Have you ever looked up at the sky and witnessed a mesmerizing ring of light encircling the sun? This captivating optical phenomenon is known as a halo. Halos are formed by the collective glints of millions of tiny ice crystals suspended in the atmosphere. These crystals, with their specific orientation and position, refract sunlight to create a stunning display of light. In this article, we will delve deeper into the fascinating world of atmospheric halos and explore how these intricate patterns come to be.
The most common type of halo is the 22º halo, characterized by a bright ring located 22 degrees away from the sun. This halo is created by ice crystals whose prism axes are roughly perpendicular to the direction of sunlight. When sunlight passes through these crystals, it undergoes refraction, causing it to deviate by 22 degrees or more. Each individual crystal sparkles when it reaches this specific angular distance from the sun. The collective sparkle of millions of these crystals forms the awe-inspiring 22º halo we observe in the sky.
You might wonder how such disorderly and randomly oriented ice crystals can give rise to the structured and ordered pattern of a halo. The key lies in crystal orientation. Only those crystals with their prism axes approximately perpendicular to the sun's rays can allow light to pass through two side faces. These specific crystals refract light at an angle of 22º to 50º, forming an illuminated disk with a 22º hole at its center. The brightness of the halo is most pronounced near the hole, creating a captivating spectacle.
When poorly aligned ice crystals are scattered throughout the sky, one might wonder which crystals glint and where these glints occur. The answer lies in the angular position of the crystals relative to the sun. Crystals that are anywhere on a line 22º to 50º away from the sun, as seen by the observer, will shine light into their eyes, creating glints. These glints collectively illuminate the disk formed by the halo, resulting in its ethereal appearance. It is important to note that each observer perceives their own halo, and someone standing just a short distance away may witness a completely different halo formed by a distinct set of crystals.
While we may marvel at the halos we observe, it is intriguing to consider the millions of other ice crystals that remain unseen but potentially contribute to the formation of another person's halo. These crystals, which chance has neither placed nor aligned favorably for our view, might be helping create a spectacle for someone else. The existence of a halo relies on the presence of an observer or a receiving lens, such as a camera. Without someone to witness it, a halo remains an unperceived phenomenon.
One of the enchanting aspects of atmospheric halos is that they are highly individualistic. Each person perceives their own halo, shaped by their specific vantage point and the interplay of light with the ice crystals. Even slight variations in viewing angle or crystal alignment can result in distinct patterns and colors within the halo. This uniqueness adds to the allure of these optical phenomena, making them a source of wonder and fascination for sky gazers worldwide.
While halos are among the most well-known atmospheric optical phenomena, they are just one facet of a vast and captivating field. Atmospheric optics encompasses a myriad of awe-inspiring phenomena, including sundogs, rainbows, and iridescent clouds. Each of these phenomena offers its own enchanting display of light and color, inviting us to explore the wonders of our atmosphere and deepen our understanding of the physics behind these captivating events.
In conclusion, the disk with a hole in the sky, known as the 22º halo, is a mesmerizing manifestation of light interacting with ice crystals in the atmosphere. Through the intricate dance of crystal orientation and refraction, millions of glints come together to form this ethereal spectacle. While each observer sees their own unique halo, the collective beauty of atmospheric halos unites sky gazers around the world in awe and wonder. So next time you look up at the sky and witness a halo, take a moment to appreciate the delicate interplay of light and crystals that creates this celestial masterpiece.
Rays and crystals forming a 22º halo. Crystals whose prism axes are roughly perpendicular to the direction of sunlight refract its rays through 22º or more. Each crystal sparkles in the sky when it is at this angular distance from the sun. The collective sparkles make the halo. Millions of other unlit crystals are not visible to you but they might be helping to form someone else's halo!
Halos are the collective glints of millions of crystals which happen to have the right orientation and angular position to direct their refracted light into the your eye.
When the sky contains "poorly aligned" crystals pointing in nearly all directions, which ones glint and where?
How does so disorderly a host produce something so structured and ordered as a 22º halo?
The key is still crystal orientation. Only those crystals with their prism axes roughly perpendicular to the sun's rays allow light to pass through two side faces.
These all deviate light by 22º to 50º to form an illuminated disk with a 22º hole in its centre. The halo is brightest near the hole.
When, as seen by the eye, they are also anywhere on a line 22º - 50º from the sun their light shines into the eye - they glint. But all the lines of those angles form an annulus around the sun. The glints 'light up' the disk to form the halo.
And the other crystals that chance has neither placed nor aligned in positions favourable for you? They are not seen but they could be helping to create another person's halo.
A halo only exists if something is there to see it. It is the effect of a collection of light rays travelling in particular directions and converging on a receiving lens be it an eye or camera. Each person sees their own halo. Someone standing only a short distance away sees another halo made from the collective glints of another set of crystals.
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"Disk with a hole in the sky". Atmospheric Optics. Accessed on November 30, 2023. https://atoptics.co.uk/blog/disk-with-a-hole-in-the-sky/.
"Disk with a hole in the sky". Atmospheric Optics, https://atoptics.co.uk/blog/disk-with-a-hole-in-the-sky/. Accessed 30 November, 2023
Disk with a hole in the sky. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/disk-with-a-hole-in-the-sky/.