For reasons I do not fully understand, this is my second post about weird lights in the atmosphere this month. As an environmental geologist, I generally get more excited about the ground than about the air; that said, it’s hard to pass up a chance to investigate cool science when it comes my way.
The following photos were taken of the sky above several Finnish cities on January 19, 2014.
Photo credit: Atacan Ergin
Photo credit: Jari Kankaanpaa
These multi-coloured vertical light ‘pillars’ are neither fireworks nor Northern lights, but an entirely different phenomenon that occurs in northern cities under very particular atmospheric conditions. To form, these pillars require two ingredients – light pollution and ice crystals in the atmosphere. The stress here falls on the latter, since light pollution on its own is hardly a cause for excitement.
The process is, in fact, quite elegant in its simplicity. Bright, unshielded light from a source such as a streetlamp is reflected downwards by thin, flat ice crystals in the air. Such ice crystals commonly exist in high clouds, but under the right conditions may find their way close to the ground. The tricky bit to grasp is that the pillars aren’t actually there. They result from an interaction between the light source and the observer, potentially a long distance away. Another observer standing a few meters away from the same streetlamp would likely see nothing at all.
The closest analogy that can be drawn is that of a mirror. When you look in a mirror and see a reflection of your hand, it’s obvious that your hand is still attached to you and not on the other side of the mirror as it actually appears. This makes sense to you, since your brain has grasped the concept of this visual illusion. To an infant or a puppy, however, there is no association between the faces in the mirror and their own. A baby can happily spend an eternity investigating his or her own reflection. In fact, your reflection is a function of a incorrect reconstruction made by your brain. Imagine drawing a ray of light that connects your hand to your eye. The ray would travel from your hand to the mirror, bounce off, and reach your eyes. What your brain does instead is follow the ray backwards from your eye and through the mirror, making the incorrect assumption that your hand is, in fact, behind the mirror.
The same ‘brain trick’ is what creates the light pillars. Light from a streetlamp is reflected downwards by ice crystals roughly halfway in between the source and the observer. The eyes (and camera) of the observer instead follow the light ray further upwards, reconstructing the light in the sky. When many ice crystals, one above another, act in unison, this illusory light is stretched into a pillar.
This link provides a diagram of how light rays move between the source and the observer in this scenario.
Be warned: you may have just sustained a lethal dose of mostly harmless science.
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Cover photo courtesy of Bo Insogna, Flickr Creative Commons.