A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon

Optical phenomenon

An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....

 produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky. They can also form around artificial lights in very cold weather when ice crystals called diamond dust

Diamond dust

Diamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ice crystals and is reported in the METAR code as IC. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as...

 are floating in the nearby air.
There are many types of ice halos. They are produced by the ice crystals in cirrus cloud

Cirrus cloud

Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...

s high (5–10 km, or 3–6 miles) in the upper troposphere

Troposphere

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

. The particular shape and orientation of the crystals is responsible for the type of halo observed. Light

Light

Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 is reflected

Reflection (physics)

Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

 and refracted

Refraction

Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

 by the ice crystals

Ice crystals

Ice crystals are a small crystalline form of ice including hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. The highly symmetric shapes are due to depositional growth, namely, direct deposition of water vapour onto the ice crystal...

 and may split up into color

Color

Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...

s because of dispersion

Dispersion (optics)

In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...

. The crystals behave like prism

Prism (optics)

In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...

s and mirror

Mirror

A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

s, refracting and reflecting sunlight between their faces, sending shafts of light in particular directions.

Atmospheric

Earth's atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 phenomena such as halos were used as part of weather lore as an empirical

Empirical

The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....

 means of weather forecasting

Weather forecasting

Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Human beings have attempted to predict the weather informally for millennia, and formally since the nineteenth century...

 before meteorology

Meteorology

Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

 was developed.

Other common optical phenomena involving water droplets rather than ice crystals include the
glory

Glory (optical phenomenon)

A glory is an optical phenomenon that resembles an iconic saint's halo about the shadow of the observer's head. The effect is produced by light backscattered towards its source by a cloud of uniformly sized water droplets...

 and the rainbow

Rainbow

A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...

.

Light pillar

A light pillar, or sun pillar, appears as a vertical pillar or column of light rising from the sun near sunset or sunrise, though it can appear below the sun, particularly if the observer is at a high elevation or altitude. Hexagonal plate- and column-shaped ice crystals cause the phenomenon. It is often caused by water droplets in the sky. Plate crystals generally cause pillars only when the sun is within 6 degrees of the horizon, or below it; column crystals can cause a pillar when the sun is as high as 20 degrees above the horizon. The crystals tend to orient themselves near-horizontally as they fall or float through the air, and the width and visibility of a sun pillar depends on crystal alignment.

Light pillars can also form around the moon, and around street lights or other bright lights. Pillars forming from ground-based light sources may appear much taller than those associated with the sun or moon. Since the observer is closer to the light source, crystal orientation matters less in the formation of these pillars.

Circular halo

A diffraction

Diffraction

Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

 disc or Airy disc

Airy disc

In optics, the Airy disk and Airy pattern are descriptions of the best focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light....

 has similar appearance, but is a disk, rather than a ring, and has a red border on the inside. Its size depends on the size of the ice or water particles that cause it. These are also known as coronas

Corona (meteorology)

In meteorology, a corona is produced by the diffraction of light from either the Sun or the Moon by individual small water droplets of a cloud....

, but are not to be confused with the thin streaming luminous gas that makes up the sun's own corona

Corona

A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...

.

See also

• 120° parhelion
  120° parhelion
  A 120° parhelion is a relatively rare halo, an optical phenomenon occasionally appearing along with very bright sun dogs as ice crystal-saturated cirrus clouds fill the atmosphere...
  
  
• 22° halo
  22° halo
  A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere...
  
  
• 46° halo
  46° halo
  A 46° halo is a rare optical halo centred on the sun. At sun elevations between 15-27°, it is often confused with the more colourful and frequently observed supralateral and infralateral arcs. It is named for crossing the parhelic circle 46° from the sun....
  
  
• Anthelion
  Anthelion
  An anthelion is a rare optical phenomenon appearing on the parhelic circle opposite to the sun as a faint white halo, not unlike a sundog.How anthelions are formed is disputed...
  
  
• Atmospheric optics
  Atmospheric optics
  Atmospheric optics deals with how the unique optical properties of the Earth's atmosphere cause a wide range of spectacular optical phenomena. The blue color of the sky is a direct result of Rayleigh scattering which redirects higher frequency sunlight back into the field of view of the observer...
  
  
• Circumhorizontal arc
  Circumhorizontal arc
  A circumhorizontal arc is an optical phenomenon - an ice-halo formed by plate-shaped ice crystals in high level cirrus clouds.The current accepted names are circumhorizon arc or lower symmetric 46° plate arc The complete halo is a huge, multi-coloured band running parallel to the horizon with its...
  
  
• Circumscribed halo
  Circumscribed halo
  A circumscribed halo is a type of halo, an optical phenomenon that circumscribes a related phenomenon, the 22° halo centred on the sun.When observed, a circumscribed halo is normally oval in shape and, at the points directly below and above the sun, lies tangential to the 22° halo...
  
