SUN HALO

SUN HALO

Recent Context:

An iridescent halo was spotted around the sun in the skies over Bengaluru, mid-morning today. Known as a sun halo, it appears like a circular rainbow around the sun, visible bright during the day.

Introduction

Staying confined in your homes for very long can be difficult. This pandemic has taught us how to get through this period quite effortlessly. One of the methods includes marvelling the nature around us.

A rainbow-like halo has encircled the sun in a perfect circle and this phenomenon is called a halo and it happens because of light interacting with ice crystals in the atmosphere. It is also called ’22-degree halo because of its radius around the sun. 

About Halos

  • Halos are a sign of high, thin cirrus clouds drifting 20,000 feet or more above our heads. These clouds contain millions of tiny ice crystals.
  • The halos seen are caused by both refraction, or splitting of light, and also by reflection, or glints of light from these ice crystals.
  • The crystals have to be oriented and positioned just so with respect to your eye, in order for the halo to appear.
  • Just like a rainbow, a halo is visible when viewed from the right angle — sometimes appearing just white but often with colours of the spectrum also clearly present.
  • Such a halo could also occur around the moon at night, which is formed due to the same phenomenon. Lunar halos are signs that storms are nearby.
  • The halo is the brightest at the inner edge of the circular disk, with no light inside the disk as no light is refracted at smaller angles.
  • Red light is refracted less than other colours of light, so the halo’s inner edge is reddish. Other shades typically tend to overlap and wash out.
  • However, the exact mechanism behind the orientation of these crystals by the winds and the shape in which they do so are unclear and is the subject of much ongoing research.

Different Formation of Halos

  1. Halos formed by low level diamond dust during very cold weather. Such  halos only occur in Polar regions or countries with very cold winters (Canada for example is not high latitude).
  2. Halos formed by ice crystals in high cirrus cloud. Such Halos can occur anywhere on the planet during winter or summer. Their frequency depends on the frequency of cirrus coverage and whether it has had a history such that it contains halo forming crystals. The latter is hard to predict. For example, there are major differences in halo frequencies and types of halos across even 200 miles [300 km] in the UK.

22 degree Halos

  • Circular halos around the sun are large, with a typical radius of a human arm, when viewed from the Earth. These halos are also called 22-degree halos, as the halo or ring has an apparent radius of 22 degrees around the sun.
  • This is measured in the form of angular or apparent distance, where the opposite ends of a line of the halo’s radius is traced back to our line of sight at an angle of 22 degrees.
  • A much larger halo with an apparent radius of 46 degrees is also produced, called the 46-degree halo. This is much rarer than the 22-degree halo, which, in turn, is actually more frequent than rainbows in a year.
  • The exact orientation required for hexagonal ice crystals to cause a 22-degree halo is not fully understood.
  • However, it is thought that the sun’s rays pass through two faces of a crystal, inclined at 60 degrees to each other. As a result, the light is bent by 22 degrees from its original position.

Other types of halos

  • There are also other forms a halo could take, such as a sun dog or a parhelion. A sun dog is a bright spot that appears on a halo on one or both sides of the sun. Typically, two sun dogs are spotted on opposite sides of the sun over a 22-degree halo when this phenomenon occurs.
  • There are also sun pillars where vertical beams of light appear to extend above and below a light source. Light pillars can also be caused by lights from the moon, or more commonly, from street lamps.
  • There are also other phenomenon like a 9-degree smaller halo, multiple concentric circular halos, a tangent arc which appears tangential to a halo, a parhelia circle which is a horizontal line that appears at the same height as the sun, a circum zenithal arc that looks like an upside down rainbow, or a circum horizontal arc that looks like a horizontal straight rainbow.

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