Halo (optical phenomenon)

A halo (from Ancient Greek ἅλως (hálōs) 'threshing floor, disk')[1] is an optical phenomenon produced by light (typically from the Sun or Moon) interacting with ice crystals suspended in the atmosphere.

The ice crystals responsible for halos are typically suspended in cirrus or cirrostratus clouds in the upper troposphere (5–10 km (3.1–6.2 mi)), but in cold weather they can also float near the ground, in which case they are referred to as diamond dust.

Other common types of optical phenomena involving water droplets rather than ice crystals include the glory and the rainbow.

While Aristotle had mentioned halos and parhelia, in antiquity, the first European descriptions of complex displays were those of Christoph Scheiner in Rome (c. 1630), Johannes Hevelius in Danzig (1661), and Tobias Lowitz in St Petersburg (c. 1794).

[3] The 22° halo is not to be confused with the corona, which is a different optical phenomenon caused by water droplets rather than ice crystals, and which has the appearance of a multicolored disk rather than a ring.

It has a small diameter, which makes it very difficult to see in the Sun's glare and more likely to be noticed around the dimmer subsun, often seen from mountain tops or airplanes.

It is suggested that they are formed by very flat pyramidal ice crystals with faces at uncommonly low angles, suspended horizontally in the atmosphere.

[4] In the Cornish dialect of English, a halo around the sun or the moon is called a cock's eye and is an omen of bad weather.

[5] In Nepal, the halo round the sun is called Indrasabha with a connotation of the assembly court of Lord Indra – the Hindu god of lightning, thunder, and rain.

The earliest chemical recipes to generate artificial halos has been put forward by Brewster and studied further by A. Cornu in 1889.

Even earlier than Bravais, the Italian scientist F. Venturi experimented with pointed water-filled prisms to demonstrate the circumzenithal arc.

[25][29] Putting such machines inside spherical projection screens, and by the principle of the so-called sky transform,[30] the analogy is nearly perfect.

A realization using micro-versions of the aforementioned machines produces authentic distortion-free projections of such complex artificial halos.

The resulting superposition image is then a representation of complex natural halo displays containing many different orientation sets of ice prisms.

[24][25] The experimental reproduction of circular halos is the most difficult using a single crystal only, while it is the simplest and typically achieved one using chemical recipes.

The first one using pneumatics and a sophisticated rigging,[29] and a second one using an Arduino-based random walk machine which stochastically reorients a crystal embedded in a transparent thin-walled sphere.