Why infrequent?

Last updated on September 16, 2023

Why are Some Halos Infrequent?

Halos, the beautiful optical phenomena that occur in the Earth's atmosphere, can vary in frequency. While some halos, like the 22° halo, can be observed on about 100 days each year in Europe, others are much more infrequent. In this article, we will explore the reasons behind the infrequency of certain halos, particularly focusing on the 46° halo.

Crystal Quality and Imperfections

One of the main factors contributing to the infrequency of halos is the quality of the ice crystals that form in the atmosphere. Cloud crystals rarely achieve perfect clarity, as they often contain inclusions or imperfections. These inclusions can extend out to and damage the crystal faces, making them less suitable for halo formation. Additionally, crystal faces themselves can be imperfect, further hindering the formation of specific halos.

Alignment and Crystal Size

Precise alignment of ice crystals is crucial for the formation of many halos. Larger crystals, measuring 0.1mm or more, tend to align better but are also more prone to imperfections. This means that even if larger crystals are present, their imperfections may prevent the formation of certain halos. The alignment of crystals plays a significant role in determining the visibility and distinctiveness of halos.

Size and Color Dispersion

Some halos are inherently faint due to their large size and dispersed colors. The 46° halo, for example, is both larger and has more dispersed colors than the 22° halo. As a result, it is naturally fainter, even if the crystal faces were perfect. The size and dispersion of colors within a halo can greatly affect its visibility.

Effects of Imperfect Alignment

The alignment of ice crystals not only affects the formation of halos but also influences their appearance. When crystal alignment is near perfection, halos like the upper tangent and Parry arcs appear clear and distinct. However, as crystal alignment becomes poorer, the visibility and clarity of these halos diminish. Even a slight tilt of the crystals can significantly impact the visibility of specific halos.

Factors Affecting Halo Visibility

Apart from crystal quality and alignment, other factors can affect the visibility of halos and contribute to their infrequency. These factors include location, season, and the observer's ability to see a large portion of the sky. Certain halos may be more common in specific regions or during certain seasons, while others may require dedicated sky watching to catch a glimpse.

The Fascination of Rare Halos

Despite their infrequency, rare halos continue to captivate sky watchers and researchers alike. Their elusive nature adds to their allure, encouraging dedicated observers to search for them whenever cirrus clouds are present. By understanding the factors that contribute to their infrequency, we can appreciate the rarity and beauty of these optical phenomena.

In conclusion, the infrequency of certain halos can be attributed to factors such as crystal quality, alignment, crystal size, and the dispersion of colors within the halo. Imperfections in ice crystals can impair halo clarity, while the size and dispersion of colors can make certain halos inherently faint. Crystal alignment plays a crucial role in determining the visibility and distinctiveness of halos. Factors like location, season, and sky visibility also affect the frequency of halo sightings. Despite their infrequency, rare halos continue to captivate our fascination with their beauty and mystery.

How infrequent are some halos?

The relative frequencies of halo sightings at right were calculated from 10 years of observations by the German Halo Research Group. In Europe a 22� halo occurs on about 100 days each year so dedicated sky watching might catch a Parry arc on average once a year.

Visibility also depends on location, season and how much of the sky can be seen. Whatever, always search for rare halos whenever there are cirrus clouds.

Why are some halos infrequent? Why is the 46� halo seen much less often than the 22�?

Crystal quality. Cloud crystals rarely attain gemlike perfection. They have inclusions which impair halo clarity. Sometimes the inclusions extend out to and damage crystal faces. Sometimes the faces are imperfect anyway. These faults tend to impair particular halos. For example, column crystal end faces are most often imperfect thus blocking rays which would otherwise form the 46� halo, infralateral and supralateral arcs.

Alignment & crystal size. Precise crystal alignment is needed for many halos. Large crystals (0.1mm or larger) are the best aligned but these are the most prone to imperfections.

Large halos & dispersed colours. Some halos are so large and their colours so dispersed that they are inherently faint. Even if column crystals had perfect end faces the 46� halo would always be at least six times fainter than the 22�.

The effects of imperfect alignment. At top right clear and distinct upper tangent and Parry arcs are formed by column crystals with tilts of only 0.1� std dev.

As the crystal alignment becomes poorer the Parry arc becomes less and less distinct. With tilts over 1� it is hardly visible and even the Upper Tangent Arc is mediocre.

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