Pillars & Columns

Last updated on September 16, 2023

Pillars & Columns: Exploring the Phenomenon of Atmospheric Optics

Atmospheric optics is a fascinating field that encompasses a range of optical phenomena occurring in the Earth's atmosphere. One such phenomenon is the formation of pillars and columns, which can create stunning visual displays in the sky. In this article, we will delve into the intricacies of pillars and columns, exploring their formation, characteristics, and the science behind their mesmerizing appearance.

The Beauty of Pillars and Columns

Pillars and columns are optical phenomena that occur when light interacts with ice crystals suspended in the atmosphere. These crystals act as tiny prisms, bending and reflecting light to create various optical effects. Pillars appear as vertical shafts of light extending above or below the light source, while columns manifest as vertical beams of light extending from the horizon. These ethereal displays can be observed in diverse weather conditions, adding an enchanting touch to the natural world.

Unraveling the Mystery

To understand the formation of pillars and columns, it is crucial to examine the characteristics of the ice crystals involved. Most pillars are created by gently tilted plate crystals, which reflect light in a glinting manner. However, there are instances where column crystals, lacking permanently near-horizontal faces, produce pillars. This raises the perplexing question: How can column crystals without such faces generate these mesmerizing vertical shafts of light?

The Role of Column Side Faces

In the case of column crystals, it is the side faces that play a pivotal role in pillar formation. These crystals have rotational positions about two axes: one that is near vertical and another that is near horizontal along the length of the column. When visualizing this, imagine a strong red arrow perpendicular to one of the side faces. The closer the arrow tip is to the zenith (the point directly overhead), the more horizontal the face becomes, increasing the likelihood of sunlight glinting into a pillar.

The Dance of Crystal Rotations

As column crystals rotate about the vertical axis, the arrow tip sweeps out circles, known as lines of latitude, centered on the zenith. Simultaneously, as the crystal rotates about its long axis, the tip sweeps out lines of longitude. By simulating the rotational positions of each axis, we can observe the movement of the arrow tip and gain insights into pillar formation. Surprisingly, these simulations reveal that the tip spends more time at the zenith than anywhere else, suggesting that a side face spends more time nearly horizontal than in any other position.

Mirror-Like Reflections

When a side face of a column crystal reaches a near-horizontal position, it acts as a mirror-like surface that reflects sunlight. This reflection results in the formation of a sun pillar, adding to the beauty and complexity of atmospheric optics. It is not just columns that can create pillars; Lowitz-oriented crystals can also produce them through the same mechanism.

Conclusion

Pillars and columns are captivating atmospheric optical phenomena that offer a glimpse into the intricacies of light interaction with ice crystals in the atmosphere. Through a deeper understanding of the rotational positions of column crystals and the role of their side faces, we can appreciate the science behind these awe-inspiring displays. The next time you witness a pillar or column in the sky, take a moment to marvel at the dance of light and ice, reminding us of the boundless wonders of our natural world.

Sun Pillar at Oslo, Norway by Steinar Midtskogen December 3, '08. ©Steinar Midtskogen, shown with permission.

Steinar produces daily time-lapse movies of the Oslo sky and these images are individual frames. The video is here.

The temperature was -3C and there was light snow before an ice fog which later cleared.

The pillar is unusually tall. It stretches up almost to the bright but diffuse upper tangent arc 22° from the sun.

No sundogs are visible in the movie. This combined with the tangent arc and pillar's abnormal height suggest that it might have been produced by horizontal column crystals rather than plates.

The majority of pillars are the collective glints from gently tilted plate crystals. How can column crystals with no permanently near horizontal faces produce a pillar?

The column side faces are responsible. The diagram below shows how a column crystal takes rotational positions about two axes (1) near vertical and (2) a near horizontal one lengthways through the column.

Now visualize the strong red arrow perpendicular to one of the side faces. The closer the arrow tip is to the zenith the more horizontal is the face and the more likely will it glint sunlight into a pillar.

As the crystal rotates about the vertical axis the tip sweeps out circles (lines of latitude) centred on the zenith. As the crystal rotates about its long axis the tip sweeps out lines of longitude.

We can simulate the tip positions by drawing a dot for random rotational positions of each axis.

The diagram has 7000 points computed in this way. They give a surprise, they show that the combination of the two motions leads to the tip spending more time at the zenith than anywhere else.

The corollary is that a side face spends more time nearly horizontal than at any other position.

Each crystal side face acts for a moment as a near horizontal mirror and glints the sun to form a sun pillar.

Columns can make pillars. Lowitz oriented crystals also produce them by the same mechanism.

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Reference Atmospheric Optics

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