Plate crystal sun pillars - effect of solar altitude

Last updated on September 16, 2023

Plate Crystal Sun Pillars: Exploring the Effect of Solar Altitude

When it comes to atmospheric optics, sun pillars are a fascinating phenomenon that never fails to captivate our attention. These vertical columns of light appear to extend upward from the sun, creating a mesmerizing display in the sky. However, did you know that the appearance of sun pillars can change as the sun descends? In this article, we will delve deeper into the effect of solar altitude on plate crystal sun pillars and unravel the mysteries behind their ever-changing nature.

The Dynamics of Sun Pillars

At first glance, you may notice that the lower pillar of a sun pillar is stronger than the upper pillar. This observation holds true when the sun is only a few degrees above the horizon. During this time, the sunlight reflects off the underside of plate crystals, illuminating their lower faces and creating a prominent lower pillar. On the other hand, the upper faces of these crystals are easily illuminated by sunlight, resulting in a relatively faint upper pillar.

As the sun begins its descent, something remarkable happens. The upper pillar gradually grows taller and brighter while the lower pillar remains strong. This phenomenon occurs because the slanting downward rays of the sun can only illuminate strongly tilted plate crystals, causing their lower faces to be lit. In contrast, sunlight effortlessly reaches the upper faces of these crystals, intensifying the brightness of the lower pillar.

The Enchanting Subhorizon Components

Have you ever noticed subhorizon components in nearby diamond dust or while flying in an aircraft? These subhorizon components add an enchanting touch to sun pillars. Diamond dust particles, often found near mountains or in the wake of aircraft, can reflect sunlight and contribute to the overall appearance of sun pillars. As these particles interact with the descending sun, they become visible and enhance the beauty of the phenomenon.

The Subsun: A Bright Companion

One of the most intriguing aspects of sun pillars is the presence of the subsun. The subsun is located at the same distance below the horizon as the sun is above it. This luminous point appears especially bright and adds a captivating element to the overall scene. As the sun sinks further, the upper pillar lengthens and brightens, eventually reaching its peak at sunset when both pillars are equal in strength.

The Afterglow Effect

Even after the sun has set, its illuminating power extends to the high clouds in the western sky. This afterglow effect occurs due to almost horizontal or slightly upward-going rays caused by atmospheric refraction. These rays interact with the lower crystal faces, reflecting downwards and creating a strong upper pillar. Observing a sun pillar strengthen after sunset becomes an awe-inspiring experience, especially when aided by reduced glare and skylight.

Exploring Crystal Tilts and Pillar Appearance

The height of sun pillars is influenced by the tilts of plate crystals. Large crystal tilts tend to produce taller pillars, while smaller tilts result in shorter ones. However, it's not just the size of the tilts that affects pillar appearance; the distribution of tilts also plays a crucial role. In simulations conducted using HaloSim, crystals with a uniform distribution of tilts up to ±3° from horizontal were observed, with a few crystals having tilts of ±6°. Altering the tilt distribution can lead to sun pillars of varying appearances, providing a fascinating avenue for exploration.

In conclusion, plate crystal sun pillars exhibit a captivating interplay between solar altitude and their visual characteristics. As the sun descends, the upper pillar grows taller and brighter, while the lower pillar remains strong due to the unique properties of plate crystals. The presence of subhorizon components and the subsun add further enchantment to this atmospheric optical phenomenon. Moreover, the afterglow effect and the influence of crystal tilts on pillar appearance contribute to the ever-changing nature of sun pillars. Exploring the intricacies of these phenomena allows us to appreciate the beauty and complexity of the natural world that surrounds us.

Sun Pillars change as the sun descends. At first the lower pillar is stronest but as the sun decends the upper pillar grows taller and brighter. The subhorizon componets are visible in nearby diamond dust from mountains or from aircrafts. Simulations by HaloSim: plate crystals of various tilts.., 10 million rays traced per pillar.

Upper pillars are short and faint when the sun is a few degrees high. The sun needs to reflect off the underside of plate crystals to make an upper pillar and when its rays are slanting downwards a few degrees, only strongly tilted crystals have their lower faces lit. In contrast, sunlight easily shines on upper faces and so the lower pillar is strong. Most of the lower pillar is of course beneath the horizon. The subsun - at the same distance below the horizon as the sun is above it - is especially bright.As the sun sinks, the upper pillar lengthens and brightens. At sunset both pillars are equal.After the sun has set it still illuminates high clouds in the west. These almost horizontal or even slightly upward going (from atmospheric refraction) rays illuminate the lower crystal faces. They reflect downwards to form a strong upper pillar. Watch a pillar strengthen after sunset (the reduced glare and skylight also helps make the pillar more apparent) and creep slowly northward following the sun hidden below the horizon.

** Large crystal tilts produce tall pillars but the distribution of tilts also affects pillar appearance. Crystals in the simulation had a uniform distribution of tilts up to ±3° from horizontal then tailing off so that a few crystals had tilts of ±6°. Other tilt distributions produce pillars of different appearance. Try it in HaloSim.

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Reference Atmospheric Optics

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