Halo Atmospheric Phenomenon

The Halo phenomenon is said to be a rare phenomenon but not an unheard one. It is one of the beautiful phenomena of the atmosphere that is worth observing. At different locations and at different times, this phenomenon has been observed by people. It occurred in the sky because of the ice crystals present in the clouds and due to the reflection or refraction of light through these crystals. It can be formed in various forms among which the circular type phenomenon is the most common. In this article, we will learn about the halo atmospheric phenomenon. We will cover what is halo, its definition and various types of halo light phenomena.

What is Halo?

Halo is a kind of optical atmospheric phenomenon which occurs in the sky when the Sun or the Moon shines in the thin clouds which are composed of ice crystals. It occurred because either because of the refraction of light which passes from these ice crystals or due to the reflection of light from these crystal faces. It can occur because of the combination of both the effects. The refraction phenomena lead to the separation of colours whereas the reflection is said to be whitish in colour due to the incident light which is not broken up into component colours. Various kinds of halo can occur in the atmosphere. 22° halo is considered as the most common one whereas tangent arc, sun pillars, parhelia, etc are considered as those which occurred less frequently and others occur with respect to reflection as well as refraction of light of the crystals of the Sun and the Moon.

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Definition

We Can Define Halo in the Following Ways:

• According to the World Meteorological Organisation, " A group of optical phenomena in the form of rings, arcs, pillars or bright spots, produced by the refraction or reflection of light by ice crystals suspended in the atmosphere (cirriform clouds, diamond dust, etc.)."

• "Ice crystals lead to the formation of a halo of light because of reflection or refraction or combination of both the effects." - General definition.

Types of Halo Atmospheric Phenomenon

We Have Read What is a Halo and Its Various Types are Mentioned below:

22° Halo

• The small halo is known as the 22° halo and it is often observed.

• It is white in colour and there is a white luminous ring of 22° around the sun and moon which is situated in the centre.

• It has a red border on the inside and sometimes it shows a violet border on the outside.

• The part of the sky which is in the ring is quite darker than the whole of the sky.

• It always has a 22° radius and it is also known as the Sun Halo effect or Sun Halo.

• On some days, the complete circle may be visible depending on the amount of cirriform cloud.

•  We can get to know about the radius if we use our hand and we see if the approximate angle between the thumb and little finger, with an outstretched finger and arms spread, is 20°. 

• If the thumb is seen over the sun then the 22° halo will be found near the tip of the little finger.

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46° Halo

• The large halo is known as the 46° halo.

• It is circular in shape and has a radius of 46°.

• Sometimes it is observed.

• It is a common halo that is usually less bright than the 22° halo.

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Luminous Pillar or Light Pillar

• It is a luminous pillar generally white in colour which is shown as a broken or continuous trail of light.

• It is also known as a light pillar.

• It can be seen above and below the sun and moon.

• Pillars of light can be seen occasionally above the light sources but can be seen below the source of light as well.

• Small light pillars may be seen extending above and below a bright planet such as venus.

• When the sun is the light source then the phenomenon is known as a sun pillar.

• There are two types of pillars. One is the upper pillar which means when a pillar is above the light source and the other is the lower pillar which means when the pillar is below the light source.

• From mountains, hills and or an aircraft, light pillars can be seen.

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Tangent Arc

• Tangent Arc is the form of halo which is made from many types of luminous arcs. It forms tangentially to other halos.

• It can be seen from outside on 22° and 46° halo.

• These arcs touch the circular halo at its highest and lowest points.

• The arcs vary from the angular elevation of the sun and moon.

• They are frequently short and seen as a dark spot.

• The tangent arc can be seen as a narrow v shape when the light source is above the horizon.

• When the sun and moon rise in the sky, the v shape of the upper tangent arc resembles the outstretched wings of a light bird.

• The lower tangent arc can be seen below the horizon when the sun and moon are within 22° of the horizon. It can be seen from a high location.

• Like mountains, aircraft -deeper cirrus clouds.

• The v shape of the lower tangent arc narrows and then broadens as the upgrading of the light source increases.

• Circumscribed halo is the halo which is formed when the sun and moon reach 32° above the horizon and upper and lower arcs are linked with each other.

• The oval halo which is rough is outside the 22° and touches at the high and lower points.

• Sometimes the both- upper and lower points are visible.

• When the sun and moon rise above 50° then the circumscribed halo forms circular.

• It is generally brighter and has pure colours.

