Recently I led a photography workshop in Italian Dolomites. One of the evenings we were treated with most beautiful conditions one can imagine, heavy storm with dark clouds, followed by a rainbow, anticrepuscular rays and amazing sunset colours.
At one point we noticed beautiful god rays coming from behind the mountain. Normally I would say these are sun rays, but the sun was setting behind our backs. So what were these rays if not from sun?
Anticrepuscular rays, also known as antisolar rays, are a fascinating atmospheric phenomenon that occurs opposite the sun’s direction. Here’s a detailed explanation of what they are and how they form:
What are Anticrepuscular Rays?
Anticrepuscular rays manifest as lines of light and shadow converging at a point directly opposite the sun in the sky, known as the antisolar point. Unlike the more commonly observed crepuscular rays, which extend from the horizon towards the sun at sunrise or sunset, anticrepuscular rays seem to meet at the antisolar point, creating the illusion that they radiate from a point below the horizon on the side opposite the sun.
Formation of Antisolar Rays
The formation of anticrepuscular rays involves several atmospheric conditions and optical effects:
- Sunset or Sunrise: People most often see anticrepuscular rays during sunrise or sunset. At these times, the sun sits low on the horizon. This low position creates long shadows and sharp contrasts between light and dark in the atmosphere.
- Obstructions: Clouds, mountains, or distant objects (such as mountains) can block some sunlight to form anticrepuscular rays. These obstructions cast long shadows.
- Scattering of Light: Sunlight scatters when it passes through Earth’s atmosphere. Molecules and particles scatter short wavelengths like blue and violet more than long wavelengths like red and orange. That’s why the sky looks blue during the day. At sunrise and sunset, sunlight travels a longer path through the atmosphere. This longer path scatters short wavelengths and leaves longer wavelengths, making the sky look reddish. This effect also enhances the visibility of rays and shadows.
- Perspective: Anticrepuscular rays meeting at the antisolar point is an optical illusion. It’s due to linear perspective. The rays run parallel, but our eyes perceive them as converging in the distance. This effect is similar to how parallel railroad tracks seem to meet at the horizon.
The contrast between the light rays and the shadows they cast through the atmosphere creates the visible streaks that we see as antisolar rays. The conditions need to be just right for these rays to be visible, which makes them a less commonly observed phenomenon than their counterparts, the crepuscular rays.
The only other time I witnessed this phenomena was three years ago, in Dolomites, Italy as well. It seems I need to go there more often – high mountains are ideal for unusual weather.
Have you ever witnessed these rays yourself? Where? Send the photos and I will publish them here along with a link to your website.
Photos of Anticrepuscular Rays
Recently, Bill Lea sent me an email with his photos of this phenomena from the Great Smoky Mountains National Park. Thank you Bill for connecting and sharing your images! So here they are.
