Real Crystals

Last updated on September 16, 2023

Real Crystals: Unveiling the Secrets of Atmospheric Optics

Have you ever gazed up at the sky and been captivated by the mesmerizing halo displays? These ethereal phenomena, caused by the interaction of light with ice crystals in the atmosphere, create a spectacle that never fails to leave us in awe. However, not all cirrus clouds produce halo displays. Why is that? Let's delve into the fascinating world of real crystals and explore the factors that determine the formation of these enchanting optical wonders.

When it comes to halo displays, the size, alignment, and optical quality of ice crystals play a crucial role. Crystals smaller than approximately 0.01 mm (10 microns) diffract light significantly, resulting in weak and diffuse halos. On the other hand, crystals larger than 0.05 mm refract and reflect light more efficiently, giving rise to more distinct halos. The sharpest and most vivid halos occur when the crystals have precise alignments, which typically requires crystals larger than about 0.1 mm.

Interestingly, crystal size alone does not guarantee optimal halo formation. Very large crystals measuring 1 mm or more are rarely of sufficient optical quality, as they tend to possess defects that diminish the clarity of their halos. Moreover, these larger crystals have a higher tendency to wobble and lose their alignments, further compromising the formation of well-defined halo displays.

Now, let's take a closer look at two sets of crystals collected during different halo displays to understand their distinct characteristics:

1. Crystals from a Superb South Pole Display (17th January 1986): The crystals obtained during this display closely resemble their ideal hexagonal plate and column shapes, with minimal air bubble inclusions. These crystals exemplify the optimal conditions required for creating stunning halo displays.

2. Crystals from a Mediocre Halo Display (16 days earlier): In contrast to the crystals from the superb South Pole display, the crystals collected during this event exhibit larger inclusions and imperfect faces. These imperfections significantly affect the optical quality of the crystals, resulting in less impressive halo displays.

It is important to note that not all cirrus clouds possess the necessary ice crystals to produce halo displays. In certain atmospheric conditions, the ice crystals present may not meet the criteria for generating well-defined halos. However, it is always worth checking the skies, as perfect conditions can occasionally align, offering a glimpse into the enchanting world of atmospheric optics.

To summarize the key points about real crystals and halo formation:

• Crystal size plays a vital role in determining the quality of halo displays. Crystals larger than 0.05 mm are more effective at refracting and reflecting light, resulting in clearer halos.
• The alignment of ice crystals is crucial for creating sharp and well-defined halos. Crystals larger than approximately 0.1 mm tend to have more precise alignments.
• Very large crystals measuring 1 mm or more often lack sufficient optical quality and are prone to defects that diminish their halo displays.
• Imperfections in crystal faces and the presence of inclusions can significantly impact the optical quality of ice crystals, affecting the clarity of halo displays.
• Halo displays are not guaranteed with every cirrus cloud. The size, alignment, and optical quality of ice crystals must align perfectly to create captivating halo phenomena.

Next time you find yourself gazing at the sky, remember that the beauty of halo displays lies in the intricate properties of real crystals. Each crystal has its own story to tell, revealing a symphony of light and ice suspended in our atmosphere.

Crystals collected during a superb South Pole display on 17th January 1986. Apart from a few small air bubble inclusions, the crystals really are like their hexagonal plate and column ideals. Crystals from a mediocre halo display 16 days earlier. They have large inclusions and their faces are imperfect. Photographs from 'Atmospheric Halos by Walter Tape' reproduced with permission.Not all cirrus clouds produce halo displays. Why is this? When their ice crystals are smaller than ~0.01 mm (10 micron) light is significantly diffracted and any halos are weak and diffuse. Crystals larger than 0.05 mm refract and reflect light cleanly to produce halos. The sharpest halos occur when the crystals have precise alignments and this needs crystals larger than about 0.1 mm. Very large crystals of 1 mm or more are rarely of sufficient optical quality and have defects which spoil their halos. They also have an increasing tendency to wobble and lose their alignments. So the crystal sizes, alignments and optical qualities are sometimes not right to give good halos. But always check the skies for sometimes the conditions are perfect!

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Reference Atmospheric Optics

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