Halos are optical phenomena that are caused due to the dispersion of light by ice crystals in the atmosphere. There can be many types of halos depending on the type of light available, the number of ice crystals in the air and so on.
The ice crystals that cause halos can be suspended anywhere between the immediate surface of the Earth and up to 10 km from it, among the cirrus clouds. In the former case, these crystals are known as Diamond Dust.
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What are the different types of halos?
There can be many types depending on the local conditions. Some of the most common types are enlisted below. In all of these different iterations of what is essentially the same phenomenon, the ice crystals reflect and refract light rays from any source and send them ricocheting into different directions. The light rays can also get a tinge of color if dispersion takes place. Interestingly, as the size of the ice crystals grows, the phenomenon gets more pronounced.
22-degree halo or the Circular Halo
This is the most common form of halos. Also known as the 22-degree halo because this optical illusion generally forms a ring of 22 degrees, around the sun or the moon. Moreover, it can also be seen around artificial lights like streetlights in cold weather.
The formation of this halo involves a certain orientation of the crystals of ice in the atmosphere. Though it is still up for debate, the general consensus is that the hexagonal wafers of ice are arranged in random orientations in clusters. Think of it as a very thin cloud of random ice crystals.
Within these crystals, light rays undergo deflection twice in accordance with the ray theory of light. The range of the deviation angles lies between 22-50 degrees. So why is it called the 22-degree halo? Because the circular ring visible to us is formed only by the light rays deflecting at 22 degrees. The other deflections are not involved in the formation of the ring. As 22 degrees is the minimum angle of deviation, there are no light rays inside the circular ring. Hence the inside of the ring appears slightly darker than the rest of the sky.
Sun dogs
Known as a parhelion in meteorological terms, it’s a unique phenomenon where bright spots of lights are visible on both the sides of the sun. These spots of light are often within a 22-degree circle so you can generally observe two sun dogs and a circular halo at the same time. The two bright spots lie on the same horizontal line that passes through the sun.
Sun dogs are also formed due to hexagonal and flat-surfaced ice crystals situated above immediate land and below cirrus clouds. These crystals act as prisms to refract and scatter the light at a deviation of 22 degrees. As a result, circular halos are also generally viewable with sun dogs.
Light pillars
These are just heavenly. As the name suggests, light pillars are pillars of light that seem like they have descended from a UFO or are respawning some dead game character.
Light pillars are formed by either natural light from the sun or the moon or from terrestrial artificial lights. The ice crystals that cause this spectacular phenomenon are usually hexagonal and have a flat surface. However, in some rare cases, column-shaped ice crystals can also generate these halos. Light pillars are formed when the orientation of these ice crystals in the atmosphere is parallel to Earth’s surface. The light bouncing off of these crystals form a virtual image that extends in a direction perpendicular to the light source. Here is a diagram explaining their formation.
Sources
- Pillars of light
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