If you caught Wednesday’s morning sunrise, chances are you may have noticed a stunning solar phenomenon. It’s not frightening, and actually far more intriguing... crepuscular rays shining on a steamy September day.
NBC Boston photojournalist Mark Garfinkel captured the view from Roxbury’s Fort Hill at sunrise.
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In Latin, the word crepusculum means twilight, and can be seen at sunrise and sunset. While the name sounds technical, their formation is a fairly straight-forward explanation, and not too science-intensive. They form when taller and thicker clouds, or large features like mountains or buildings, partially block sunlight, and breaks in clouds allow sunlight to beam through. When there are numerous breaks, you see the fanned out effect, alternating between lighter blue, and darker blue skies.
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The American Meteorological Society defines crepuscular rays as shadow band. These “twilight rays” are alternating dark and light bands (shadows and light scattered from sunbeams, respectively) that seem to diverge fanlike from the sun’s position during twilight.
Let’s look at Mark’s photo more. We know it was taken at twilight on Wednesday morning. The crepuscular rays appear to converge on the horizon and diverge out (away) from the rising sun. The effect is exacerbated more with haze and dust particles, that scatters light in unshadowed areas.
In fact, Wednesday’s haze has been brought about by a combination of pollutants, from wildfire smoke and ground-level zone, that’s accelerated due to the heat. So while the sunrise, is pretty, it’s been induced because of pollutants in the air.
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It’s also worth noting that we see sunlight because of gases, water vapor and dust in the atmosphere that scatters the light. If we didn’t have this, the sky would appear black, like if you were observing Earth from the moon. But because there is an atmosphere on our planet, we see blue sky, with yellow and red hints at sunrise and sunset.
The sunbeams, or rays, are actually parallel, and only appear to converge at the sun (behind Fort Hill tower in Mark’s photo). This illuminated illusion is nothing more than perspective. Think about standing on one end of a long straight road, a row of crops, or train tracks, and they converge in the distance. This is very similar.
Crepuscular rays converge toward the sun, but in the inverse, anti-crepuscular rays converge opposite the sun. And like its counterparts, they also form with shadows cast by clouds and made visible by dust, haze and gases in the atmosphere. This type of ray is dependent on reflected sunlight and is therefore weaker.
Spotting crepuscular and anti-crepuscular rays can be tricky. An easy way to distinguish between the two is to understand if the rays are converging toward the sun, or opposite the solar point.
