging sides, standing on the water. There was no longer any doubt as to the appearance of the phenomenon. Nothing could now interfere with this glorious sunset! Nothing could prevent its last ray from being seen!

The sun was just half way below the horizon, and its powerful rays were shot across the sky like golden arrows; in the distance the cliffs of Mull and the summit of Ben More were bathed in brilliant, purple light.

At last only a faint rim of gold skimmed the surface of the sea.

‘The Green Ray! The Green Ray!’ cried in one breath the brothers, Dame Bess, and Partridge, whose eyes for one second had revelled in the incomparable tint of liquid jade.

Fire rainbows!

Fire rainbows, illustrated in the photo by “Dehk” above, are actually not rainbows at all, but a halo phenomenon related to the sun dogs previously discussed.

Fire rainbows (or, more accurately, fragments of circumhorizontal arcs) are formed at an angle of 46° from the Sun, and result from light refracting through ice crystals in high-altitude cirrus clouds. In particular, they result from light refracting through one of the 90° angles of an ice crystal, like so:

Fire rainbows are on my bucket list too, given the many beautiful pictures of the phenomenon on Flickr and on Wikimedia Commons.