  
• Circumzenithal arc
  Circumzenithal arc
  The circumzenithal arc or circumzenith arc , also called the Bravais' arc, is an optical phenomenon similar in appearance to a rainbow; but it arises from refraction of sunlight through horizontally-oriented ice crystals, generally in cirrus clouds, rather than from raindrops...
  
  
• Corona
  Corona (meteorology)
  In meteorology, a corona is produced by the diffraction of light from either the Sun or the Moon by individual small water droplets of a cloud....
  
  

• Diamond dust
  Diamond dust
  Diamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ice crystals and is reported in the METAR code as IC. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as...
  
  
• False sunrise
  False sunrise
  A false sunrise or dawn sundog is a very particular kind of parhelion, belonging to the optical phenomenon family of halos.It is an atmospheric optical phenomenon associated with the reflection or refraction of sunlight by small ice crystals making up cirrus or cirrostratus clouds in the very...
  
  
• Glory
  Glory (optical phenomenon)
  A glory is an optical phenomenon that resembles an iconic saint's halo about the shadow of the observer's head. The effect is produced by light backscattered towards its source by a cloud of uniformly sized water droplets...
  
  
• Heiligenschein
  Heiligenschein
  Heiligenschein is an optical phenomenon which creates a bright spot around the shadow of the viewer's head. It is created when the surface on which the shadow falls has special optical characteristics. Dewy grass is known to exhibit these characteristics, and creates a Heiligenschein...
  
  
• Infralateral arc
  Infralateral arc
  An infralateral arc is a rare halo, an optical phenomenon appearing similar to a rainbow under a white parhelic circle...
  
  
• Kern arc
  Kern arc
  The Kern arc is the extremely rare atmospheric optical phenomenon related to the circumzenithal arc. It is a complete and faint circle around the zenith. In contrast, the related and much more common circumzenithal arc is only ever a partial circle....
  
  
• Light pillar
  Light pillar
  A light pillar is a visual phenomenon created by the reflection of light from ice crystals with near horizontal parallel planar surfaces. The light can come from the Sun in which case the phenomenon is called a sun pillar or solar pillar...
  
  
• Liljequist parhelion
  Liljequist parhelion
  A Liljequist parhelion is a rare halo, an optical phenomenon appearing on the parhelic circle approximately ±150-160° from the sun between a 120° parhelion and the anthelion ....
  
  
• Lower tangent arc
  Lower tangent arc
  A lower tangent arc is a rarely observable halo, an optical phenomenon appearing under and tangent to a 22° halo centred around the sun.Just like upper tangent arcs, the shape of a lower tangent arc is dependent on the altitude of the sun. As the sun slips over the horizon the lower tangent arc...
  
  

• Parhelic circle
  Parhelic circle
  A parhelic circle is a halo, an optical phenomenon appearing as a horizontal white line on the same altitude as the sun, or occasionally the Moon. If complete, it stretches all around the sky, but more commonly it only appears in sections....
  
  
• Parry arc
  Parry arc
  A Parry arc is a rare halo, an optical phenomenon which occasionally appears over a 22° halo together with an upper tangent arc.- Discovery :...
  
  
• Rainbow
  Rainbow
  A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...
  
  
• Subparhelic circle
  Subparhelic circle
  The subparhelic circle is a rare halo, an optical phenomenon located below the horizon. It passes through both the subsun, below the sun, and the antisolar point, opposite to the sun...
  
  
• Sun dog
  Sun dog
  A sun dog or sundog, scientific name parhelion ; , also called a mock sun or a phantom sun, is an atmospheric phenomenon that creates bright spots of light in the sky, often on a luminous ring or halo on either side of the sun.Sundogs may appear as a colored patch of light to the left or right of...
  
  
• Supralateral arc
  Supralateral arc
  A supralateral arc is a rare halo, an optical phenomenon often confused with the indeed infrequently appearing 46° halo. Distinguishing between the two is furthermore difficult as supralateral arcs typically only appears in fragments while the 46° halo is very faint.In contrast to the static 46°...
  
  
• Upper tangent arc
  Upper tangent arc
  An upper tangent arc is a halo, an atmospheric optical phenomenon which appears over and tangent to the 22° halo around the sun.The shape of an upper tangent arc varies with the elevation of the sun; while the sun is low it appears as an arc over the sun forming a sharp angle...
  
  

External links

• Halo explanations and image galleries at Atmospheric Optics
• Meteoros AKM - Halo explanations and image galleries
• Halo reports of interesting halo observations around the World
• Southern Hemisphere Halo and other atmospheric phenomena
• Moon Halo Gallery
• Halo in Chisinau Moldova (photo and video
• Sun Halo appeared in Padang after the earthquake