A 22° halo is an atmospheric
optical phenomenon
Optical phenomena are any observable events that result from the interaction of light and matter.
All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the Sun or Moon with ...
that consists of a
halo
HALO, halo, halos or haloes may refer to:
Most common meanings
* Halo (optical phenomenon)
* Halo (religious iconography), a ring of light around the image of a head
* ''Halo'' (franchise), a sci-fi video game series (2001–2021)
Arts and en ...
with an
apparent radius of approximately 22° around the
Sun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
or
Moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
. Around the Sun, it may also be called a sun halo. Around the Moon, it is also known as a moon ring, storm ring, or winter halo. It forms as
sunlight
Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...
or
moonlight
Moonlight consists of mostly sunlight (with little earthlight) reflected from the parts of the Moon's surface where the Sun's light strikes.
History
The ancient Greek philosopher Anaxagoras was aware that "''the sun provides the moon with its ...
is
refracted
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
by millions of
hexagonal
In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°.
Regular hexagon
A regular hexagon is d ...
ice crystals
Ice crystals are solid water (known as ice) in crystal structure, symmetrical shapes including hexagonal crystal family, hexagonal columns, hexagonal plates, and dendrite (crystal), dendritic crystals. Ice crystals are responsible for various at ...
suspended in the atmosphere. Its radius, as viewed from Earth, is roughly the length of an outstretched hand at arm's length.
Formation

Even though it is one of the most common types of halo, the shape and orientation of the ice crystals responsible for the 22° halo are the topic of debate. Hexagonal, randomly oriented columns are usually put forward as the most likely candidate, but this explanation presents problems, such as the fact that the aerodynamic properties of such crystals leads them to be oriented horizontally rather than randomly. Alternative explanations include the involvement of clusters of bullet-shaped ice columns.
[ ]
As light passes through the 60° apex angle of the hexagonal ice
prisms, it is deflected twice, resulting in deviation angles ranging from 22° to 50°. Given the
angle of incidence onto the hexagonal ice prism
and the
refractive index
In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
inside the prism
, then the angle of deviation
can be derived from
Snell's law
Snell's law (also known as the Snell–Descartes law, the ibn-Sahl law, and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing th ...
:
:
For
= 1.309, the angle of
minimum deviation
In a Prism (optics), prism, the angle of deviation () decreases with increase in the angle of incidence () up to a particular angle. This angle of incidence where the angle of deviation in a prism is minimum is called the minimum deviation positio ...
is almost 22° (21.76°, when
= 40.88°). More specifically, the angle of minimum deviation is 21.84° on average (
= 1.31); 21.54° for red light (
= 1.306) and 22.37° for blue light (
= 1.317). This
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish.
The ice crystals in the clouds all deviate the light similarly, but only the ones from the specific ring at 22 degrees contribute to the effect for an observer at a set distance. As no light is refracted at angles smaller than 22°, the sky is darker inside the halo.
Another way to intuitively understand the formation of the 22° halo is to consider the following logic:
* All rays from the Sun/Moon are incoming in a parallel manner towards the observer.
* We can consider a specific case when the source is right on top of the sky.
* Hexagonal water crystals can take on any orientation. But any rotation beyond 30° would be redundant when analyzing the angles subtended by the emerging rays. This means that for all the incoming vertical rays, we only need to consider incident angles in the range 30° to 60° that are incumbent on one edge of the hexagonal crystal; these are the ones that will reach the observer.
* For the above range of incident angles, we can find the angle of the outgoing ray with respect to the vertical—which in fact is the angle subtended at the eye of the observer.
* Outgoing ray angles (in the graphs on the right in the figure below) were obtained from the equation at the bottom. For a majority of rotation angles, the average of outgoing ray angles for red hovers around 22° and is slightly higher for blue.

Angle of rotation =
:
Another phenomenon resulting in a ring around the Sun or Moon—and therefore sometimes confused with the 22° halo—is the
corona. Unlike the 22° halo, however, it is produced by water droplets instead of ice crystals and is much smaller and more colorful.
Weather relation
In folklore, moon rings are said to warn of approaching storms.
Like other ice halos, 22° halos appear when the sky is covered by thin
cirrus or
cirrostratus cloud
Cirrostratus () is a high-altitude, very thin, and generally uniform stratiform genus-type of cloud. It is composed of ice crystals, which are particles of frozen water. Cirrostratus is difficult to see and can produce halos. These optical ...
s that often come a few days before a large storm front.
However, the same clouds can also occur without any associated weather change, making a 22° halo unreliable as a sign of bad weather.
See also
*
46° halo
*
Circumzenithal arc
*
Circumhorizontal arc
*
Sun dog
A sun dog (or sundog) or mock sun, also called a parhelion (plural parhelia) in atmospheric science, is an atmospheric optical phenomenon that consists of a bright spot to one or both sides of the Sun. Two sun dogs often flank the Sun within ...
*
Parhelic circle
*
Moon dog
*
Moonbow
A moonbow (also known as a moon rainbow or lunar rainbow) is a rainbow produced by moonlight rather than direct sunlight. Other than the difference in the light source, its formation is the same as for a solar rainbow: It is caused by the refracti ...
References
{{DEFAULTSORT:22 Halo
Atmospheric optical phenomena