Atmospheric Optical Phenomena (except Rainbows and Auroras)
halo		Sun pillar

Below is a checklist of Atmospheric optical phenomena (except Rainbows and Auroras) on postal items (stamps, souvenir sheets, aerogrammes, postal cards, etc.). Catalog numbers, years of issue, and notes on the items featured are given when available. If readers know of additional information or images, please contact the authors using the e-mail addresses at the bottom of this page.

Atmospheric Optical Phenomena: Definition

We define atmospheric optical phenomena to be the visual results of:

1. The interaction of light from the Sun or the Moon with certain components of the atmosphere (cloud water droplets or ice crystals, liquid or solid precipitation, dust, water vapour and the gases of the atmosphere itself) or with certain atmospheric structures (such as the vertical distribution of temperature and/or humidity); or
2. The interaction of charged particles from the sun (the solar wind) with certain atmospheric gases.

Atmospheric Optical Phenomena included in this page:

The following phenomena fall under part 1 of the above definition, and philatelic items referring to them are included in this page.

	Halo The term for various circles or arcs of light around the sun or Moon, caused by reflection and refraction of light as it moves through ice crystals in the atmosphere. If colors are present the reddish tones are at the inner part of the rings. Halos are large - in the most common halo, the angle between the centre and the ring is 22 degrees (approximately the angle defined by the span of a hand at arm's length, as in this example), but other even larger halos are also possible.
	Corona (diffraction ring around the Sun or Moon, not the corona composed of plasma surrounding the Sun) One or more diffuse concentric rings of light around the Sun or Moon, formed by diffraction of light as it moves through water droplets in the atmosphere; the rings may have more color than halos. In a corona the reddish tones are at the outer part of the rings. Coronas usually occur in the presence of thin clouds, but a particular type, known as the Bishop's ring corona, occurs in the absence of clouds. It is caused by scattering of light from very small (~ 1 micrometer, or 1/1000 of a mm) particles, usually stratospheric dust or ash or sulphate droplets from volcanic eruptions that are suspended in the atmosphere. Bishop's ring coronas can be larger than the usual coronas associated with thin clouds, which are smaller than halos. The central bright area of a corona is called the aureole. It appears as a whitish or bluish-white disk that fades to a pale reddish-brown toward the circumference of the disk. It may be the only visible part of the corona, particularly for those associated with the Moon. The aureole of a lunar corona can be enhanced if seen through a cloud layer that is not too thick and/or has breaks.
	Sundog (mock Sun, or parahelion) A type of halo consisting of a pair of bright spots to the left and right of the Sun.
	Sun pillar A vertical shaft of light extending upward or downward from the Sun, generally seen when the Sun is very low in the sky, and caused by reflection of sunlight from ice crystals that form high, thin cirrostratus clouds.
	Glory One or more diffuse concentric rings of colored light around a shadow image of an object projected on a cloud of refracting water droplets. Each ring is red on the outside and bluish toward the centre. To see a glory, the observer must be between the Sun and the cloud, so it often seen from aircraft. C.T.R. Wilson saw a glory while working as an observer at the Ben Nevis weather station in Scotland, and that sight inspired him to study clouds and eventually led to the development of his cloud chamber.
	Subsun (virtual Sun) A special type of halo that appears as an enlongated mirror image of the Sun in the clouds. It is seen only when the clouds are observed from above, and is caused by reflection of sunlight from the flat ice crystals of which the clouds are composed.
	Crepuscular rays Rays of light that appear to radiate from the location of the Sun in the sky, caused by sunlight streaming through cloud gaps and emphasized by the cloud-shadowed regions where no sunlight is streaming. Their apparent convergence toward the sun is a perspective effect.
	Cloud iridescence Colors in clouds similar to those seen in oil films in puddles, due to diffraction of light by small water droplets or ice crystals. Such colors can occur in high-level cirrus clouds or in mid-level altocumulus clouds or in some polar stratospheric clouds known as nacreous clouds or mother-of-pearl clouds. The color patterns tend to be unordered or chaotic, with a pastel look to them. Image credit: C Messier - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14697774
	Circumhorizontal arc Circumhorizontal arcs are visually similar in some ways to cloud iridescence, but are in fact part of the halo family. Their color patterns are ordered, in approximately horizontal bands, with red at the top and colors changing downward to blue at the bottom. The misleading term "fire rainbow" is sometimes used for this phenomenon.
	Atmospheric refraction effects and mirages (related to unusual vertical temperature gradients near the surface) If the surface is very hot and the temperature decreases rapidly upward (e.g. above asphalt on a hot day under the hot sun of the desert) then light curves upward and an observer may see a mirage of apparent water (actually light from the sky) on the ground. Conversely, in a strong inversion in which the temperature increases rapidly with height (this is common in polar areas), light is bent downward in varying ways and amounts so that an observer may see misleading images of distant objects (larger, smaller, stretched, compressed, floating or distorted). The Fata Morgana is one example of such an effect.
	Airglow Airglow (also called nightglow) is the faint emission of light by the upper atmosphere. The phenomenon was first identified in 1868 by Swedish physicist Anders Ångström. Airglow is caused by various upper atmospheric processes, such as the recombination of atoms which were photoionized by the Sun during the day, luminescence caused by cosmic rays striking the upper atmosphere, and chemiluminescence caused mainly by oxygen and nitrogen reacting with hydroxyl free radicals at heights of a few hundred kilometres. Airglow is visible only at night.

Atmospheric Optical Phenomena not included in this page

Sky Color

The usual blue sky color (or the red/orange sky when the sun is very low) can be considered as an atmospheric optical phenomenon. It is caused by Rayleigh scattering of sunlight by air molecules. Horace de Saussure invented the cyanometer, an instrument that measures the blueness of the sky. Clouds have different colors as well: usually white in sunlight, they can appear grey by contrast where sunlight does not strike them, and red or orange in the light of the rising or setting Sun. Philatelic items that depict sky color and cloud color, despite their potential beauty, are not included in this page. However, items that mention sky color or cloud color in the context of scientific observations and research are included.

Lightning

Lightning can be considered as a type of atmospheric optical phenomenon, but it does not fall under the definition given above. Selected philatelic items referring to lightning are included in the Thunderstorms and Lightning page. No attempt will be made to provide a complete listing of the large number of such items.

Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights)

The Earth's auroras fall under the second part of the definition above. Selected philatelic items referring to the auroras as seen either from the Earth's surface or from space are found in the Auroras page. No attempt will be made to provide a complete listing of the large number of such items. Auroras are also related to the topic of space weather. In that page a few aurora items are included: those showing auroras as seen from space or for which the reference to them is secondary to the space weather topic itself.

Rainbows

Part 1 of the general definition of "Atmospheric Optical Phenomena" (near the top of this page) includes rainbows. Selected philatelic items referring to rainbows are found in the Rainbows page, in which we include only a small sample of all rainbow-related items (either those which also refer to an independent meteorological topic, or a few others that strike our fancy). No attempt will be made to provide a complete listing of the large number of such items.

Below is a list of Atmospheric optical phenomena on postal items (stamps, souvenir sheets, aerogrammes, postal cards, etc.).

Below is a list of Atmospheric optical phenomena on launch covers (including anniversary-of-launch covers, and launch-related event covers).

¹ATLAS: ATmospheric Laboratory for Applications and Science

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