A false sunset or sunset sundog is a very particular kind of parhelion, belonging to the optical phenomenon family of halos

Halo (optical phenomenon)

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

.

It is an atmospheric 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....

 associated with the reflection

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 refraction

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...

 of sunlight

Sunlight

Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

 by small 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...

 making up cirrus

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...

 or cirrostratus cloud

Cirrostratus cloud

Cirrostratus clouds are thin, generally uniform clouds, composed of ice-crystals. They are difficult to detect and if capable of forming halos the cloud takes the form of thin cirrostratus nebulosus. The cloud has a fibrous texture with no haloes if it is thicker cirrostratus fibratus. On the...

s in the very particular case when the late-afternoon sun is still a few degrees above the horizon. The reflection or refraction is caused by large numbers of flat ice crystals in the high-altitude clouds. Unless disturbed by turbulence

Turbulence

In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...

, flat ice crystals such as snow

Snow

Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...

flakes will orient themselves parallel to the Earth's

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 surface as they fall. If the ice crystals are randomly oriented due to turbulence, a halo is seen; if they are oriented parallel to earth's surface, two sundogs of the same latitude as the sun is seen; if they are oriented perpendicular to the earth's surface, a lone sundog is seen below the true sun. The region of ice crystals acts as a large 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...

, creating a virtual image

Virtual image

In optics, a virtual image is an image in which the outgoing rays from a point on the object always diverge. It will appear to converge in or behind the optical device . A simple example is a flat mirror where the image of oneself is perceived at twice the distance from oneself to the mirror...

 of the sun which appears to be touching the horizon

Horizon

The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...

. Refraction happens also as sunlight enters the ice crystal, and may cause the virtual image of the sun to have a different chromacity.

The light of a false sunset has less power than that of a real sun, but sometimes is amazingly similar in its visual behaviour at distance. If the real sun is obstructed from view by clouds or other objects, the false sunset appears like a minuscule sun touching the horizon. It is similar to a subsun

Subsun

A subsun or sub-sun is a glowing spot seen within clouds when the clouds are observed from above. The subsun appears directly below the sun, as if it were caused by sunlight reflecting from a body of water. The reflection is caused by large numbers of flat ice crystals...

, because in a false sunset the sun is above the horizon and the ice crystals are below it.

See also

• 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...
  
  
• Belt of Venus
  Belt of Venus
  The Belt of Venus or Venus's Girdle is the Victorian-era name for an atmospheric phenomenon seen at sunrise and sunset. Shortly after sunset or shortly before sunrise, the observer is, or is very nearly, surrounded by a pinkish glow or antitwilight arch that extends roughly 10°–20° above the horizon...
  
  
• Crepuscular rays
  Crepuscular rays
  Crepuscular rays , in atmospheric optics, are rays of sunlight that appear to radiate from a single point in the sky. These rays, which stream through gaps in clouds or between other objects, are columns of sunlit air separated by darker cloud-shadowed regions...
  
  
• Gegenschein
  Gegenschein
  The gegenschein is a faint brightening of the night sky in the region of the antisolar point.- Explanation :Like the zodiacal light, the gegenschein is sunlight reflected by interplanetary dust...
  
  
• 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...
  
  
• Milky Way
  Milky Way
  The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
  
  
• Mirage
  Mirage
  A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky. The word comes to English via the French mirage, from the Latin mirare, meaning "to look at, to wonder at"...
  
  
• Noctilucent cloud
  Noctilucent cloud
  Night clouds or Noctilucent clouds are tenuous cloud-like phenomena that are the "ragged-edge" of a much brighter and pervasive polar cloud layer called polar mesospheric clouds in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. The name means roughly night...
  
  
• 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....
  
  
• Skyglow
  Skyglow
  Skyglow is the illumination of the night sky or parts of it. The most common cause of skyglow is artificial light that emits light pollution, which accumulates into a vast glow that can be seen from miles away and from high in the sky...
  
  
• 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 pillar
• Zodiacal light
  Zodiacal light
  Zodiacal light is a faint, roughly triangular, whitish glow seen in the night sky which appears to extend up from the vicinity of the sun along the ecliptic or zodiac. Caused by sunlight scattered by space dust in the zodiacal cloud, it is so faint that either moonlight or light pollution renders...