22° Halo in Kolkata

It was about 12 noon today. Interrupting me from trying to wrap my head around Planck’s Quantum Theory and black body radiation on the Internet, my mom prodded me to “go outside and look at the Sun” immediately. Clearly, she’d received a WhatsApp message about something ‘odd’.

This was what I saw.

What I saw. Image Credit: Sagnik Sarkar (CC-BY-SA)

This, is what is called a 22° halo. 

Such a halo is formed in a very specific circumstance, when hexagonal ice crystals in the upper atmosphere line up in a particular arrangement.


Remember the types of clouds? One of them is cirrostratus clouds, if you recall.

Cirrus clouds in the atmosphere. Image Credit: Lisa Gardiner, UCAR

Cirrus as well as cirrostratus clouds are found at altitudes greater than 6 km (that’s higher than most other types of clouds).

Cirrostratus clouds, in particular, are thin, sheet-like clouds composed of water vapour evaporated from the Earth’s surface condensed into ice crystals (remember, it’s way cold up there in the atmosphere). However, they are pretty transparent, and quite often, the only way to detect them is by the formation of halos like these.

A 22° halo is formed when sunlight (or moonlight) is refracted through hexagonal ice crystals with diameters less than 20.5 micrometers present in such cirrostratus clouds . 

The hexagonal ice crystals form part of a equilateral prism, and thus behave like one.

Image Credit: Hyperphysics, UIUC

Just like with a prism, the incident sunlight gets split into its constituent colours (violet through red, violet being deviated the most and red the least) by the phenomenon of dispersion, thus forming a rainbow-like feature at the edge of the halo.

Dispersion of white light in a prism. Image Credit: tutorvista.com

The wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish and the outer edge to be bluish.

The hexagonal ice crystals need to be aligned in a specific position, such that the deviation produced in the path of the light rays is about 22 degrees. A numbers of ice crystal so aligned create a 22° halo at a particular distance.

Another view of the same phenomenon from today afternoon.  Image Credit: Sagnik Sarkar (CC-BY-SA)

So long! So, how else in Kolkata noticed this today (I mean; Monday, April 30, 2016). 


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