Is it a 46° halo or a supra/infralateral arc?

Last updated on September 16, 2023

Is it a 46° Halo or a Supra/Infralateral Arc?

Atmospheric optics is a fascinating field that encompasses a variety of optical phenomena occurring in the Earth's atmosphere. One such phenomenon is the occurrence of halos and arcs, which can create stunning displays in the sky. In this article, we will delve into the topic of differentiating between a 46° halo and a supra/infralateral arc, two optical phenomena that can sometimes be confused with each other.

Supralateral Arc and 46° Halo

Let's start by understanding the characteristics of a supralateral arc. This brightly colored arc is always seen touching the circumzenithal arc (CZA), which is a halo encircling the zenith. A supralateral arc is typically located a short distance below the CZA. In contrast, a 46° halo appears as a faint halo approximately 46 degrees away from the sun.

When observing these optical phenomena, it is essential to consider the altitude of the sun. The separation between the 46° halo and the CZA becomes apparent when the sun is less than 15° or greater than 27° high. This distinction can help in distinguishing between the two phenomena, as the supralateral arc is always connected to the CZA, while the 46° halo appears separate from it.

Differentiating Factors

Distinguishing between a 46° halo and a supra/infralateral arc can be challenging, but there are several criteria that can aid in making an accurate determination. Let's explore some of these factors:

• Sun Altitude: A supralateral arc cannot form when the sun is higher than 32°. However, a very high sun can produce infralateral and circumhorizon arcs approximately 46° below the sun. Therefore, observing the position of the halo in relation to the sun's altitude can provide valuable clues.

• Distance from the Circumzenithal Arc: A supralateral arc always touches the circumzenithal arc, while the 46° halo is located below the CZA and separated from it when the sun is at specific altitudes. By measuring the separation between the 46° halo and the CZA, it becomes possible to differentiate between the two phenomena.

• Other Halos: The presence of other halos can also serve as an indicator. A strong upper tangent arc and a weak or equal intensity 22° halo are characteristic of a supralateral arc. On the other hand, a 46° halo is often accompanied by a strong 22° halo and no (or a weak) upper tangent arc. Observing these additional halos can help in making a more informed determination.

• Cusps at Parhelic Circle: Supra/infralateral arcs intersect at the parhelic circle, whereas a 46° halo would cross it without any discontinuity. Therefore, observing the presence or absence of cusps at the parhelic circle can provide valuable insights into the nature of the optical phenomenon being observed.

• Brightness Distribution: The brightness distribution of the halo can also be indicative. While a 46° halo appears uniformly bright around its circle, a supralateral arc is brightest at its top for solar altitudes greater than 15°. However, it is essential to consider variations in cloud cover, as they can sometimes mislead in interpreting the brightness distribution.

• Colors: Another factor to consider is the coloration of the halo. A supralateral arc tends to have more saturated and brighter colors compared to a 46° halo. The presence of blues and greens in the halo is a strong indicator that it is a supralateral arc. However, it is important to note that colors alone are not definite evidence for determining the type of halo.

In conclusion, distinguishing between a 46° halo and a supra/infralateral arc can be challenging due to their similarities. However, by considering factors such as sun altitude, distance from the circumzenithal arc, presence of other halos, cusps at the parhelic circle, brightness distribution, and colors, it becomes possible to differentiate between these optical phenomena. Observing and understanding these atmospheric optics phenomena adds to the wonder and beauty of the natural world around us.

Supralateral arc + 46° halo.

The brightly coloured arc touching the circumzenithal arc is a supralateral. This arc always touches the CZA.

A short distance below it is a faint 46° halo. Mouse over the image for an enhanced view.

The 46° halo is clearly separated from the CZA when the sun is less than 15° or greater than 27° high. This can help in distinguishing between 46° and supralateral arcs.

Brian Hartmann took this image of the Fairbanks, Alaska display of 6th March 2002. The sun was 11° high. Other arcs, from base upwards, are a weak 22° halo, intense upper tangent arc and a faint Parry arc.

Image ©2002 Brian Hartmann, shown with permission.

46° halo or a supralateral/infralateral arc? The 46° halo is very rare. Most halo fragments seen two hand widths from the sun are actually supralateral or infralateral arcs. Here are some guides to help in distinguishing them from the 46° halo. Try to apply as many criteria as possible when making a decision.

Sun altitude:

When the sun is higher than 32° a supralateral arc cannot form .

However, a very high sun can produce infralateral and circumhorizon arcs approximately 46° below the sun.

The sun was high when Mitsy Krzywicki of California imaged these halos. The lower colourful halo some 46° below the sun is an infralateral or circumhorizon arc. Note the greens and blues which would not be present in a 46° halo.

The intense upper halo is a circumscribed rather than a 22° halo.

Image ©Mitsy Marx

Distance from a circumzenithal arc

A supralateral arc always touches a circumzenithal arc. The 46° halo is below the CZA and separated from it when the sun is lower than 15° or higher than 27°.

The graph shows the separation between the 46° halo and the CZA. At solar elevations of 15 to 27° it is not possible to tell the 46° halo and supralateral arc apart by this method.

Other halos:

A strong upper tangent arc and weak or equal intensity 22° halo indicates a supralateral arc. No (or a weak) upper tangent arc and a strong 22° halo indicates a 46° halo.

The 22° halo is very weak compared to the tangent arc in this diamond dust display imaged by Franck Schwitter near Crans Montana in Switzerland. This, together with the bright colours of the upper arc indicate that it was a supralateral.

Image ©Franck Schwitter

Cusps at parhelic circle

Indicate supralateral and infralateral arcs, the two arcs cross at the parhelic circle whereas the 46° halo would cross it without any discontinuity.

Intensely coloured supra- and infralateral arcs cross at the parhelic circle in this display seen in Switzerland by Alastair Adams.

Image ©Alastair Adams

Brightness distribution

The 46° is uniformly bright around its circle. A supralateral arc is brightest at its top for solar altitudes greater than 15° - but variations in clouds can mislead.

Colours:

A supralateral arc tends to have more saturated (brighter) colours than the 46° halo. The presence of blues and greens are strong indicators that the halo is a supralateral arc. Colours are not definite evidence for one or the other.

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