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Circumzenithal Arc

• It is also known as the upper circumzenithal arc.

• It is near its zenith and has a colourful half circle. It is at 48° above the sun.

• It has bright colours and red is at its lower side, outside of the arc and violet on the upper inside.

• This arc is only shown when the elevation of the light source is less than 32°.

• It has a small radius.

• If the radius decreases then the arc will steadily enlarge.

• The circumzenithal arc touches the 46° halo and it is visible if the angular elevation of the light source is about 22°.

• These two features are separated with a light source ahead from 22°

• The arc may also be visible if the 46° halo is not present.

• The circumzenithal arc will be visible if the elevation of the sun is between 15° and 25° or if there are parhelia because both are caused by the plate-shaped ice crystals that occur horizontally.

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Circumhorizontal Arc

• This arc is known as the lower circumhorizontal arc. It is a near horizon and extends parallel to the horizon.

• This light phenomena has a colourful & bright circumzenithal arc.

• It occurs only when the light source is more than 58°.

• When the sun's elevation is about 68° then the circumhorizontal arc reaches its maximum intensity.

• It is one of those halos which are not seen everywhere on the earth. In the north and south, the sun is always lower than 58°. That's why they are not seen at these places.

• The circumhorizontal arc touches the lower part of the 46° halo if visible when the angular elevation of the light source is about 68°.

• The two halos are increasingly separated with a light source further from 46°. 

• The arc is visible if the 46° halo is not visible.

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Parhelic Circle

• It is a white and horizontal circle, the same as the sun.

• Some of the bright spots are seen at certain points.

• These halos occur outside the 22° halo.

• Bright spots are seen at an azimuthal distance of 120° from the sun and opposite the sun.

• The parhelia or the anthelion are bright then they are often called mock suns.

• The phenomenon which is produced by the moon is called the paraselenic circle and anti selene and when they are sometimes bright then these are called mock moons.

• Parhelia and paraselenae are connected with the 22° halo by obliquely oriented arcs of lowitz arcs.

• The group of the rare arcs that occur outside the 22° halo is known as the lowitz arcs.

• It can be only seen when the sun's elevation is high.

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Subsun or Undersun

• With its name, it is clear that it forms below the Sun and it forms vertically.

• It is formed because of the reflection of the light from the Sun on the plate-shaped crystals. 

• It looks like a white spot that forms below the Sun.

• It can only be observed when looking downwards such as from the aeroplane or in the mountains in the winters. 

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Supralateral Arc

• This arc is said to be large as well as rainbow-coloured in nature.

• It forms when the source of light is below 32° of elevation.

• This arc is said to appear as a fragment of the circle as well as lies above the parhelic circle's elevation.

• Sometimes this arc can be mistaken for a 46° halo. This arc is brighter as well as more colourful than the 46° halo.

• Its shape is said to vary with respect to the elevation of the source of light.

• The apex of this arc is seen to be near as well as touching the circumzenithal arc. 

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Infralateral Arc

• These are an example of rare and coloured pairs of arcs which are outside as well as a tangent to 46° halo.

• This arc is found always below the parhelic circle.

• The shape of the arc can be changed with respect to the elevation of the source of light.

• The contact points at 46° halo due to an increase in the elevation of the Sun move downwards and then it forms a common arc.

• At 68° elevation, this arc touches 46° halo below the Sun vertically. 

• The vertex of the arc lies below the 46° halo when the sun gets higher.

• This arc can be bright as well as colourful like a circumzenithal arc.

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Other Halo

Besides these mentioned halos various other rare halo phenomena can also be seen or observed in the sky occasionally. For example, Parry arc, tape arcs or 46° tape arcs, anthlic arcs, helic arcs and subhelic arcs, moilanen arc, etc. Haloes on pyramidal ice crystals with different radii such as 9°, 18°, 20°, 23°, 24°, 35° as well as other forms of tangent or parroid arcs and elliptical halo, etc. can also be observed occasionally.

Conclusion

To sum up, at the end of the discussion about the halo light phenomena or what is halo, we can say that it is one of the most beautiful phenomena that can be observed in the sky due to the presence of the Moon and the Sun and due to their shining in the sky which basically occurred because of the reflection or refraction of light. Several kinds of such halo optical phenomena can be seen that we have covered in this article such as 22° halo, 46° halo, etc. This article will help you to learn about one of the atmospheric phenomena that help in Geography, Earth Sciences, Climatology, Geophysics, etc.