Crespuscular Rays - What Are They?

Nothing like driving home from a long day of work only to notice a pillar of light radiating from the horizon. Have you ever wondered what creates this heart-warming phenomenon? 

The rays of light that you see beaming from the sky is known as crepuscular rays. The word “crepuscular” has Latin roots to “crepusculum” which stands for twilight. These sunbeam rays often are formed when the sun is at its lowest level in the sky. This is typically in the morning just before dawn or in the evening just before or during twilight.

Speaking of which, these sunbeams are not just confined to the start and finish of the day, but also can be seen when the sun is high in the sky and it’s at least partly cloudy. Once seen, these rays are often referred to “Heaven opening up” as they’ve been known to be called “God rays.” They appear heavenly to the naked eye. 

The science behind this type of sunbeam starts with the atmosphere itself. Specific particles such as dust and water vapor can influence how prominent these rays are in the sky. Like the colorful sunrises and sunsets, when the sun’s light hits these particles, it produces a scattering effect that focusses the light into parallel lines, birthing the brilliant rays. Another factor involves cloud cover and how dark they are. The darker the clouds, the brighter these rays will appear. It is a simple result of the contrast between light and dark. 

Like any good yin and yang relationship, another type of sunbeam called an anti-crepuscular ray also provides similar illusions. Unlike the name implies, these anti-crepuscular rays are essentially the same as crepuscular rays except that their direction is opposite from the sun. Ironically, this is known as backscattering which happens when light travels back to its source after being reflected from the air particles. When this takes place, a parallel line of light is seen radiating through the concentrated air. 

The same optical illusion is seen in rainbows. To illustrate this, image a carriage or bike wheel.  The wheel’s rim is the rainbow and the sunbeam rays are the spokes, creating a “wagon-wheel spokes” anti-crepuscular rays. Another cool example of these “anti” rays is a shadow effect that is created by a mountain. As the sun sets, the tip of the mountain acts as a focal point of the suns rays and creates a triangular anti-crepuscular ray in its wake. 

It is important to note that anti-crepuscular rays are far dimmer than the crepuscular rays, but the conditions are the same. These rays can be witnessed during the times the sun is at is lowest level in the sky, such as at sunrise and sunset. Not to mention, if there are a few dark clouds around then there is a good chance of seeing both “sunbeams” fighting for the spotlight.

The next time you are out for a sunset walk, keep an eye out for crepuscular rays. 

Image: Crepuscular rays of Taipei, Taiwan photographed in July 2018 (Steven Weng via Wikimedia Commons)