Crepuscular rays in Pune and a surprise from Western Ghats

What you see in this image (observed on 6th April 2023, 7.13am, Pune, India) is the emergence of sunlight from clouds in a peculiar expanding beam of rays. Such rays are called crepuscular rays.

The etymology of the word –

crepuscular (adj.)

figurative use, “dim, indistinct,” is attested from 1660s; literal use, “pertaining to or resembling twilight,” from 1755, from Latin crepusculum “twilight, dusk,” related to creper “obscure, uncertain,”

source : “Crepuscular | Search Online Etymology Dictionary.” Accessed April 7, 2023.

These rays are generally observed during the twilight and persist for a short time. The location of observation is close to the horizon, although I sighted it at a reasonable angle above the horizon (on Dr. Homi Bhabha road, in front of IISER-Pune Pashan gate). 

There is a large class of optical phenomena including sunbeams and moonshines, which are exclusively observed at twilight. For more details read this article by Shields, Janet. “Sunbeams and Moonshine.” Optics and Photonics News 5, no. 7 (July 1, 1994): 57.

One may think this phenomenon is related to some kind of diffraction, but it is not. It mainly arises due to the linear perspective of how we visualize this optical effect in an open sky. Of course light scattering has some interesting role in this observation. A very nice discussion on this can be found in an article by Bohren, Craig, and Mark L. Sowers. “Simple Experiments in Atmospheric Physics.” Weatherwise 45, no. 2 (May 1, 1992): 34–38.

A Surprise in Western Ghats

Western ghats are known to show this optical effect. In the year 1984, J. L. Monteith, a researcher who was visiting India from Goddard space center, USA (which is part of NASA) made an interesting and surprising observation. Below I reproduce his text –

For more details on this observation, read this paper by Monteith, J. L. “Crepuscular Rays Formed by the Western Ghats.” Weather 41, no. 9 (September 1986): 292–99.

Interestingly, such rays have also been periodically found in China and was reported as early as 1888 ! See this remarkable observation by Doberck, W. “Crepuscular Rays in China.” Nature 37, no. 959 (March 1888): 464–464. Doberck also makes a connection to a historical text by Homer, in which Greek mariners describe the beautiful skies with peculiar rays. 

Around 2011, the crepuscular rays were simulated in a laboratory and further corroborated using numerical simulations. In order to mimic the behavior of clouds, “artificial fogs and milk–water solutions” were used and interesting quantitative observations were made.  For more details read : Gedzelman, Stanley David, and Michael Vollmer. “Crepuscular Rays: Laboratory Experiments and Simulations.” Applied Optics 50, no. 28 (October 1, 2011): F142–51.

Atmospheric and meteorological optics are fascinating topics with some historical connections. For interested readers I highly recommend the following (fascinating and readable) books : 

  • Bohren, Craig F. Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics. Courier Corporation, 2001.
  •  What Light Through Yonder Window Breaks?: More Experiments in Atmospheric Physics. Courier Corporation, 2006.
  •        Minnaert, M. The Nature of Light and Colour in the Open Air. Revised ed. edition. New York,       N.Y: Dover Publications, 1954.
  • Minnaert, Marcel. Light and Color in the Outdoors. Translated by L. Seymour. Corrected edition. New York: Springer, 1993.

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